Papers by Amir Haghighirad
Physical Review B, 2021
Strain is a powerful experimental tool to explore new electronic states and understand unconventi... more Strain is a powerful experimental tool to explore new electronic states and understand unconventional superconductivity. Here, we investigate the effect of uniaxial strain on the nematic and superconducting phase of single crystal FeSe using magnetotransport measurements. We find that the resistivity response to the strain is strongly temperature dependent and it correlates with the sign change in the Hall coefficient being driven by scattering, coupling with the lattice and multiband phenomena. Band structure calculations suggest that under strain the electron pockets develop a large in-plane anisotropy as compared with the hole pocket. Magnetotransport studies at low temperatures indicate that the mobility of the dominant carriers increases with tensile strain. Close to the critical temperature, all resistivity curves at constant strain cross in a single point, indicating a universal critical exponent linked to a strain-induced phase transition. Our results indicate that the superconducting state is enhanced under compressive strain and suppressed under tensile strain, in agreement with the trends observed in FeSe thin films and overdoped pnictides, whereas the nematic phase seems to be affected in the opposite way by the uniaxial strain. By comparing the enhanced superconductivity under strain of different systems, our results suggest that strain on its own cannot account for the enhanced high Tc superconductivity of FeSe systems.
Physical Review Letters, 2020
We study the elastoresistance of the highly correlated material CsFe2As2 in all symmetry channels... more We study the elastoresistance of the highly correlated material CsFe2As2 in all symmetry channels. Neutralizing its thermal expansion by means of a piezoelectric-based strain cell is demonstrated to be essential. The elastoresistance response in the in-plane symmetric channel is found to be large, while the response in the symmetry-breaking channels is weaker and provides no evidence for a divergent nematic susceptibility. Rather, our results can be interpreted naturally within the framework of a coherence-incoherence crossover, where the low-temperature coherent state is sensitively tuned by the in-plane atomic distances.
Seminar Universtität Heidelberg (2020), Heidelberg, Deutschland, 10.01.2020, 2020
We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled ... more We measure the electronic structure of FeSe from within individual orthorhombic domains. Enabled by an angle-resolved photoemission spectroscopy beamline with a highly focused beamspot (nano-ARPES), we identify clear stripe-like orthorhombic domains in FeSe with a length scale of approximately 1-5 m. Our photoemission measurements of the Fermi surface and band structure within individual domains reveal a single electron pocket at the Brillouin zone corner. This result provides clear evidence for a one-electron pocket electronic structure of FeSe, observed without the application of uniaxial strain, and calls for further theoretical insight into this unusual Fermi surface topology. Our results also showcase the potential of nano-ARPES for the study of correlated materials with local domain structures.
Physical Review Research, 2020
2D Materials, 2022
Among two-dimensional materials, Fe3GeTe2 has come to occupy a very important place owing to its ... more Among two-dimensional materials, Fe3GeTe2 has come to occupy a very important place owing to its ferromagnetic nature with one of the highest Curie temperatures among known van der Waals materials and the potential for hosting skyrmions. In this combined experimental and theoretical work, we investigate the magnetic bubble domains as well as the microscopic domain wall profile using spin-polarized scanning tunneling microscopy in combination with atomistic spin dynamics simulations performed with parameters from density functional theory calculations. We find a weak magneto-electric effect influencing the domain wall width by the electric field in the tunneling junction and determine the critical magnetic field for the collapse of the bubble domains. Our findings shed light onto the origins of complex magnetism that Fe3GeTe2 exhibits.
Physical Review Research
{"data":{"abstract":{"value":"<p>We present a combined study from angle-resolved photoemission an... more {"data":{"abstract":{"value":"<p>We present a combined study from angle-resolved photoemission and density-functional-theory calculations of the temperaturedependent electronic structure in the excitonic insulator candidate <math xmlns=\". Our experimental measurements unambiguously establish the normal state as a semimetal with a significant band overlap of <math xmlns=\"http://www.w3.org/1998/Math/MathML\"><mrow><mo>></mo><mn>100</mn></mrow> </math> meV. Our temperature-dependent measurements indicate how these low-energy states hybridize when cooling through the well-known 327 K phase transition in this system. From our calculations and polarization-dependent photoemission measurements, we demonstrate the importance of a loss of mirror symmetry in enabling the band hybridization, driven by a shearlike structural distortion which reduces the crystal symmetry from orthorhombic to monoclinic. Our results thus point to the key role of the lattice distortion in enabling the phase transition of <math xmlns=\"
Nano Letters
Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in th... more Magnetic skyrmions are two-dimensional magnetization swirls that stack in the form of tubes in the third dimension and which are proposed as prospective information carriers for nonvolatile memory devices due to their unique topological properties. From resonant elastic X-ray scattering measurements on Cu 2 OSeO 3 with an in-plane magnetic field, we find that a state of perpendicularly ordered skyrmions forms, in stark contrast to the well-studied bulk state. The surface state is stable over a wide temperature range, unlike the bulk state in out-of-plane fields which is confined to a narrow region of the temperaturefield phase diagram. In contrast to ordinary skyrmions found in the bulk, the surface state skyrmions result from the presence of magnetic interactions unique to the surface which stabilize them against external perturbations. The surface guiding makes the robust state particular interesting for racetracklike devices, ultimately allowing for much higher storage densities due to the smaller lateral footprint of the perpendicular skyrmions.
Journal of Materials Chemistry A
The power conversion efficiency (PCE) of lead-halide perovskite solar cells (PSCs) is reported to... more The power conversion efficiency (PCE) of lead-halide perovskite solar cells (PSCs) is reported to increase over a period of days after their fabrication while they are stored in dark. Thus...
Physical Review B
Superconductivity in FeSe has recently attracted a great deal of attention because it emerges out... more Superconductivity in FeSe has recently attracted a great deal of attention because it emerges out of an electronic nematic state of elusive character. Here we study both the electronic normal state and the superconducting gap structure using heat-capacity measurements on high-quality single crystals. The specific-heat curve, from 0.4 K to Tc = 9.1 K, is found to be consistent with a recent gap determination using Bogoliubov quasiparticle interference [P. O. Sprau et al., Science 357, 75 (2017)], however only if nodes are introduced on either the electron or the hole Fermi-surface sheets. Our analysis, which is consistent with quantum-oscillation measurements, relies on the presence of only two bands, and thus the fate of the theoretically predicted second electron pocket remains mysterious.
Crystal Growth & Design
Understanding the growth mechanism of nanostructures is key to tailoring their properties. Many c... more Understanding the growth mechanism of nanostructures is key to tailoring their properties. Many compounds form nanowires following the vapor−liquid−solid (VLS) growth mechanism, and the growth of Bi 2 Te 3 nanobelts was also explained following the VLS route. Here, we present another growth mechanism of Bi 2 Te 3 nano-and submicron belts and ribbons. The samples were grown by physical vapor transport from Bi 2 Te 3 precursors using TiO 2 nanoparticles as a catalyst and analyzed by scanning electron microscopy and scanning transmission electron microscopy. The growth starts from a Te-rich cluster and proceeds via a thin, tip-catalyzed primary layer growing in the [110] direction. The primary layer serves as a support for subsequent step-flow growth. The precursor predominantly absorbs on the substrate and reaches the belt by migration from the base to the tip. Terrace edges pose energy barriers that enhance the growth rate of secondary layers compared to the primary layer. Broadening of the sidewalls is commonly observed and leads to triangular voids that can even result in a branching of the growing belts.
Nanoscale
Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to... more Vapour deposition of metal halide perovskite by co-evaporation of precursors has the potential to achieve large-area high-efficiency solar cells on an industrial scale, yet little is known about the growth...
Advanced Energy Materials
Advanced Energy Materials
Journal of the American Chemical Society
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Papers by Amir Haghighirad