Papers by Vitaly Golovach
Physical Review A, 2009
We calculate the dynamic structure factor S(q, ω) of a one-dimensional (1D) interacting Bose gas ... more We calculate the dynamic structure factor S(q, ω) of a one-dimensional (1D) interacting Bose gas confined in a harmonic trap. The effective interaction depends on the strength of the confinement enforcing the 1D motion of atoms; interaction may be further enhanced by superimposing an optical lattice on the trap potential. In the compressible state, we find that the smooth variation of the gas density around the trap center leads to softening of the singular behavior of S(q, ω) at Lieb-1 mode compared to the behavior predicted for homogeneous 1D systems. Nevertheless, the density-averaged responseS(q, ω) remains a non-analytic function of q and ω at Lieb-1 mode in the limit of weak trap confinement. The exponent of the power-law non-analyticity is modified due to the inhomogeneity in a universal way, and thus, bears unambiguously the information about the (homogeneous) Lieb-Liniger model. A strong optical lattice causes formation of Mott phases. Deep in the Mott regime, we predict a semi-circular peak in S(q, ω) centered at the on-site repulsion energy, ω = U. Similar peaks of smaller amplitudes exist at multiples of U as well. We explain the suppression of the dynamic response with entering into the Mott regime, observed recently by D. Clément et al., Phys. Rev. Lett. 102, 155301 (2009), based on an f-sum rule for the Bose-Hubbard model.
Physical Review B
A ferromagnetic insulator (FI) in contact with a superconductor (S) is known to induce a spin spl... more A ferromagnetic insulator (FI) in contact with a superconductor (S) is known to induce a spin splitting of the BCS density of states at the FI/S interface. This spin splitting causes the Cooper pairs to reduce their singlet-state correlations and acquire odd-in-frequency triplet correlations. We consider a diffusive FI/S bilayer with a sharp magnetic domain wall in the FI and study the local quasiparticle density of states and triplet superconducting correlations. In the case of collinear alignment of the domains, we obtain analytical results by solving the Usadel equation. For a small enough exchange field, we also find an analytical expression for arbitrary magnetic textures, which reveals how the triplet component vector depends on the local magnetization of the FI. For an arbitrary angle between the magnetizations and the strength of the exchange field, we numerically solve the problem of a sharp domain wall. We finally propose two different setups based on FI/S/F stacks, where F is a ferromagnetic layer, to filter out singlet pairs and detect the presence of triplet correlations via tunneling differential conductance measurements.
Spin Hall magnetoresistance (SMR) refers to a resistance change in a metallic film reflecting the... more Spin Hall magnetoresistance (SMR) refers to a resistance change in a metallic film reflecting the magnetization direction of a magnet attached to the film. The mechanism of this phenomenon is spin exchange between conduction-electron spins and magnetization at the interface. SMR has been used to read out information written in a small magnet and to detect magnetization dynamics, but it has been limited to magnets; magnetic ordered phases or instability of magnetic phase transition has been believed to be indispensable. Here, we report the observation of SMR in a paramagnetic insulator Gd_3Ga_5O_12 (GGG) without spontaneous magnetization combined with a Pt film. The paramagnetic SMR can be attributed to spin-transfer torque acting on localized spins in GGG. We determine the efficiencies of spin torque and spin-flip scattering at the Pt/GGG interface, and demonstrate these quantities can be tuned with external magnetic fields. The results clarify the mechanism of spin-transport at a m...
Abstract. We review our recent contributions on shot noise for entangled electrons and spin-polar... more Abstract. We review our recent contributions on shot noise for entangled electrons and spin-polarized currents in novel mesoscopic geometries. We first discuss some of our recent proposals for electron entanglers involving a superconductor coupled to a double dot in the Coulomb blockade regime, a superconductor tunnel-coupled to Luttinger-liquid leads, and a triple-dot setup coupled to Fermi leads. We briefly survey some of the available possibilities for spin-polarized sources. We use the scattering approach to calculate current and shot noise for spin-polarized currents and entangled/unentangled electron pairs in a novel beamsplitter geometry with a local Rashba spin-orbit (s-o) interaction in the incoming leads. For single-moded incoming leads, we find continuous bunching and antibunching behaviors for the entangled pairs – triplet and singlet – as a function of the Rashba rotation angle. In addition, we find that unentangled triplets and the entangled one exhibit distinct shot n...
We predict a double-resonant feature in the magnetic field dependence of the phonon-mediated long... more We predict a double-resonant feature in the magnetic field dependence of the phonon-mediated longitudinal conductivity σxx of a two-subband quasi-twodimensional electron system in a quantizing magnetic field. The two sharp peaks in σxx appear when the energy separation between two Landau levels belonging to different size-quantization subbands is favorable for acoustic-phonon transitions. One-phonon and two-phonon mechanisms of electron conductivity are calculated and mutually compared. The phonon-mediated interaction between the intersecting Landau levels is considered and no avoided crossing is found at thermal equilibrium. I.
Alexander Khaetskii, Vitaly Golovach, 3, 4, 5 and Arnold Kiefer Air Force Research Laboratory, Wr... more Alexander Khaetskii, Vitaly Golovach, 3, 4, 5 and Arnold Kiefer Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433, USA Centro de F́ısica de Materiales (CFM-MPC), Centro Mixto CSIC-UPV/EHU, 20018 Donostia-San Sebastián, Spain Departamento de Poĺımeros y Materiales Avanzados: F́ısica, Qúımica y Tecnoloǵıa, Facultad de Qúımica, University of the Basque Country UPV/EHU, 20080 Donostia-San Sebastián, Spain Donostia International Physics Center (DIPC), 20018 Donostia-San Sebastián, Spain IKERBASQUE, Basque Foundation for Science, 48013 Bilbao, Spain (Dated: July 13, 2021)
Physical Review B
We study the magnetic and superconducting proximity effects in a semiconducting nanowire (NW) att... more We study the magnetic and superconducting proximity effects in a semiconducting nanowire (NW) attached to superconducting leads and a ferromagnetic insulator (FI). We show that a sizable equilibrium spin polarization arises in the NW due to the interplay between the superconducting correlations and the exchange field in the FI. The resulting magnetization has a nonlocal contribution that spreads in the NW over the superconducting coherence length and is opposite in sign to the local spin polarization induced by the magnetic proximity effect in the normal state. For a Josephson-junction setup, we show that the nonlocal magnetization can be controlled by the superconducting phase bias across the junction. Our findings are relevant for the implementation of Majorana bound states in state-of-the-art hybrid structures.
Physical Review Materials
A ferromagnetic insulator in contact with a superconductor is known to induce exchange fields ran... more A ferromagnetic insulator in contact with a superconductor is known to induce exchange fields ranging from few to tens of Tesla driven splitting of the Bardeen-Cooper-Schrieffer (BCS) density of states singularity by a magnitude proportional to the magnetization, and the exchange field penetrating into the superconductor to a depth comparable with the superconducting coherence length. This magnetic proximity effect in EuS/Al bilayers was found to be influenced by the domain structure of the EuS that affects both, the position and the intensity of the exchange-splitted BCS peaks. This splitting is observed even in the unmagnetized state of the EuS suggesting that the domain size of the EuS is comparable with the superconducting coherence length. Upon magnetizing the EuS the splitting was enhanced while peaks became symmetric. Conductance measurements as a function of bias voltage at the lowest temperatures could theoretically relate the line shape of the split BCS DoS with the characteristic domain structure in the ultra thin EuS layer. These results pave the way to engineering triplet superconducting correlations at domain walls in EuS/Al bilayers. Furthermore, the clear gap and splitting observed in our tunneling spectroscopy measurements show that it can be an excellent candidate for substituting strong magnetic fields in experiments studying Majorana bound states.
Physical Review B
We investigate transport properties of junctions between two spin-split superconductors linked by... more We investigate transport properties of junctions between two spin-split superconductors linked by a spin-polarized tunneling barrier. The spin-splitting fields in the superconductors (S) are induced by adjacent ferromagnetic insulating (FI) layers with arbitrary magnetization. The aim of this study is twofold: On the one hand, we present a theoretical framework based on the quasiclassical Green's functions to calculate the Josephson and quasiparticle current through the junctions in terms of the different parameters characterizing it. Our theory predicts qualitative new results for the tunneling differential conductance, dI/dV , when the spin-splitting fields of the two superconductors are non-collinear. We also discuss how junctions based on FI/S can be used to realize anomalous Josephson junctions with a constant geometric phase shift in the current-phase relation. As a result, they may exhibit spontaneous triplet supercurrents in the absence of a phase difference between the S electrodes. On the other hand, we show results of planar tunneling spectroscopy of a EuS/Al/AlOx/EuS/Al junction and use our theoretical model to reproduce the obtained dI/dV curves. Comparison between theory and experiment reveals information about the intrinsic parameters of the junction, such as the size of the superconducting order parameter, spin-splitting fields and spin relaxation, and also about properties of the two EuS films, such as their morphology, domain structure, and magnetic anisotropy.
Physical Review B
We observe an unusual behavior of the spin Hall magnetoresistance (SMR) in Pt deposited on a tens... more We observe an unusual behavior of the spin Hall magnetoresistance (SMR) in Pt deposited on a tensile-strained LaCoO3 (LCO) thin film, which is a ferromagnetic insulator with the Curie temperature Tc=85K. The SMR displays a strong magnetic-field dependence below Tc, with the SMR amplitude continuing to increase (linearly) with increasing the field far beyond the saturation value of the ferromagnet. The SMR amplitude decreases gradually with raising the temperature across Tc and remains measurable even above Tc. Moreover, no hysteresis is observed in the field dependence of the SMR. These results indicate that a novel lowdimensional magnetic system forms on the surface of LCO and that the LCO/Pt interface decouples magnetically from the rest of the LCO thin film. To explain the experiment, we revisit the derivation of the SMR corrections and relate the spin-mixing conductances to the microscopic quantities describing the magnetism at the interface. Our results can be used as a technique to probe quantum magnetism on the surface of a magnetic insulator. Introduction.-Magnetoresistance has been key for understanding spin-dependent transport in solids [1]. In the last years, new magnetoresistance phenomena were discovered in thin ferromagnetic/normal metal(FM/NM)-based heterostructures [2-18], which originate from the interplay of the spin currents generated in the heterostructure (via the spin Hall effect [19-22] or the Rashba-Edelstein effect [23,24]) with the magnetic moments of the FM layer. Among 2 many applications, these magnetoresistance effects have been used for quantifying spin transport properties such as the spin diffusion length and the spin Hall angle SH of different NM layers, or the spin-mixing conductance ↑↓ of FM/NM interfaces. More interestingly, unlike other surface-sensitive techniques that suffer from a bulk contribution due to a finite penetration depth, the spin Hall magnetoresistance (SMR) [4-11] uses the spin accumulation at interfaces for sensing the magnetic properties of the very first atomic layer of magnetic insulators (MIs) [25,26]. For instance, SMR has been employed for probing the surface of complex magnetic systems such as ferrimagnetic spinel oxides [11,27], spin-spiral multiferroics [28,29], canted ferrimagnets [30], Y3Fe5O12/antiferromagnetic (YIG/AFM) bilayers [31,32], and synthetic AFMs [33]. LaCoO3 (LCO) presents an intriguing magnetic behavior, which has been studied for decades and is still under debate [34-49]. Bulk LCO is a diamagnetic insulator at low temperature, owing to the low-spin (LS) configuration of Co 3+. The relatively small crystal-field splitting of the Co 3+ 3d-shell results in an increasing population of high-spin (HS) Co 3+ with temperature, reaching 1:1 (LS:HS) above ~150K. The close proximity between crystal-field splitting and exchange energy makes the magnetic properties of LCO particularly susceptible to small changes in inter-ionic distances and coordination. For this reason, tensile-strained LCO thin films grown on particular substrates [such as SrTiO3 (STO)] exhibit FM order at low temperatures [42-49]. However, the magnetic properties of the surface of these films-where the crystal-field symmetry is lowered because of a different stoichiometry at the surface-have not been addressed yet. In this letter, we take the first steps towards understanding the magnetic behavior of the surface of strained LCO films by performing magnetoresistance measurements in STO/LCO/Pt. We find that SMR depends strongly on the magnetic field at all temperatures, both above and below the Curie temperature (Tc) of the film, and more strikingly, no hysteresis in the magnetoresistance is observed. These observations clearly show that the surface magnetism of the LCO film is radically different from its bulk counterpart. We support our measurements with a theoretical model that extends the known expressions for SMR [7,50] and HMR [51,52] in MI/NM bilayers for an arbitrary magnetic ordering (para-, ferri-, ferro-, antiferro-magnet) of the localized magnetic moments at the MI/NM interface. We provide expressions for ↑↓ Gr+iGi [25,53] and the effective spin conductance Gs [54,55] in terms of surface spincorrelators. The experimental data evidence that the surface of LCO behave as a lowdimensional Heisenberg FM. Experimental details.-Growth of epitaxial LCO thin films via polymer-assisted deposition on (001) STO substrates, as well as their structural, electrical, and magnetic characterization, is described in Ref. [46]. The LCO films exhibit a tetragonal distortion, which induces FM ordering below Tc~85K and with a coercive field below 1T at 10K, in agreement to other reports [43-45,47,56]. The films exhibit low surface roughness (<1nm) and are insulating [46]. Pt Hall bar structures (width W 100m, length L 800m and thickness dN 7nm) were
Nano Letters
We present a theory of the spin Hall magnetoresistance of metals in contact with magnetic insulat... more We present a theory of the spin Hall magnetoresistance of metals in contact with magnetic insulators. We express the spin mixing conductances, which govern the phenomenology of the effect, in terms of the microscopic parameters of the interface and the spin-spin correlation functions of the local moments on the surface of the magnetic insulator. The magnetic-field and temperature dependence of the spin mixing conductances leads to a rich behaviour of the resistance due to an interplay between the Hanle effect and spin mixing at the interface. Our theory provides a useful tool for understanding the experiments on heavy metals in contact with magnetic insulators of different kinds, and it predicts striking behaviours of the magnetoresistance.
Aps Meeting Abstracts, Mar 1, 2008
ABSTRACT We study theoretically transport through an electronic Mach-Zehnder interferometer in th... more ABSTRACT We study theoretically transport through an electronic Mach-Zehnder interferometer in the presence of Coulomb interaction inside the interferometer, using a discrete wave-packet model. We find that the mutual capacitance between the arms of the interferometer leads to a suppression of the visibility of the Aharonov-Bohm oscillations at a large source-drain bias deltamuvF/L, where L is the length of the arms and vF is the electron drift speed. Our numerical simulations indicate that the visibility of the Aharonov-Bohm oscillations is a non-analytic function of the mutual capacitance strength, in the limit deltamu-&gt;∞.
Room-temperature crystal structure of the transition-metal coordination polymers MCl2(bpy) (M = F... more Room-temperature crystal structure of the transition-metal coordination polymers MCl2(bpy) (M = Fe, Co, Ni, bpy = 4, 4'-bipyridine) belongs to the orthorhombic crystal system, space group Cmmm (#65). Magnetic susceptibility M(T)/H and isothermal magnetization M(H) have been measured. Spontaneous antiferromagnetic ordering was observed in M(T)/H of all compounds, with transition temperatures 10.0 K, 5.0 K, and 8.5 K, for M = Fe, Co, and Ni, respectively. A metamagnetic transition was found in M(H) of each compound. The metamagnetic-transition critical fields for Fe, Co, and Ni compounds are 3.5 kG, 2.5 kG, and 12 kG, respectively. High temperature M(T)/H data of all compounds were fit to a modified Curie-Weiss law. The values of effective moment yielded from the fitting indicate the high spin states for all M ions. The magnetic behaviors exhibited in this system are attributed to the ferromagnetic intra-chain M-M exchange interaction through the Cl2 bridges along the c axis and t...
I would like to thank Prof. Dr. Daniel Loss for giving me the opportunity to work on this interes... more I would like to thank Prof. Dr. Daniel Loss for giving me the opportunity to work on this interesting topic in physics, which I believe will keep fascinating everyone for quite some time. I started as a graduate student at the Department of Physics and Astronomy of the University of Basel about four years ago and at that time I knew very little about quantum computing and about spins of electrons in quantum dots. I learned a lot from Prof. Loss and from the research group here in Basel, and I find it great that I have now this occasion to thank everyone for their help. I thank Prof. Dr. D. Loss for guiding my research work and supporting me in writing this thesis. It has been a useful experience for me to work with Prof. Loss. The topics we chose to work on were timely and realistic to observe in experiment. I thank Prof. Loss for countless discussions on physics and for always being available to talk to and give advice. I thank Prof. Dr. C. Bruder and Prof. Dr. L. Glazman for kindly agreeing to co-referee this thesis. Next, I would like to thank the people I worked with during my time as a graduate student in Basel. I thank M. Borhani for collaborating with me on QPC-induced spin decay in quantum dots, Prof. Dr. G. Burkard and B. Coish for collaborating on Pauli-blockaded double dots, Prof. Dr. M.-S. Choi for collaborating on Kondo effect,
We calculate the dynamic structure factor S(q,omega) of a one-dimensional (1D) interacting Bose g... more We calculate the dynamic structure factor S(q,omega) of a one-dimensional (1D) interacting Bose gas confined in a harmonic trap. The effective interaction depends on the strength of the confinement enforcing the 1D motion of atoms; interaction may be further enhanced by superimposing an optical lattice on the trap potential. In the compressible state, we find that the smooth variation of the gas density around the trap center leads to softening of the singular behavior of S(q,omega) at Lieb-1 mode compared to the behavior predicted for homogeneous 1D systems. Nevertheless, the density-averaged response remains a non-analytic function of q and omega at Lieb-1 mode in the limit of weak trap confinement. The exponent of the power-law non-analyticity is modified due to the inhomogeneity in a universal way, and thus, bears unambiguously the information about the (homogeneous) Lieb-Liniger model. A strong optical lattice causes formation of Mott phases. Deep in the Mott regime, we predict...
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Electron and hole spectra in-HgS cylindrical quantum wire superlattice in-CdS matrix are calculat... more Electron and hole spectra in-HgS cylindrical quantum wire superlattice in-CdS matrix are calculated within the method of augmented plane waves. The energy term determined by the movement of quasiparticles in direction perpendicular to the axial axis of the wire is presented by the alternating zones with a positive and a negative effective mass. The degeneration on the magnetic quantum number outside ¡ point of Brillouin zone is taken off when the potential of the superlattice quantum wires is taken into account. The ground zones energy dependence of plane quasiparticle movement on the radii of the quantum wires as well as the distance between the wires are investigated.
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Papers by Vitaly Golovach