Alexander Balatsky - Academia.edu (original) (raw)

Papers by Alexander Balatsky

Physical review, Mar 4, 2020

Superconductor-ferromagnetic heterostructures have been suggested as one of the most promising al... more Superconductor-ferromagnetic heterostructures have been suggested as one of the most promising alternatives of realizing odd-frequency superconductivity. In this work we consider the limit of shrinking the ferromagnetic region to the limit of a single impurity embedded in a conventional superconductor, which gives rise to localized Yu-Shiba-Rusinov (YSR) bound states with energies inside the superconducting gap. We demonstrate that all the sufficient ingredients for generating odd-frequency pairing are present at the vicinity of these impurities. We investigate the appearance of all possible pair amplitudes in accordance with the Berezinskii SP * OT * = −1 rule, with the symmetry under the exchange of spin, spatial, orbital (in our case O = +1), and time index, respectively. We study the spatial and frequency dependence of the possible pairing amplitudes, analyzing their evolution with impurity strength and identifying a reciprocity between different symmetries related through impurity scattering. We show that the odd-frequency spin-triplet pairing amplitude dominates at the critical impurity strength, where the YSR states merge at the middle of the gap, while the even components are quenched close to the impurity. We also show that the spin-polarized local density of states exhibits the same spatial and frequency behavior as the odd-ω spin-triplet component at the critical impurity strength.

Bulletin of the American Physical Society, Mar 16, 2017

magnetoinfrared spectroscopy, we have explored the charge dynamics of (Bi, Sb) 2 T e 3 thin films... more magnetoinfrared spectroscopy, we have explored the charge dynamics of (Bi, Sb) 2 T e 3 thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that the electron-and hole-type Dirac fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.

Bulletin of the American Physical Society, Mar 22, 2005

Physical Review B

We investigate the effects of two electronic bands at the negative electronic compressibility (NE... more We investigate the effects of two electronic bands at the negative electronic compressibility (NEC) in a two-dimensional electron gas (2DEG). We use a simple homogeneous model with Coulombic interactions and first-order multi-band coupling to examine the role of effective mass and relative permittivity in relation to the critical carrier density, where compressibility turns negative. We demonstrate that the population of a second band, along with the presence of inter-band coupling, can dramatically change the cross-over carrier density. Given the difficulty in determining and confirming multi-band electronic systems, this model provides a potential method for identifying multi-band electronic systems using precise bulk electronic properties measurements. To help illustrate this method, we apply our results to the observed NEC in the 2D electron gas at the interface of LaAlO3/SrTiO3 (LAO/STO) and determine that, for the known parameters of LAO/STO, the system is likely a realization of a two-band 2D electron gas. Furthermore, we provide general limits on the inter-band coupling with respect to the electronic band population.

Physical Review B, 2021

Superconductivity and ferroelectricity are typically incompatible because the former needs free c... more Superconductivity and ferroelectricity are typically incompatible because the former needs free carriers, but the latter is usually suppressed by free carriers, unless their concentration is low. In the case of strontium titanate with low carrier concentration, unconventional superconductivity and ferroelectricity were shown to be correlated. Here, we report theoretically and experimentally evaluated Grüneisen parameters whose divergence under tensile stress indicates that the dominant phonon mode that enhances the superconducting order is the ferroelectric transverse soft-mode. This finding rules out all other phonon modes as the main contributors to the enhanced superconductivity in strained strontium titanate. This methodology shown here can be applied to many other quantum materials.

Annalen der Physik, 2020

We are witnessing rapid developments in the field of quantum materials with the focus on some of ... more We are witnessing rapid developments in the field of quantum materials with the focus on some of the most profound concepts in condensed matter, including entangled orders, quantum coherence, and quantum topology. Quantum correlations in these materials naturally reveal themselves in the time domain, since their temporal evolution is governed by the full Hamiltonian, which contains multiple interactions. Therefore, nonequilibrium quantum dynamics emerges as a design principle to create desired quantum materials and functionalities. We see a growing focus on dynamics as a way to understand and control the fundamental physical processes that emerge due to quantum coherence of entangled quantum matter. Commensurate with the developments in theory and modeling of quantum materials, we see a rapid rise of new probes for the examination of quantum matter. These new probes include ultra-fast optics, free-electron lasers, and new neutron scattering facilities with capabilities to probe quantum matter at the increasingly short time scale while maintaining high-spatial resolution. Reflecting on recent progress and excitement in the field, we organized a workshop on Dynamic Quantum Matter Dec 10-14, 2018 at the Nordic Institute for Theoretical Physics (NORDITA, https://indico.fysik.su.se/event/6504/). As a result of this conference, we put together a special issue of the Annalen der Physik on "Dynamic Quantum Matter". We summarize the contributions in this volume. This special issue contains a rapid research letter, four feature articles, five original papers, and a perspective on the dynamics of various forms of quantum matter and materials ranging from superconductivity to magnetism, bosonic and fermionic Dirac materials ferro-states in 2D materials.

Physical Review B, 2018

We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice... more We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the collinear phase diagram as well as the change in the spin dynamics with various exchange interactions. We show that the ferromagnetic structure produces bosonic Dirac and Weyl points due to the competition between the interactions. Furthermore, it is shown that the criteria for magnetic Dirac nodes are coupled to the magnetic structure and not the overall crystal symmetry, where the breaking of inversion symmetry greatly affects the antiferromagnetic configurations. The tunability of the nodal points through variation of the exchange parameters leads to the possibility of controlling Dirac symmetries through an external manipulation of the orbital interactions.

Physical Review B, 2018

We investigate the effects of strain on superconductivity with particular reference to SrTiO3. As... more We investigate the effects of strain on superconductivity with particular reference to SrTiO3. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviours under uniaxial and biaxial strain can be understood.

Journal of Applied Physics, 2018

This study examines the magnetic interactions between spatially variable manganese and chromium t... more This study examines the magnetic interactions between spatially variable manganese and chromium trimers substituted into a graphene superlattice. Using density functional theory, we calculate the electronic band structure and magnetic populations for the determination of the electronic and magnetic properties of the system. To explore the super-exchange coupling between the transition-metal atoms, we establish the magnetic ground states through a comparison of multiple magnetic and spatial configurations. Through an analysis of the electronic and magnetic properties, we conclude that the presence of transition-metal atoms can induce a distinct magnetic moment in the surrounding carbon atoms as well as produce a Ruderman-Kittel-Kasuya-Yosida-like super-exchange coupling. It is hoped that these simulations can lead to the realization of spintronic applications in graphene through electronic control of the magnetic clusters.

Physical Review B, 2017

2I3 charge transfer salts are reported to show superconductivity and pressure induced quasi two-d... more 2I3 charge transfer salts are reported to show superconductivity and pressure induced quasi two-dimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases α, β and κ. We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and discuss the origin of band crossings. Additionally, we discuss the chemically induced strain in κ-(BEDT-TTF)2I3 achieved by replacing the Iodine layer with other Halogens: Fluorine, Bromine and Chlorine. In the case of κ-(BEDT-TTF)2F3, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the non-symmorphic nature of the crystal. The calculated electronic structures presented here are added to the organic materials database (OMDB).

Physical Review B, 2016

We demonstrate how a Dirac-like magnon spectrum is generated for localized magnetic moments formi... more We demonstrate how a Dirac-like magnon spectrum is generated for localized magnetic moments forming a two-dimensional honeycomb lattice. The Dirac crossing point is proven to be robust against magnon-magnon interactions, as these only shift the spectrum. Local defects induce impurity resonances near the Dirac point, as well as magnon Friedel oscillations. The energy of the Dirac point is controlled by the exchange coupling, and thus a two-dimensional array of magnetic dots is an experimentally feasible realization of Dirac magnons with tunable dispersion.

PLOS ONE, 2017

We present an organic materials database (OMDB) hosting thousands of Kohn-Sham electronic band st... more We present an organic materials database (OMDB) hosting thousands of Kohn-Sham electronic band structures, which is freely accessible online at http://omdb.diracmaterials.org. The OMDB focus lies on electronic structure, density of states and other properties for purely organic and organometallic compounds that are known to date. The electronic band structures are calculated using density functional theory for the crystal structures contained in the Crystallography Open Database. The OMDB web interface allows users to retrieve materials with specified target properties using non-trivial queries about their electronic structure. We illustrate the use of the OMDB and how it can become an organic part of search and prediction of novel functional materials via data mining techniques. As a specific example, we provide data mining results for metals and semiconductors, which are known to be rare in the class of organic materials.

Physical Review B, 2016

We consider a superconductor proximity coupled to a two-dimensional ferromagnetic film with a sky... more We consider a superconductor proximity coupled to a two-dimensional ferromagnetic film with a skyrmion texture. Using the T-matrix calculations and numerical modeling we calculate the spinpolarized local density of states in the superconductor in the vicinity of the skyrmion. We predict the skyrmion bound states that are induced in the superconductor, similar to the well-known Yu-Shiba-Rusinov (YSR) states. The bound state wavefunctions have spatial power-law decay. It is suggested that superconductivity could facilitate an effective long-range interaction between skyrmions when bound state wavefunctions overlap.

AIP Advances, 2016

Rashba spin-orbit effects and electron correlations in the two-dimensional cylindrical lattices o... more Rashba spin-orbit effects and electron correlations in the two-dimensional cylindrical lattices of square geometries are assessed using mesoscopic two-, three- and four-leg ladder structures. Here the electron transport properties are systematically calculated by including the spin-orbit coupling in tight binding and Hubbard models threaded by a magnetic flux. These results highlight important aspects of possible symmetry breaking mechanisms in square ladder geometries driven by the combined effect of a magnetic gauge field spin-orbit interaction and temperature. The observed persistent current, spin and charge polarizations in the presence of spin-orbit coupling are driven by separation of electron and hole charges and opposite spins in real-space. The modeled spin-flip processes on the pairing mechanism induced by the spin-orbit coupling in assembled nanostructures (as arrays of clusters) engineered in various two-dimensional multi-leg structures provide an ideal playground for un...

Physics Letters A, 2015

The VCA ground state of the 2D Hubbard model is examined for possible phase separation under hole... more The VCA ground state of the 2D Hubbard model is examined for possible phase separation under hole doping manifested by spatial inhomogeneities of coexisting different electron densities at equilibrium. Phase separation is accompanied by spectral weight loss and first Brillouin zone boundary deformation. Such instability is observed in square structures and it is absent in honeycomb lattices. To our knowledge, no previous publications have revealed relationship between Fermi surface instability and phase separation. Our VCA calculations provide strong support for spontaneous instability, driven by electron correlations in specific lattice geometries, proposed in our earlier publications using exact quantum cluster calculations.

We report 29 Si nuclear magnetic resonance measurements in a single crystal of URu 2 Si 2 in the ... more We report 29 Si nuclear magnetic resonance measurements in a single crystal of URu 2 Si 2 in the hidden order, antiferromagnetic, and paramagnetic phases under pressure. We find evidence for partial suppression of the density of states below 30 K at ambient pressure. We study how this behavior varies under pressure as hidden order gives way to antiferromagnetism. We analyze the data in light of various recent models.

Physical Review Letters, 1999

Physical review, Mar 4, 2020

Superconductor-ferromagnetic heterostructures have been suggested as one of the most promising al... more Superconductor-ferromagnetic heterostructures have been suggested as one of the most promising alternatives of realizing odd-frequency superconductivity. In this work we consider the limit of shrinking the ferromagnetic region to the limit of a single impurity embedded in a conventional superconductor, which gives rise to localized Yu-Shiba-Rusinov (YSR) bound states with energies inside the superconducting gap. We demonstrate that all the sufficient ingredients for generating odd-frequency pairing are present at the vicinity of these impurities. We investigate the appearance of all possible pair amplitudes in accordance with the Berezinskii SP * OT * = −1 rule, with the symmetry under the exchange of spin, spatial, orbital (in our case O = +1), and time index, respectively. We study the spatial and frequency dependence of the possible pairing amplitudes, analyzing their evolution with impurity strength and identifying a reciprocity between different symmetries related through impurity scattering. We show that the odd-frequency spin-triplet pairing amplitude dominates at the critical impurity strength, where the YSR states merge at the middle of the gap, while the even components are quenched close to the impurity. We also show that the spin-polarized local density of states exhibits the same spatial and frequency behavior as the odd-ω spin-triplet component at the critical impurity strength.

Bulletin of the American Physical Society, Mar 16, 2017

magnetoinfrared spectroscopy, we have explored the charge dynamics of (Bi, Sb) 2 T e 3 thin films... more magnetoinfrared spectroscopy, we have explored the charge dynamics of (Bi, Sb) 2 T e 3 thin films on InP substrates. From the magneto-transmission data we extracted three distinct cyclotron resonance (CR) energies that are all apparent in the broad band Faraday rotation (FR) spectra. This comprehensive FR-CR data set has allowed us to isolate the response of the bulk states from the intrinsic surface states associated with both the top and bottom surfaces of the film. The FR data uncovered that the electron-and hole-type Dirac fermions reside on opposite surfaces of our films, which paves the way for observing many exotic quantum phenomena in topological insulators.

Bulletin of the American Physical Society, Mar 22, 2005

Physical Review B

We investigate the effects of two electronic bands at the negative electronic compressibility (NE... more We investigate the effects of two electronic bands at the negative electronic compressibility (NEC) in a two-dimensional electron gas (2DEG). We use a simple homogeneous model with Coulombic interactions and first-order multi-band coupling to examine the role of effective mass and relative permittivity in relation to the critical carrier density, where compressibility turns negative. We demonstrate that the population of a second band, along with the presence of inter-band coupling, can dramatically change the cross-over carrier density. Given the difficulty in determining and confirming multi-band electronic systems, this model provides a potential method for identifying multi-band electronic systems using precise bulk electronic properties measurements. To help illustrate this method, we apply our results to the observed NEC in the 2D electron gas at the interface of LaAlO3/SrTiO3 (LAO/STO) and determine that, for the known parameters of LAO/STO, the system is likely a realization of a two-band 2D electron gas. Furthermore, we provide general limits on the inter-band coupling with respect to the electronic band population.

Physical Review B, 2021

Superconductivity and ferroelectricity are typically incompatible because the former needs free c... more Superconductivity and ferroelectricity are typically incompatible because the former needs free carriers, but the latter is usually suppressed by free carriers, unless their concentration is low. In the case of strontium titanate with low carrier concentration, unconventional superconductivity and ferroelectricity were shown to be correlated. Here, we report theoretically and experimentally evaluated Grüneisen parameters whose divergence under tensile stress indicates that the dominant phonon mode that enhances the superconducting order is the ferroelectric transverse soft-mode. This finding rules out all other phonon modes as the main contributors to the enhanced superconductivity in strained strontium titanate. This methodology shown here can be applied to many other quantum materials.

Annalen der Physik, 2020

We are witnessing rapid developments in the field of quantum materials with the focus on some of ... more We are witnessing rapid developments in the field of quantum materials with the focus on some of the most profound concepts in condensed matter, including entangled orders, quantum coherence, and quantum topology. Quantum correlations in these materials naturally reveal themselves in the time domain, since their temporal evolution is governed by the full Hamiltonian, which contains multiple interactions. Therefore, nonequilibrium quantum dynamics emerges as a design principle to create desired quantum materials and functionalities. We see a growing focus on dynamics as a way to understand and control the fundamental physical processes that emerge due to quantum coherence of entangled quantum matter. Commensurate with the developments in theory and modeling of quantum materials, we see a rapid rise of new probes for the examination of quantum matter. These new probes include ultra-fast optics, free-electron lasers, and new neutron scattering facilities with capabilities to probe quantum matter at the increasingly short time scale while maintaining high-spatial resolution. Reflecting on recent progress and excitement in the field, we organized a workshop on Dynamic Quantum Matter Dec 10-14, 2018 at the Nordic Institute for Theoretical Physics (NORDITA, https://indico.fysik.su.se/event/6504/). As a result of this conference, we put together a special issue of the Annalen der Physik on "Dynamic Quantum Matter". We summarize the contributions in this volume. This special issue contains a rapid research letter, four feature articles, five original papers, and a perspective on the dynamics of various forms of quantum matter and materials ranging from superconductivity to magnetism, bosonic and fermionic Dirac materials ferro-states in 2D materials.

Physical Review B, 2018

We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice... more We examine the presence and evolution of magnetic Dirac nodes in the Heisenberg honeycomb lattice. Using linear spin theory, we evaluate the collinear phase diagram as well as the change in the spin dynamics with various exchange interactions. We show that the ferromagnetic structure produces bosonic Dirac and Weyl points due to the competition between the interactions. Furthermore, it is shown that the criteria for magnetic Dirac nodes are coupled to the magnetic structure and not the overall crystal symmetry, where the breaking of inversion symmetry greatly affects the antiferromagnetic configurations. The tunability of the nodal points through variation of the exchange parameters leads to the possibility of controlling Dirac symmetries through an external manipulation of the orbital interactions.

Physical Review B, 2018

We investigate the effects of strain on superconductivity with particular reference to SrTiO3. As... more We investigate the effects of strain on superconductivity with particular reference to SrTiO3. Assuming that a ferroelectric mode that softens under tensile strain is responsible for the coupling, an increase in the critical temperature and range of carrier densities for superconductivity is predicted, while the peak of the superconducting dome shifts towards lower carrier densities. Using a Ginzburg-Landau approach in 2D, we find a linear dependence of the critical temperature on strain: if the couplings between the order parameter and strains in different directions differ while their sum is fixed, different behaviours under uniaxial and biaxial strain can be understood.

Journal of Applied Physics, 2018

This study examines the magnetic interactions between spatially variable manganese and chromium t... more This study examines the magnetic interactions between spatially variable manganese and chromium trimers substituted into a graphene superlattice. Using density functional theory, we calculate the electronic band structure and magnetic populations for the determination of the electronic and magnetic properties of the system. To explore the super-exchange coupling between the transition-metal atoms, we establish the magnetic ground states through a comparison of multiple magnetic and spatial configurations. Through an analysis of the electronic and magnetic properties, we conclude that the presence of transition-metal atoms can induce a distinct magnetic moment in the surrounding carbon atoms as well as produce a Ruderman-Kittel-Kasuya-Yosida-like super-exchange coupling. It is hoped that these simulations can lead to the realization of spintronic applications in graphene through electronic control of the magnetic clusters.

Physical Review B, 2017

2I3 charge transfer salts are reported to show superconductivity and pressure induced quasi two-d... more 2I3 charge transfer salts are reported to show superconductivity and pressure induced quasi two-dimensional Dirac cones at the Fermi level. By performing state of the art ab initio calculations in the framework of density functional theory, we investigate the structural and electronic properties of the three structural phases α, β and κ. We furthermore report about the irreducible representations of the corresponding electronic band structures, symmetry of their crystal structure, and discuss the origin of band crossings. Additionally, we discuss the chemically induced strain in κ-(BEDT-TTF)2I3 achieved by replacing the Iodine layer with other Halogens: Fluorine, Bromine and Chlorine. In the case of κ-(BEDT-TTF)2F3, we identify topologically protected crossings within the band structure. These crossings are forced to occur due to the non-symmorphic nature of the crystal. The calculated electronic structures presented here are added to the organic materials database (OMDB).

Physical Review B, 2016

We demonstrate how a Dirac-like magnon spectrum is generated for localized magnetic moments formi... more We demonstrate how a Dirac-like magnon spectrum is generated for localized magnetic moments forming a two-dimensional honeycomb lattice. The Dirac crossing point is proven to be robust against magnon-magnon interactions, as these only shift the spectrum. Local defects induce impurity resonances near the Dirac point, as well as magnon Friedel oscillations. The energy of the Dirac point is controlled by the exchange coupling, and thus a two-dimensional array of magnetic dots is an experimentally feasible realization of Dirac magnons with tunable dispersion.

PLOS ONE, 2017

We present an organic materials database (OMDB) hosting thousands of Kohn-Sham electronic band st... more We present an organic materials database (OMDB) hosting thousands of Kohn-Sham electronic band structures, which is freely accessible online at http://omdb.diracmaterials.org. The OMDB focus lies on electronic structure, density of states and other properties for purely organic and organometallic compounds that are known to date. The electronic band structures are calculated using density functional theory for the crystal structures contained in the Crystallography Open Database. The OMDB web interface allows users to retrieve materials with specified target properties using non-trivial queries about their electronic structure. We illustrate the use of the OMDB and how it can become an organic part of search and prediction of novel functional materials via data mining techniques. As a specific example, we provide data mining results for metals and semiconductors, which are known to be rare in the class of organic materials.

Physical Review B, 2016

We consider a superconductor proximity coupled to a two-dimensional ferromagnetic film with a sky... more We consider a superconductor proximity coupled to a two-dimensional ferromagnetic film with a skyrmion texture. Using the T-matrix calculations and numerical modeling we calculate the spinpolarized local density of states in the superconductor in the vicinity of the skyrmion. We predict the skyrmion bound states that are induced in the superconductor, similar to the well-known Yu-Shiba-Rusinov (YSR) states. The bound state wavefunctions have spatial power-law decay. It is suggested that superconductivity could facilitate an effective long-range interaction between skyrmions when bound state wavefunctions overlap.

AIP Advances, 2016

Rashba spin-orbit effects and electron correlations in the two-dimensional cylindrical lattices o... more Rashba spin-orbit effects and electron correlations in the two-dimensional cylindrical lattices of square geometries are assessed using mesoscopic two-, three- and four-leg ladder structures. Here the electron transport properties are systematically calculated by including the spin-orbit coupling in tight binding and Hubbard models threaded by a magnetic flux. These results highlight important aspects of possible symmetry breaking mechanisms in square ladder geometries driven by the combined effect of a magnetic gauge field spin-orbit interaction and temperature. The observed persistent current, spin and charge polarizations in the presence of spin-orbit coupling are driven by separation of electron and hole charges and opposite spins in real-space. The modeled spin-flip processes on the pairing mechanism induced by the spin-orbit coupling in assembled nanostructures (as arrays of clusters) engineered in various two-dimensional multi-leg structures provide an ideal playground for un...

Physics Letters A, 2015

The VCA ground state of the 2D Hubbard model is examined for possible phase separation under hole... more The VCA ground state of the 2D Hubbard model is examined for possible phase separation under hole doping manifested by spatial inhomogeneities of coexisting different electron densities at equilibrium. Phase separation is accompanied by spectral weight loss and first Brillouin zone boundary deformation. Such instability is observed in square structures and it is absent in honeycomb lattices. To our knowledge, no previous publications have revealed relationship between Fermi surface instability and phase separation. Our VCA calculations provide strong support for spontaneous instability, driven by electron correlations in specific lattice geometries, proposed in our earlier publications using exact quantum cluster calculations.

We report 29 Si nuclear magnetic resonance measurements in a single crystal of URu 2 Si 2 in the ... more We report 29 Si nuclear magnetic resonance measurements in a single crystal of URu 2 Si 2 in the hidden order, antiferromagnetic, and paramagnetic phases under pressure. We find evidence for partial suppression of the density of states below 30 K at ambient pressure. We study how this behavior varies under pressure as hidden order gives way to antiferromagnetism. We analyze the data in light of various recent models.

Physical Review Letters, 1999