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Surajit Saha

IISER Bhopal, Physics, Faculty Member

National University of Singapore, Department of Physics, Post-Doc

Indian Institute of Science, Physics Department, Alumnus

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Presently, I am a faculty member at the Department of Physics, IISER Bhopal.

Oxides and their interfaces reveal fascinating properties which may be tailored to achieve novel functionalities for electronic and optoelectronic applications. I try to explore various oxides in search of their functionalities besides trying to understand their fundamentals. In addition, I am working toward integrating oxides with 2D materials (graphene & TMDCs) for multi-functional device applications and basic science.

Education:

PhD (Physics) from Indian Institute of Science (IISc), Bangalore, in 2010.

MSc (Physics) from Indian Institute of Technology (IIT), Guwahati, Assam, in 2002.

BSc (Physics) from Goalpara College (Gauhati University), Assam, in 2000.
Phone: +91-755-6692517
Address: Department of Physics
Indian Institute of Science Education and Research (IISER), Bhopal
Indore-Bypass Road, Bhauri,
Bhopal 462066, INDIA

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IIT Kanpur

National Chiayi University

McMaster University

Dr. Anjali Rathore

Ayush University Of Health Sciences

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Papers by Surajit Saha

Raman mode softening in SrMnO3 induced by optical excitation

Physical Review B

In recent years, photosensitive materials have been in huge demand because of their fascinating a... more In recent years, photosensitive materials have been in huge demand because of their fascinating ability to convert absorbed photon energy to generate strain and henceforth tuning physical properties. In this work we report the photosensitive activity of SrMnO 3. Using a power-dependent and temperature-dependent Raman study with different laser sources having wavelengths across the optical band gap of SrMnO 3 , we divulge the photosensitive character of SrMnO 3 thin film. Upon laser light illumination, Raman modes soften, and softening increases further with an increase in laser power. A similar kind of mode variation is observed with increasing temperature at fixed laser power. A soft x-ray absorption near-edge structure in the presence of laser illumination reveals the change in crystal field splitting associated with mode softening.

Electron–phonon coupling in APd3O4: A = Ca, Sr, and Sr0.85Li0.15

Journal of Physics: Condensed Matter, 2020

Here we have investigated the role of electron phonon coupling on the Raman spectrum of narrow ba... more Here we have investigated the role of electron phonon coupling on the Raman spectrum of narrow bandgap semiconductors APd3O4 (A = Ca, Sr) and hole-doped system Sr0.85Li0.15Pd3O4. Four Raman active phonons are observed at room temperature for all three compounds as predicted by factor group analysis. The lowest energy phonon (∼190/202 cm−1) associated with Pd vibrations is observed to exhibit an asymmetric Fano-like lineshape in all the three compounds, indicating the presence of an interaction between the phonon and the electronic continuum. The origin of the electronic continuum states and electron–phonon coupling are discussed based on our laser power- and temperature-dependent Raman results. We have observed an enhanced strength of electron–phonon coupling in Sr0.85Li0.15Pd3O4 at low temperatures which can be attributed to the metallicity in this doped compound.

Tailoring the phase transition and electron-phonon coupling in 1T′−MoTe2 by charge doping: A Raman study

Physical Review B, 2020

Transition metal dichalcogenides (TMDs) are a class of widely studied 2D layered materials which ... more Transition metal dichalcogenides (TMDs) are a class of widely studied 2D layered materials which exist in various polymorphs. The 1T′ phase of MoTe2 is of prime importance as it has been reported to show quantum spin hall (QSH) behavior with a fairly large band-gap of ~ 60 meV, in contrast to most QSH materials known. It is noteworthy that though the monolayer 1T′-MoTe2 was initially predicted to show the QSH behavior, recent theoretical studies claim that the few-layered counterparts also exhibit higher order topological behavior. Besides, 1T′-MoTe2 also undergoes a hysteretic phase transition to the Td phase (which is a type-II Weyl semimetal) by breaking the inversion symmetry of the crystal. While the phase transition between these two topological phases is of utmost importance, its study has been mostly restricted to bulk single crystal flakes, thereby not sufficiently exploring the effect of dimensionality. We have studied the phase transition in 1T′-MoTe2 as a function of flake-thickness. Though our Raman studies show a suppression of the phase transition in the thin (thickness <10 nm) flakes [similar to the report Phys. Rev. B 97, 041410 (2018)], we have experimentally demonstrated the possibility of stabilizing the desired phase (1T′ or Td) at room temperature by charge doping. Further, we have observed clear signatures of electron-phonon coupling in MoTe2, which evolves as a function of flake-thickness and charge doping.

Electron-phonon and phonon-phonon anharmonic interactions in 2H−MoX2 ( X=S,Te ): A comprehensive resonant Raman study

Physical Review B, 2021

Transition metal dichalcogenides (TMDs) are layered materials which show excellent potential for ... more Transition metal dichalcogenides (TMDs) are layered materials which show excellent potential for nanoelectronic and optoelectronic applications. However, as many of the exciting features of these materials are controlled by the anharmonic effects, a proper understanding of the phonon properties and anharmonicity associated with these materials is essential f or the proposed applications to be realized. We present here a comprehensive study on the phonon properties of two different TMDs; viz., MoS 2 and MoTe 2 , as a function of temperature, laser excitations, and polarization, as well as flake-thickness. Our study includes the measurement of anharmonicity in the first-order and higher order Raman scattering processes. The variations in anharmonicity with the thickness of MoS 2 and MoTe 2 have been explained in terms of their phonon symmetries , electronphonon coupling, and phonon-phonon interactions. Further, the effect of the underlying substrate on the anharmonic properties of the in-plane and out-of-plane phonons has also been estimated from the Raman measurements, thus elucidating the intrinsic phonon properties of the 2D layered materials.

Superconductivity in doped Weyl semimetal Mo0.9Ir0.1Te2 with broken inversion symmetry

Superconductor Science and Technology, 2021

This work presents the emergence of superconductivity in Ir—doped Weyl semimetal T d —MoTe2 with ... more This work presents the emergence of superconductivity in Ir—doped Weyl semimetal T d —MoTe2 with broken inversion symmetry. Chiral anomaly induced planar Hall effect and anisotropic magneto-resistance confirm the topological semimetallic nature of Mo 1 − x Ir x Te2. Observation of weak anisotropic, moderately coupled type-II superconductivity in T d -Mo 1 − x Ir x Te2 makes it a promising candidate for topological superconductor.

Engineering crystal structure and spin-phonon coupling in Ba1-xSrxMnO3

Journal of Magnetism and Magnetic Materials, 2021

The interplay between different degrees of freedom such as charge, spin, orbital, and lattice has... more The interplay between different degrees of freedom such as charge, spin, orbital, and lattice has received a great deal of interest due to its potential to engineer materials properties and their functionalities for device applications. In this work, we have explored the crystallographic phase diagram of Ba1-xSrxMnO3 and studied the correlation between two degrees of freedom, namely phonons and spins using magnetization and inelastic light scattering measurements. The system undergoes a series of crystallographic phase transitions 2H ➔ 9R ➔ 4H as a function of doping (Sr) as observed by X-ray diffraction measurements. Investigation of their temperature-dependent magnetization reveals a para-to antiferro-magnetic transition for all the compositions. An Eg phonon in the 9R phase and an E1g phonon in the 4H phase involving Mn/O-vibrations, show anomalous temperature-dependence in the antiferromagnetic phase arising due to spin-phonon coupling.

Signatures of magnetostriction and spin-phonon coupling in magnetoelectric hexagonal15R−BaMnO3

Physical Review B, 2020

Spin-phonon coupling, the interaction of spins with surrounding lattice is a key parameter to und... more Spin-phonon coupling, the interaction of spins with surrounding lattice is a key parameter to understand the underlying physics of multiferroics and engineer their magnetization dynamics. Elementary excitations in multiferroic materials are strongly influenced by spin-phonon interaction, making Raman spectroscopy a unique tool to probe these coupling(s). Recently, it has been suggested that the dielectric and magnetic properties of 15R-type hexagonal BaMnO3 are correlated through the spin-lattice coupling. Here, we report the observation of an extensive renormalization of the Raman spectrum of 15R-BaMnO3 at 230 K, 280 K, and 330 K. Magnetic measurements reveal the presence of a long-range and a short-range magnetic ordering in 15R-BaMnO3 at 230 K and 330 K, respectively. The Raman spectrum shows the appearance of new Raman modes in the magnetically ordered phases. Furthermore, an additional Raman phonon appears below ~ 280 K, possibly arising from a local latticedistortion due to the displacement of Mn-ions, that exhibits anomalous shift with temperature. The origin of the observed renormalization and phonon anomalies in Raman spectra are discussed based on the evidences from temperature-and magnetic-field-dependent Raman spectra, temperature-dependent x-ray diffraction, magnetization, and specific heat measurements. Our results indicate the presence of magnetostriction and spin-phonon coupling in 15R-BaMnO3 thus suggesting that the optical phonons are strongly correlated to its magnetoelectric properties.

Direct Bandgap-like Strong Photoluminescence from Twisted Multilayer MoS2 Grown on SrTiO3

ACS Nano, 2020

While direct bandgap monolayer 2D transition metal dichalcogenides (TMDs) have emerged as an impo... more While direct bandgap monolayer 2D transition metal dichalcogenides (TMDs) have emerged as an important optoelectronic material due to strong light−matter interactions, their multilayer counterparts exhibit an indirect bandgap resulting in poor photon emission quantum yield. We report strong direct bandgap-like photoluminescence at ∼1.9 eV from multilayer MoS 2 grown on SrTiO 3 , whose intensity is significantly higher than that observed in multilayer MoS 2 / SiO 2. Using high-resolution electron microscopy we observe interlayer twist and >8% increase in the van der Waals gap, which leads to weaker interlayer coupling. This affects the evolution of the band structure in multilayer MoS 2 as probed by transient absorption spectroscopy, causing higher photo carrier recombination at the direct gap. Our results provide a platform that could enable multilayer TMDs for robust optical device applications.

Hexagonal Boron Nitride–Graphene Heterostructures with Enhanced Interfacial Thermal Conductance for Thermal Management Applications

ACS Applied Nano Materials, 2021

Atomically thin monolayers of graphene show excellent electronic properties which have led to a g... more Atomically thin monolayers of graphene show excellent electronic properties which have led to a great deal of research on their use in nanoscale devices. However, heat management of such nanoscale devices is essential in order to improve their performance. Graphene supported on hexagonal boron nitride (h-BN) substrate has been reported to show enhanced (opto)electronic and thermal properties as compared to extensively used SiO 2 /Si supported graphene. Motivated by this, we have performed temperature-and power-dependent Raman Spectroscopic measurements on four different types of (hetero)structures: (a) h-BN (BN), (b) graphene (Gr), (c) h-BN on graphene (BG), and (d) graphene encapsulated by h-BN layers from both top and bottom (BGB), all supported on SiO 2 /Si substrate. We have estimated the values of thermal conductivity (κ) and interfacial thermal conductance per unit area (g) of these four (hetero)structures to demonstrate the structure-activity (thermal) relationship. We report here the values of κ and g for h-BN supported on SiO 2 /Si as 280.0±58.0 Wm-1 K-1 and 25.6±0.4 MWm-2 K-1 , respectively. More importantly, we have observed an improvement in both thermal conductivity and interfacial thermal conductance per unit area in the heterostructures which ensures a better heat dissipation in devices. The κ and g of h-BN encapsulated graphene on SiO 2 /Si (BGB) sample was observed to be 850.0±81.0 Wm-1 K-1 and 105±1 MWm-2 K-1 , respectively, as opposed to 600.0±93.0 Wm-1 K-1 and 1.15±0.40 MWm-2 K-1 , respectively, for graphene on SiO 2 /Si substrate. Therefore, we propose that for graphene-based nanoscale devices, encapsulation with h-BN is a better alternative to address heat management issues.

Signatures of weak spin-phonon coupling in 12R-BaTi0.4Mn0.6O3: A Raman study

DAE SOLID STATE PHYSICS SYMPOSIUM 2019, 2020

Antiferromagnetic materials with strong spin-phonon coupling have the potential for spintronic ap... more Antiferromagnetic materials with strong spin-phonon coupling have the potential for spintronic applications. Here, we report the structural, magnetic, and vibrational properties of 12R-BaTi0.4Mn0.6O3. Temperature-dependent magnetization data show the presence of weak antiferromagnetic interactions. Correlation between magnetic and vibrational properties are probed by using temperature-dependent Raman spectroscopic measurements. Two of the Raman active phonons show deviation from anharmonic behaviour at low temperatures which can be attributed to spin-phonon coupling. The strength of coupling is estimated using meanfield approximation and is found to be 1.1 and 1.2 cm-1 for phonons at 530 and 717 cm-1 , respectively.

Tuning phonon anharmonicity in spin-ice Dy2Ti2O7

DAE SOLID STATE PHYSICS SYMPOSIUM 2019, 2020

Chemical dilution is an important parameter for tuning the electrical, vibrational, and magnetic ... more Chemical dilution is an important parameter for tuning the electrical, vibrational, and magnetic properties of the complex oxides. Pyrochlore titanates are known for displaying a high level of phonon anharmonicities which is attributed to the strong phonon-phonon anharmonic interactions. Therefore, these systems are perfect hosts for tuning the phonon anharmonicity with external chemical pressure. Here, we present temperature-dependent Raman studies of polycrystalline Dy2Ti2O7 and its diluted analogues Dy1.9Sm0.1Ti2O7 and Dy1.9La0.1Ti2O7. We report the reduction of phonon anharmonicities in spin-ice Dy2Ti2O7 with the incorporation of larger sized atom at rare-earth site using Raman spectroscopy. This can be explained considering an indirect effect of doped atoms on the amplitude of the phonon vibration associated to Ti4+ tetrahedra containing oxygen vacancy at its centre.

Polaronic Trions at the MoS 2 /SrTiO 3 Interface

Advanced Materials, 2019

create mixed dimensional heterostructures [5] of 2D materials and functional substrates, which pr... more create mixed dimensional heterostructures [5] of 2D materials and functional substrates, which provide an even wider opportunity to tailor diverse material functionalities and interface phenomena. In this regard, a very promising substrate is SrTiO 3 (STO), a transition metal oxide with an ABO 3-type perovskite crystal structure, that has attracted significant attention due to both its unusual bulk properties like high dielectric constant, [6] quantum paraelectricity, [7] and several interface phenomena such as the observation of 2D electron gas at interfaces with oxide insulators [8] and enhancement of superconducting T c at the interfaces with 2D FeSe. [9] The speciality of STO lies in its rich phonon spectrum centered on its AFD structural phase transition [10] (from cubic to tetragonal). The uniqueness of the phonons in STO, specifically the low energy soft phonons when compared to conventional phonons present in most materials lies in the following: i) they arise at a phase transition temperature and their frequency (energy) softens to zero near the transition ii) their intensity increases at low temperatures whereas conventionally phonon intensity decreases at lower temperatures The reduced electrical screening in 2D materials provides an ideal platform for realization of exotic quasiparticles, that are robust and whose functionalities can be exploited for future electronic, optoelectronic, and valleytronic applications. Recent examples include an interlayer exciton, where an electron from one layer binds with a hole from another, and a Holstein polaron, formed by an electron dressed by a sea of phonons. Here, a new quasiparticle is reported, "polaronic trion" in a heterostructure of MoS 2 /SrTiO 3 (STO). This emerges as the Fröhlich bound state of the trion in the atomically thin monolayer of MoS 2 and the very unique low energy soft phonon mode (≤7 meV, which is temperature and field tunable) in the quantum paraelectric substrate STO, arising below its structural antiferrodistortive (AFD) phase transition temperature. This dressing of the trion with soft phonons manifests in an anomalous temperature dependence of photoluminescence emission leading to a huge enhancement of the trion binding energy (≈70 meV). The soft phonons in STO are sensitive to electric field, which enables field control of the interfacial trion-phonon coupling and resultant polaronic trion binding energy. Polaronic trions could provide a platform to realize quasiparticle-based tunable optoelectronic applications driven by many body effects.

Effect of phonon anharmonicity on ferroelectricity in EuxBa1−xTiO3

Physical Review Materials, 2019

Investigating the competition between ferroelectric ordering and quantum fluctuations is essentia... more Investigating the competition between ferroelectric ordering and quantum fluctuations is essential to tailor the desired functionalities of mixed ferroelectric and incipient ferroelectric systems, like, (Ba,Sr)TiO3 and (Eu,Ba)TiO3. Recently, it has been shown that suppression of quantum fluctuations increases ferroelectric ordering in (Eu,Ba)TiO3 and since these phenomena are coupled to crystallographic phase transitions it is essential to understand the role of phonons. Here, we observe that the unusual temperature dependence of phonons in BaTiO3 gets suppressed when Ba 2+ is replaced by Eu 2+. This manifests in a decrease in the cubic-to-tetragonal (i.e., para-to-ferroelectric) phase transition temperature (by 150 K) and a complete suppression of tetragonality of the lattice (at room temperature by 40% replacement of Ba 2+ by Eu 2+). We have quantified the anharmonicity of the phonons and observed that the replacement of Ba 2+ by Eu 2+ suppresses it (by 93%) with a resultant lowering of the ferroelectric ordering temperature in the EuxBa1-xTiO3. This suggests that tuning phonon anharmonicity can be an important route to novel ferroelectric materials.

Robust resistive memory devices using solution-processable metal-coordinated azo aromatics

Nature materials, Jan 23, 2017

Non-volatile memories will play a decisive role in the next generation of digital technology. Fla... more Non-volatile memories will play a decisive role in the next generation of digital technology. Flash memories are currently the key player in the field, yet they fail to meet the commercial demands of scalability and endurance. Resistive memory devices, and in particular memories based on low-cost, solution-processable and chemically tunable organic materials, are promising alternatives explored by the industry. However, to date, they have been lacking the performance and mechanistic understanding required for commercial translation. Here we report a resistive memory device based on a spin-coated active layer of a transition-metal complex, which shows high reproducibility (∼350 devices), fast switching (≤30 ns), excellent endurance (∼10(12) cycles), stability (>10(6) s) and scalability (down to ∼60 nm(2)). In situ Raman and ultraviolet-visible spectroscopy alongside spectroelectrochemistry and quantum chemical calculations demonstrate that the redox state of the ligands determines...

Phase pure epitaxial growth of BiFeO3 films: An effect of oxygen partial pressure

Solid State Communications, 2017

We report the growth of bismuth ferrite, BiFeO 3 (BFO), thin films by varying oxygen partial pres... more We report the growth of bismuth ferrite, BiFeO 3 (BFO), thin films by varying oxygen partial pressure (230, 100 and 10mTorr) in pulsed laser deposition system using 10% bismuth excess BFO target on SrTiO 3 (001) [STO] substrates at 700°C each for 15min duration. Xray diffraction shows the presence of an impurity phase along with BFO peaks for 230mTorr film, whereas oriented BFO film is observed for 100mTorr film with a small impurity phase. Importantly, epitaxial growth of BFO is observed without any impurity phase and with sufficiently increased strain for the 10mTorr film, confirmed by the reciprocal space mapping of (002) and (103) reflections. UV-Raman measurements have been performed on as grown films and the 10mTorr BFO film after annealing up to 500 o C to check its thermal stability. Atomic force microscopic (AFM) study reveals that the 10mTorr BFO film shows a lower roughness as compared to 100mTorr and 230mTorr films. Fast Fourier Transform (FFT) analysis in AFM shows a fourfold symmetric structure suggesting the presence of both modes of pattern-orientation in 230mTorr BFO film whereas an elongated bright spot along the y-direction in the FFT of 10mTorr BFO film suggests the growth tendency to be along the y-direction.

Fabrication of a biomimetic ZeinPDA nanofibrous scaffold impregnated with BMP‐2 peptide conjugated TiO 2 nanoparticle for bone tissue engineering

Journal of Tissue Engineering and Regenerative Medicine, 2017

A biomimetic Zein polydopamine (PDA) based nanofiber scaffold was fabricated to deliver bone morp... more A biomimetic Zein polydopamine (PDA) based nanofiber scaffold was fabricated to deliver bone morphogenic protein-2 (BMP-2) peptide conjugated titanium dioxide (TiO 2) nanoparticles in a sustained manner for investigating its osteogenic differentiation potential. To prolong its retention time at the target site, BMP-2 peptide has been conjugated to TiO 2 nanoparticles owing to its high surface to volume ratio. The effect of biochemical cues from BMP-2 peptide and nano topographical stimulation of electrospun Zein PDA nanofibers were examined for its enhanced osteogenic expression of human fetal osteoblast (hFOB) cells. The sustained delivery of bioactive signals, improved cell adhesion, mineralization and differentiation could be attributed to its highly interconnected nanofibrous matrix with unique material composition. Further, the expression of osteogenic markers revealed that the fabricated nanofibrous scaffold possess better cell-biomaterial interactions. These promising results demonstrate the potential of the composite nanofibrous scaffold as an effective biomaterial substrate for bone regeneration.

Origin and Quenching of Novel ultraviolet and blue emission in NdGaO3: Concept of Super-Hydrogenic Dopants

Scientific reports, Nov 3, 2016

In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth b... more In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth based perovskite NdGaO3 (NGO) and the systematic quench of the NGO photoluminescence (PL) by Ce doping. Study of room temperature PL was performed in both single-crystal and polycrystalline NGO (substrates and pellets) respectively. Several NGO pellets were prepared with varying Ce concentration and their room temperature PL was studied using 325 nm laser. It was found that the PL intensity shows a systematic quench with increasing Ce concentration. XPS measurements indicated that nearly 50% of Ce atoms are in the 4+ state. The PL quench was attributed to the novel concept of super hydrogenic dopant (SHD)", where each Ce(4+) ion contributes an electron which forms a super hydrogenic atom with an enhanced Bohr radius, due to the large dielectric constant of the host. Based on the critical Ce concentration for complete quenching this SHD radius was estimated to be within a range of 0.8...

Magnetic Modes in Rare Earth Perovskites: A Magnetic-Field-Dependent Inelastic Light Scattering study

Scientific reports, Nov 15, 2016

Here, we report the presence of defect-related states with magnetic degrees of freedom in crystal... more Here, we report the presence of defect-related states with magnetic degrees of freedom in crystals of LaAlO3 and several other rare-earth based perovskite oxides using inelastic light scattering (Raman spectroscopy) at low temperatures in applied magnetic fields of up to 9 T. Some of these states are at about 140 meV above the valence band maximum while others are mid-gap states at about 2.3 eV. No magnetic impurity could be detected in LaAlO3 by Proton-Induced X-ray Emission Spectroscopy. We, therefore, attribute the angular momentum-like states in LaAlO3 to cationic/anionic vacancies or anti-site defects. Comparison with the other rare earth perovskites leads to the empirical rule that the magnetic-field-sensitive transitions require planes of heavy elements (e.g. lanthanum) and oxygen without any other light cations in the same plane. These magnetic degrees of freedom in rare earth perovskites with useful dielectric properties may be tunable by appropriate defect engineering for ...

Magneto-Optical Study of Defect Induced Sharp Photoluminescence in LaAlO3 and SrTiO3

Scientific reports, Sep 13, 2016

Strongly correlated electronic systems such as Transition Metal Oxides often possess various mid-... more Strongly correlated electronic systems such as Transition Metal Oxides often possess various mid-gap states originating from intrinsic defects in these materials. In this paper, we investigate an extremely sharp Photoluminescence (PL) transition originating from such defect states in two widely used perovskites, LaAlO3 and SrTiO3. A detailed study of the PL as a function of temperature and magnetic field has been conducted to understand the behavior and origin of the transition involved. The temperature dependence of the PL peak position for SrTiO3 is observed to be opposite to that in LaAlO3. Our results reveal the presence of a spin/orbital character in these transitions which is evident from the splitting of these defect energy levels under a high magnetic field. These PL transitions have the potential for enabling non-contact thermal and field sensors.

Correlation of nanoscale behaviour of forces and macroscale surface wettability

Nanoscale, 2016

Raman mode softening in SrMnO3 induced by optical excitation

Physical Review B

In recent years, photosensitive materials have been in huge demand because of their fascinating a... more In recent years, photosensitive materials have been in huge demand because of their fascinating ability to convert absorbed photon energy to generate strain and henceforth tuning physical properties. In this work we report the photosensitive activity of SrMnO 3. Using a power-dependent and temperature-dependent Raman study with different laser sources having wavelengths across the optical band gap of SrMnO 3 , we divulge the photosensitive character of SrMnO 3 thin film. Upon laser light illumination, Raman modes soften, and softening increases further with an increase in laser power. A similar kind of mode variation is observed with increasing temperature at fixed laser power. A soft x-ray absorption near-edge structure in the presence of laser illumination reveals the change in crystal field splitting associated with mode softening.

Electron–phonon coupling in APd3O4: A = Ca, Sr, and Sr0.85Li0.15

Journal of Physics: Condensed Matter, 2020

Here we have investigated the role of electron phonon coupling on the Raman spectrum of narrow ba... more Here we have investigated the role of electron phonon coupling on the Raman spectrum of narrow bandgap semiconductors APd3O4 (A = Ca, Sr) and hole-doped system Sr0.85Li0.15Pd3O4. Four Raman active phonons are observed at room temperature for all three compounds as predicted by factor group analysis. The lowest energy phonon (∼190/202 cm−1) associated with Pd vibrations is observed to exhibit an asymmetric Fano-like lineshape in all the three compounds, indicating the presence of an interaction between the phonon and the electronic continuum. The origin of the electronic continuum states and electron–phonon coupling are discussed based on our laser power- and temperature-dependent Raman results. We have observed an enhanced strength of electron–phonon coupling in Sr0.85Li0.15Pd3O4 at low temperatures which can be attributed to the metallicity in this doped compound.

Tailoring the phase transition and electron-phonon coupling in 1T′−MoTe2 by charge doping: A Raman study

Physical Review B, 2020

Transition metal dichalcogenides (TMDs) are a class of widely studied 2D layered materials which ... more Transition metal dichalcogenides (TMDs) are a class of widely studied 2D layered materials which exist in various polymorphs. The 1T′ phase of MoTe2 is of prime importance as it has been reported to show quantum spin hall (QSH) behavior with a fairly large band-gap of ~ 60 meV, in contrast to most QSH materials known. It is noteworthy that though the monolayer 1T′-MoTe2 was initially predicted to show the QSH behavior, recent theoretical studies claim that the few-layered counterparts also exhibit higher order topological behavior. Besides, 1T′-MoTe2 also undergoes a hysteretic phase transition to the Td phase (which is a type-II Weyl semimetal) by breaking the inversion symmetry of the crystal. While the phase transition between these two topological phases is of utmost importance, its study has been mostly restricted to bulk single crystal flakes, thereby not sufficiently exploring the effect of dimensionality. We have studied the phase transition in 1T′-MoTe2 as a function of flake-thickness. Though our Raman studies show a suppression of the phase transition in the thin (thickness <10 nm) flakes [similar to the report Phys. Rev. B 97, 041410 (2018)], we have experimentally demonstrated the possibility of stabilizing the desired phase (1T′ or Td) at room temperature by charge doping. Further, we have observed clear signatures of electron-phonon coupling in MoTe2, which evolves as a function of flake-thickness and charge doping.

Electron-phonon and phonon-phonon anharmonic interactions in 2H−MoX2 ( X=S,Te ): A comprehensive resonant Raman study

Physical Review B, 2021

Transition metal dichalcogenides (TMDs) are layered materials which show excellent potential for ... more Transition metal dichalcogenides (TMDs) are layered materials which show excellent potential for nanoelectronic and optoelectronic applications. However, as many of the exciting features of these materials are controlled by the anharmonic effects, a proper understanding of the phonon properties and anharmonicity associated with these materials is essential f or the proposed applications to be realized. We present here a comprehensive study on the phonon properties of two different TMDs; viz., MoS 2 and MoTe 2 , as a function of temperature, laser excitations, and polarization, as well as flake-thickness. Our study includes the measurement of anharmonicity in the first-order and higher order Raman scattering processes. The variations in anharmonicity with the thickness of MoS 2 and MoTe 2 have been explained in terms of their phonon symmetries , electronphonon coupling, and phonon-phonon interactions. Further, the effect of the underlying substrate on the anharmonic properties of the in-plane and out-of-plane phonons has also been estimated from the Raman measurements, thus elucidating the intrinsic phonon properties of the 2D layered materials.

Superconductivity in doped Weyl semimetal Mo0.9Ir0.1Te2 with broken inversion symmetry

Superconductor Science and Technology, 2021

This work presents the emergence of superconductivity in Ir—doped Weyl semimetal T d —MoTe2 with ... more This work presents the emergence of superconductivity in Ir—doped Weyl semimetal T d —MoTe2 with broken inversion symmetry. Chiral anomaly induced planar Hall effect and anisotropic magneto-resistance confirm the topological semimetallic nature of Mo 1 − x Ir x Te2. Observation of weak anisotropic, moderately coupled type-II superconductivity in T d -Mo 1 − x Ir x Te2 makes it a promising candidate for topological superconductor.

Engineering crystal structure and spin-phonon coupling in Ba1-xSrxMnO3

Journal of Magnetism and Magnetic Materials, 2021

The interplay between different degrees of freedom such as charge, spin, orbital, and lattice has... more The interplay between different degrees of freedom such as charge, spin, orbital, and lattice has received a great deal of interest due to its potential to engineer materials properties and their functionalities for device applications. In this work, we have explored the crystallographic phase diagram of Ba1-xSrxMnO3 and studied the correlation between two degrees of freedom, namely phonons and spins using magnetization and inelastic light scattering measurements. The system undergoes a series of crystallographic phase transitions 2H ➔ 9R ➔ 4H as a function of doping (Sr) as observed by X-ray diffraction measurements. Investigation of their temperature-dependent magnetization reveals a para-to antiferro-magnetic transition for all the compositions. An Eg phonon in the 9R phase and an E1g phonon in the 4H phase involving Mn/O-vibrations, show anomalous temperature-dependence in the antiferromagnetic phase arising due to spin-phonon coupling.

Signatures of magnetostriction and spin-phonon coupling in magnetoelectric hexagonal15R−BaMnO3

Physical Review B, 2020

Spin-phonon coupling, the interaction of spins with surrounding lattice is a key parameter to und... more Spin-phonon coupling, the interaction of spins with surrounding lattice is a key parameter to understand the underlying physics of multiferroics and engineer their magnetization dynamics. Elementary excitations in multiferroic materials are strongly influenced by spin-phonon interaction, making Raman spectroscopy a unique tool to probe these coupling(s). Recently, it has been suggested that the dielectric and magnetic properties of 15R-type hexagonal BaMnO3 are correlated through the spin-lattice coupling. Here, we report the observation of an extensive renormalization of the Raman spectrum of 15R-BaMnO3 at 230 K, 280 K, and 330 K. Magnetic measurements reveal the presence of a long-range and a short-range magnetic ordering in 15R-BaMnO3 at 230 K and 330 K, respectively. The Raman spectrum shows the appearance of new Raman modes in the magnetically ordered phases. Furthermore, an additional Raman phonon appears below ~ 280 K, possibly arising from a local latticedistortion due to the displacement of Mn-ions, that exhibits anomalous shift with temperature. The origin of the observed renormalization and phonon anomalies in Raman spectra are discussed based on the evidences from temperature-and magnetic-field-dependent Raman spectra, temperature-dependent x-ray diffraction, magnetization, and specific heat measurements. Our results indicate the presence of magnetostriction and spin-phonon coupling in 15R-BaMnO3 thus suggesting that the optical phonons are strongly correlated to its magnetoelectric properties.

Direct Bandgap-like Strong Photoluminescence from Twisted Multilayer MoS2 Grown on SrTiO3

ACS Nano, 2020

While direct bandgap monolayer 2D transition metal dichalcogenides (TMDs) have emerged as an impo... more While direct bandgap monolayer 2D transition metal dichalcogenides (TMDs) have emerged as an important optoelectronic material due to strong light−matter interactions, their multilayer counterparts exhibit an indirect bandgap resulting in poor photon emission quantum yield. We report strong direct bandgap-like photoluminescence at ∼1.9 eV from multilayer MoS 2 grown on SrTiO 3 , whose intensity is significantly higher than that observed in multilayer MoS 2 / SiO 2. Using high-resolution electron microscopy we observe interlayer twist and >8% increase in the van der Waals gap, which leads to weaker interlayer coupling. This affects the evolution of the band structure in multilayer MoS 2 as probed by transient absorption spectroscopy, causing higher photo carrier recombination at the direct gap. Our results provide a platform that could enable multilayer TMDs for robust optical device applications.

Hexagonal Boron Nitride–Graphene Heterostructures with Enhanced Interfacial Thermal Conductance for Thermal Management Applications

ACS Applied Nano Materials, 2021

Atomically thin monolayers of graphene show excellent electronic properties which have led to a g... more Atomically thin monolayers of graphene show excellent electronic properties which have led to a great deal of research on their use in nanoscale devices. However, heat management of such nanoscale devices is essential in order to improve their performance. Graphene supported on hexagonal boron nitride (h-BN) substrate has been reported to show enhanced (opto)electronic and thermal properties as compared to extensively used SiO 2 /Si supported graphene. Motivated by this, we have performed temperature-and power-dependent Raman Spectroscopic measurements on four different types of (hetero)structures: (a) h-BN (BN), (b) graphene (Gr), (c) h-BN on graphene (BG), and (d) graphene encapsulated by h-BN layers from both top and bottom (BGB), all supported on SiO 2 /Si substrate. We have estimated the values of thermal conductivity (κ) and interfacial thermal conductance per unit area (g) of these four (hetero)structures to demonstrate the structure-activity (thermal) relationship. We report here the values of κ and g for h-BN supported on SiO 2 /Si as 280.0±58.0 Wm-1 K-1 and 25.6±0.4 MWm-2 K-1 , respectively. More importantly, we have observed an improvement in both thermal conductivity and interfacial thermal conductance per unit area in the heterostructures which ensures a better heat dissipation in devices. The κ and g of h-BN encapsulated graphene on SiO 2 /Si (BGB) sample was observed to be 850.0±81.0 Wm-1 K-1 and 105±1 MWm-2 K-1 , respectively, as opposed to 600.0±93.0 Wm-1 K-1 and 1.15±0.40 MWm-2 K-1 , respectively, for graphene on SiO 2 /Si substrate. Therefore, we propose that for graphene-based nanoscale devices, encapsulation with h-BN is a better alternative to address heat management issues.

Signatures of weak spin-phonon coupling in 12R-BaTi0.4Mn0.6O3: A Raman study

DAE SOLID STATE PHYSICS SYMPOSIUM 2019, 2020

Antiferromagnetic materials with strong spin-phonon coupling have the potential for spintronic ap... more Antiferromagnetic materials with strong spin-phonon coupling have the potential for spintronic applications. Here, we report the structural, magnetic, and vibrational properties of 12R-BaTi0.4Mn0.6O3. Temperature-dependent magnetization data show the presence of weak antiferromagnetic interactions. Correlation between magnetic and vibrational properties are probed by using temperature-dependent Raman spectroscopic measurements. Two of the Raman active phonons show deviation from anharmonic behaviour at low temperatures which can be attributed to spin-phonon coupling. The strength of coupling is estimated using meanfield approximation and is found to be 1.1 and 1.2 cm-1 for phonons at 530 and 717 cm-1 , respectively.

Tuning phonon anharmonicity in spin-ice Dy2Ti2O7

DAE SOLID STATE PHYSICS SYMPOSIUM 2019, 2020

Chemical dilution is an important parameter for tuning the electrical, vibrational, and magnetic ... more Chemical dilution is an important parameter for tuning the electrical, vibrational, and magnetic properties of the complex oxides. Pyrochlore titanates are known for displaying a high level of phonon anharmonicities which is attributed to the strong phonon-phonon anharmonic interactions. Therefore, these systems are perfect hosts for tuning the phonon anharmonicity with external chemical pressure. Here, we present temperature-dependent Raman studies of polycrystalline Dy2Ti2O7 and its diluted analogues Dy1.9Sm0.1Ti2O7 and Dy1.9La0.1Ti2O7. We report the reduction of phonon anharmonicities in spin-ice Dy2Ti2O7 with the incorporation of larger sized atom at rare-earth site using Raman spectroscopy. This can be explained considering an indirect effect of doped atoms on the amplitude of the phonon vibration associated to Ti4+ tetrahedra containing oxygen vacancy at its centre.

Polaronic Trions at the MoS 2 /SrTiO 3 Interface

Advanced Materials, 2019

create mixed dimensional heterostructures [5] of 2D materials and functional substrates, which pr... more create mixed dimensional heterostructures [5] of 2D materials and functional substrates, which provide an even wider opportunity to tailor diverse material functionalities and interface phenomena. In this regard, a very promising substrate is SrTiO 3 (STO), a transition metal oxide with an ABO 3-type perovskite crystal structure, that has attracted significant attention due to both its unusual bulk properties like high dielectric constant, [6] quantum paraelectricity, [7] and several interface phenomena such as the observation of 2D electron gas at interfaces with oxide insulators [8] and enhancement of superconducting T c at the interfaces with 2D FeSe. [9] The speciality of STO lies in its rich phonon spectrum centered on its AFD structural phase transition [10] (from cubic to tetragonal). The uniqueness of the phonons in STO, specifically the low energy soft phonons when compared to conventional phonons present in most materials lies in the following: i) they arise at a phase transition temperature and their frequency (energy) softens to zero near the transition ii) their intensity increases at low temperatures whereas conventionally phonon intensity decreases at lower temperatures The reduced electrical screening in 2D materials provides an ideal platform for realization of exotic quasiparticles, that are robust and whose functionalities can be exploited for future electronic, optoelectronic, and valleytronic applications. Recent examples include an interlayer exciton, where an electron from one layer binds with a hole from another, and a Holstein polaron, formed by an electron dressed by a sea of phonons. Here, a new quasiparticle is reported, "polaronic trion" in a heterostructure of MoS 2 /SrTiO 3 (STO). This emerges as the Fröhlich bound state of the trion in the atomically thin monolayer of MoS 2 and the very unique low energy soft phonon mode (≤7 meV, which is temperature and field tunable) in the quantum paraelectric substrate STO, arising below its structural antiferrodistortive (AFD) phase transition temperature. This dressing of the trion with soft phonons manifests in an anomalous temperature dependence of photoluminescence emission leading to a huge enhancement of the trion binding energy (≈70 meV). The soft phonons in STO are sensitive to electric field, which enables field control of the interfacial trion-phonon coupling and resultant polaronic trion binding energy. Polaronic trions could provide a platform to realize quasiparticle-based tunable optoelectronic applications driven by many body effects.

Effect of phonon anharmonicity on ferroelectricity in EuxBa1−xTiO3

Physical Review Materials, 2019

Investigating the competition between ferroelectric ordering and quantum fluctuations is essentia... more Investigating the competition between ferroelectric ordering and quantum fluctuations is essential to tailor the desired functionalities of mixed ferroelectric and incipient ferroelectric systems, like, (Ba,Sr)TiO3 and (Eu,Ba)TiO3. Recently, it has been shown that suppression of quantum fluctuations increases ferroelectric ordering in (Eu,Ba)TiO3 and since these phenomena are coupled to crystallographic phase transitions it is essential to understand the role of phonons. Here, we observe that the unusual temperature dependence of phonons in BaTiO3 gets suppressed when Ba 2+ is replaced by Eu 2+. This manifests in a decrease in the cubic-to-tetragonal (i.e., para-to-ferroelectric) phase transition temperature (by 150 K) and a complete suppression of tetragonality of the lattice (at room temperature by 40% replacement of Ba 2+ by Eu 2+). We have quantified the anharmonicity of the phonons and observed that the replacement of Ba 2+ by Eu 2+ suppresses it (by 93%) with a resultant lowering of the ferroelectric ordering temperature in the EuxBa1-xTiO3. This suggests that tuning phonon anharmonicity can be an important route to novel ferroelectric materials.

Robust resistive memory devices using solution-processable metal-coordinated azo aromatics

Nature materials, Jan 23, 2017

Non-volatile memories will play a decisive role in the next generation of digital technology. Fla... more Non-volatile memories will play a decisive role in the next generation of digital technology. Flash memories are currently the key player in the field, yet they fail to meet the commercial demands of scalability and endurance. Resistive memory devices, and in particular memories based on low-cost, solution-processable and chemically tunable organic materials, are promising alternatives explored by the industry. However, to date, they have been lacking the performance and mechanistic understanding required for commercial translation. Here we report a resistive memory device based on a spin-coated active layer of a transition-metal complex, which shows high reproducibility (∼350 devices), fast switching (≤30 ns), excellent endurance (∼10(12) cycles), stability (>10(6) s) and scalability (down to ∼60 nm(2)). In situ Raman and ultraviolet-visible spectroscopy alongside spectroelectrochemistry and quantum chemical calculations demonstrate that the redox state of the ligands determines...

Phase pure epitaxial growth of BiFeO3 films: An effect of oxygen partial pressure

Solid State Communications, 2017

We report the growth of bismuth ferrite, BiFeO 3 (BFO), thin films by varying oxygen partial pres... more We report the growth of bismuth ferrite, BiFeO 3 (BFO), thin films by varying oxygen partial pressure (230, 100 and 10mTorr) in pulsed laser deposition system using 10% bismuth excess BFO target on SrTiO 3 (001) [STO] substrates at 700°C each for 15min duration. Xray diffraction shows the presence of an impurity phase along with BFO peaks for 230mTorr film, whereas oriented BFO film is observed for 100mTorr film with a small impurity phase. Importantly, epitaxial growth of BFO is observed without any impurity phase and with sufficiently increased strain for the 10mTorr film, confirmed by the reciprocal space mapping of (002) and (103) reflections. UV-Raman measurements have been performed on as grown films and the 10mTorr BFO film after annealing up to 500 o C to check its thermal stability. Atomic force microscopic (AFM) study reveals that the 10mTorr BFO film shows a lower roughness as compared to 100mTorr and 230mTorr films. Fast Fourier Transform (FFT) analysis in AFM shows a fourfold symmetric structure suggesting the presence of both modes of pattern-orientation in 230mTorr BFO film whereas an elongated bright spot along the y-direction in the FFT of 10mTorr BFO film suggests the growth tendency to be along the y-direction.

Fabrication of a biomimetic ZeinPDA nanofibrous scaffold impregnated with BMP‐2 peptide conjugated TiO 2 nanoparticle for bone tissue engineering

Journal of Tissue Engineering and Regenerative Medicine, 2017

A biomimetic Zein polydopamine (PDA) based nanofiber scaffold was fabricated to deliver bone morp... more A biomimetic Zein polydopamine (PDA) based nanofiber scaffold was fabricated to deliver bone morphogenic protein-2 (BMP-2) peptide conjugated titanium dioxide (TiO 2) nanoparticles in a sustained manner for investigating its osteogenic differentiation potential. To prolong its retention time at the target site, BMP-2 peptide has been conjugated to TiO 2 nanoparticles owing to its high surface to volume ratio. The effect of biochemical cues from BMP-2 peptide and nano topographical stimulation of electrospun Zein PDA nanofibers were examined for its enhanced osteogenic expression of human fetal osteoblast (hFOB) cells. The sustained delivery of bioactive signals, improved cell adhesion, mineralization and differentiation could be attributed to its highly interconnected nanofibrous matrix with unique material composition. Further, the expression of osteogenic markers revealed that the fabricated nanofibrous scaffold possess better cell-biomaterial interactions. These promising results demonstrate the potential of the composite nanofibrous scaffold as an effective biomaterial substrate for bone regeneration.

Origin and Quenching of Novel ultraviolet and blue emission in NdGaO3: Concept of Super-Hydrogenic Dopants

Scientific reports, Nov 3, 2016

In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth b... more In this study we report the existence of novel ultraviolet (UV) and blue emission in rare-earth based perovskite NdGaO3 (NGO) and the systematic quench of the NGO photoluminescence (PL) by Ce doping. Study of room temperature PL was performed in both single-crystal and polycrystalline NGO (substrates and pellets) respectively. Several NGO pellets were prepared with varying Ce concentration and their room temperature PL was studied using 325 nm laser. It was found that the PL intensity shows a systematic quench with increasing Ce concentration. XPS measurements indicated that nearly 50% of Ce atoms are in the 4+ state. The PL quench was attributed to the novel concept of super hydrogenic dopant (SHD)", where each Ce(4+) ion contributes an electron which forms a super hydrogenic atom with an enhanced Bohr radius, due to the large dielectric constant of the host. Based on the critical Ce concentration for complete quenching this SHD radius was estimated to be within a range of 0.8...

Magnetic Modes in Rare Earth Perovskites: A Magnetic-Field-Dependent Inelastic Light Scattering study

Scientific reports, Nov 15, 2016

Here, we report the presence of defect-related states with magnetic degrees of freedom in crystal... more Here, we report the presence of defect-related states with magnetic degrees of freedom in crystals of LaAlO3 and several other rare-earth based perovskite oxides using inelastic light scattering (Raman spectroscopy) at low temperatures in applied magnetic fields of up to 9 T. Some of these states are at about 140 meV above the valence band maximum while others are mid-gap states at about 2.3 eV. No magnetic impurity could be detected in LaAlO3 by Proton-Induced X-ray Emission Spectroscopy. We, therefore, attribute the angular momentum-like states in LaAlO3 to cationic/anionic vacancies or anti-site defects. Comparison with the other rare earth perovskites leads to the empirical rule that the magnetic-field-sensitive transitions require planes of heavy elements (e.g. lanthanum) and oxygen without any other light cations in the same plane. These magnetic degrees of freedom in rare earth perovskites with useful dielectric properties may be tunable by appropriate defect engineering for ...

Magneto-Optical Study of Defect Induced Sharp Photoluminescence in LaAlO3 and SrTiO3

Scientific reports, Sep 13, 2016

Strongly correlated electronic systems such as Transition Metal Oxides often possess various mid-... more Strongly correlated electronic systems such as Transition Metal Oxides often possess various mid-gap states originating from intrinsic defects in these materials. In this paper, we investigate an extremely sharp Photoluminescence (PL) transition originating from such defect states in two widely used perovskites, LaAlO3 and SrTiO3. A detailed study of the PL as a function of temperature and magnetic field has been conducted to understand the behavior and origin of the transition involved. The temperature dependence of the PL peak position for SrTiO3 is observed to be opposite to that in LaAlO3. Our results reveal the presence of a spin/orbital character in these transitions which is evident from the splitting of these defect energy levels under a high magnetic field. These PL transitions have the potential for enabling non-contact thermal and field sensors.

Correlation of nanoscale behaviour of forces and macroscale surface wettability

Nanoscale, 2016