arXiv subjects

Alexander V. Balatsky

Publications and source records attributed to Alexander V. Balatsky.

At least 91 records · Page 5Linked to original sources

arXiv2017

Data Mining for Three-Dimensional Organic Dirac Materials: Focus on Space Group 19

We combined the group theory and data mining approach within the Organic Materials Database that leads to the prediction of stable Dirac-point nodes within the electronic band structure of 3-dimensional organic crystals. We find a particular space group $P2_12_12_1$ ($\#19$) that is conducive to the Dirac nodes formation. We prove that nodes are a consequence of the orthorhombic crystal structure. Within the electronic band structure, two different kinds of nodes can be distinguished: 8-fold degenerate Dirac nodes protected by the crystalline symmetry and 4-fold degenerate Dirac nodes protected by band topology. Mining the Organic Materials Database, we present band structure calculations and symmetry analysis for 6 previously synthesized organic materials. In all these materials, the Dirac nodes are well separated within the energy and located near the Fermi surface, which opens up a possibility for their direct experimental observation.

cond-mat.mtrl-sci↗

arXiv2017

Theory of deflagration in disordered media

The conventional theory of burning works well in the case of uniform media where all system parameters are spatially independent. We develop a theory of burning in disordered media. In this case, rare regions (hot spots) where the burning process is more effective than on average may control the heat propagation in an explosive sample. We show that most predictions of the theory of burning are quite different from the conventional case. In particular, we show that a system of randomly distributed hot spots exhibits a dynamic phase transition, which is similar to the percolation transition. Depending on parameters of the system the phase transition can be either first or second order. These two regimes are separated by a tricritical point. The above results may be applicable to dynamics of any over-heated disordered system with a first order phase transition.

cond-mat.stat-mech↗

arXiv2017

Excitonic Gap Formation in Pumped Dirac Materials

Recent pump-probe experiments demonstrate the possibility that Dirac materials may be driven into transient excited states describable by two chemical potentials, one for the electrons and one for the holes. Given the Dirac nature of the spectrum, such an inverted population allows the optical tunability of the density of states of the electrons and holes, effectively offering control of the strength of the Coulomb interaction. Here we discuss the feasibility of realizing transient excitonic instabilities in optically-pumped Dirac materials. We demonstrate, theoretically, the reduction of the critical coupling leading to the formation of a transient condensate of electron-hole pairs and identify signatures of this state. Furthermore, we provide guidelines for experiments by both identifying the regimes in which such exotic many-body states are more likely to be observed and estimating the magnitude of the excitonic gap for a few important examples of existing Dirac materials. We find a set of material parameters for which our theory predicts large gaps and high critical temperatures and which could be realized in future Dirac materials. We also comment on transient excitonic instabilities in three-dimensional Dirac and Weyl semimetals. This study provides the first example of a transient collective instability in driven Dirac materials.

cond-mat.mtrl-sci↗

arXiv2017

Faraday rotation due to surface states in the topological insulator (Bi$_{1-x}$Sb$_{x}$)$_{2}$Te$_{3}$

Using magneto-infrared spectroscopy, we have explored the charge dynamics of (Bi,Sb)$_2$Te$_3$ 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 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.

cond-mat.mes-hall↗

arXiv2017

Three-dimensional organic Dirac-line materials due to nonsymmorphic symmetry: a data mining approach

A data mining study of electronic Kohn-Sham band structures was performed to identify Dirac materials within the Organic Materials Database (OMDB). Out of that, the 3-dimensional organic crystal 5,6-bis(trifluoromethyl)-2-methoxy-1$H$-1,3-diazepine was found to host different Dirac line-nodes within the band structure. From a group theoretical analysis, it is possible to distinguish between Dirac line-nodes occurring due to 2-fold degenerate energy levels protected by the monoclinic crystalline symmetry and 2-fold degenerate accidental crossings protected by the topology of the electronic band structure. The obtained results can be generalized to all materials having the space group $P2_1/c$ (No. 14, $C^5_{2h}$) by introducing three distinct topological classes.

cond-mat.mtrl-sci↗

arXiv2016

Skyrmion-induced bound states in a superconductor

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 spin-polarized local density of states in the superconductor in the vicinity of the skyrmion. We predict the skyrmion bound states (SBS) that are induced in the superconductor, similar to the well-known Yu-Shiba-Rusinov (YSR) states. The SBS wavefunctions have spatial power law decay. Presence of the SBS suggests the mechanism by which superconductivity could facilitate an effective long-range interaction between skyrmions when their SBS wavefunctions overlap.

cond-mat.supr-con↗

arXiv2016

Odd-frequency Superconductivity in Driven Systems

We show that Berezinskii's classification of the symmetries of Cooper pair amplitudes holds for driven systems even in the absence of translation invariance. We then consider a model Hamiltonian for a superconductor coupled to an external driving potential and, treating the driving potential as a perturbation, we investigate the corrections to the anomalous Green's function, density of states, and spectral function. We find that in the presence of an external drive the anomalous Green's function develops terms that are odd in frequency and that the same mechanism responsible for these odd-frequency terms generates additional features in the density of states and spectral function.

cond-mat.supr-con↗

arXiv2016

Skyrmion-induced subgap states in p-wave superconductors

In s-wave systems, it has been theoretically shown that a ferromagnetic film hosting a skyrmion can induce a bound state embedded in the opposite-spin continuum. In this work, we consider a case of skyrmion-induced state in a p-wave superconductor. We find that the skyrmion induces a bound state that generally resides \emph{within} the spectral gap and is isolated from all other states, in contrast to the case of conventional superconductors. To this end, we derive an approximate expression for the $T$-matrix, through which we calculate the spin-polarized local density of states which is observable in scanning tunneling microscopy measurements. We find the unique spectroscopic features of the skyrmion-induced bound state and discuss how our predictions could be employed as novel experimental probes for p-wave superconducting states.

cond-mat.mes-hall↗

arXiv2016

Dynamical quantum phase transitions: Role of topological nodes in wavefunction overlaps

A sudden quantum quench of a Bloch band from one topological phase toward another has been shown to exhibit an intimate connection with the notion of a dynamical quantum phase transition (DQPT), where the returning probability of the quenched state to the initial state---i.e. the Loschmidt echo---vanishes at critical times $\{t^{*}\}$. Analytical results so far are limited to two-band models, leaving the exact relation between topology and DQPT unclear. In this work, we show that for a general multi-band system, a robust DQPT relies on the existence of nodes (i.e. zeros) in the wavefunction overlap between the initial band and the post-quench energy eigenstates. These nodes are topologically protected if the two participating wavefunctions have distinctive topological indices. We demonstrate these ideas in detail for both one and two spatial dimensions using a three-band generalized Hofstadter model. We also discuss possible experimental observations.

cond-mat.stat-mech↗

arXiv2016

General conditions for proximity-induced odd-frequency superconductivity in two-dimensional electronic systems

We obtain the general conditions for the emergence of odd-frequency superconducting pairing in a two-dimensional (2D) electronic system proximity-coupled to a superconductor, making minimal assumptions about both the 2D system and the superconductor. Using our general results we show that a simple heterostructure formed by a monolayer of a group VI transition metal dichalcogenide, such as molybdenum disulfide, and an s-wave superconductor with Rashba spin-orbit coupling will exhibit odd-frequency superconducting pairing. Furthermore, we show that in such a heterostructure the odd-frequency pairing amplitude will be proportional to the product of the Rashba spin-orbit coupling in the substrate and the spin-orbit coupling in the dichalcogenide layer.

cond-mat.mes-hall↗

arXiv2016

Landau-like states in neutral particles

We show the emergence of a new type of dispersion relation for neutral atoms with an interesting similarity with the spectrum of 2-dimensional electrons in an applied perpendicular constant magnetic field. These neutral atoms can be confined in toroidal optical traps and give quasi Landau spectra. In strong contrast to the equi-distant infinitely degenerate Landau levels for charged particles, the spectral gap for such 2-dimensional neutral particles increases in particular electric field configurations. The idea in the paper is motivated by the development in cold atom experiments and builds on the seminal paper of Aharonov and Casher.

quant-ph↗

arXiv2016

Strange Metals from Quantum Geometric Fluctuations of Interfaces

Our current understanding of strongly correlated electron systems is based on a homogeneous framework. Here we take a step going beyond this paradigm by incorporating inhomogeneity from the beginning. Specifying to systems near the Mott metal-insulator transition, we propose a real space picture of itinerant electrons functioning in the fluctuating geometries bounded by interfaces between metallic and insulating regions. In 2+1-dimensions, the interfaces are closed bosonic strings, and we have a system of strings coupled to itinerant electrons. When the interface tension vanishes, the geometric fluctuations become critical, which gives rise to non-Fermi liquid behavior for the itinerant electrons. In particular, the poles of the fermion Green's function can be converted to zeros, indicating the absence of propagating quasiparticles. Furthermore, the quantum geometric fluctuations mediate Cooper pairing among the itinerant electrons, indicating the intrinsic instability of electronic systems near the Mott transition.

cond-mat.str-el↗

arXiv2016

Voltage-dependent spin flip in magnetically-substituted graphene nanoribbons: Toward the realization of graphene-based spintronic devices

We examine the possibility of using graphene nanoribbons (GNRs) with directly substituted chromium atoms as spintronic device. Using density functional theory, we simulate a voltage bias across a constructed GNR in a device setup, where a magnetic dimer has been substituted into the lattice. Through this first principles approach, we calculate the electronic and magnetic properties as a function of Hubbard U, voltage, and magnetic configuration. By calculating of the total energy of each magnetic configuration, we determine that initial antiferromagnetic ground state flips to a ferromagnetic state with applied bias. Mapping this transition point to the calculated conductance for the system reveals that there is a distinct change in conductance through the GNR, which indicates the possibility of a spin valve. We also show that this corresponds to a distinct change in the induced magnetization within the graphene.

cond-mat.mtrl-sci↗

arXiv2016

An unusual superconducting Isotope Effect in the presence of a Quantum Criticality

We discuss the possible connection between superconductivity (SC) and quantum critical points (QCP) for any QCP that is tunable by isotopic mass substitution. We find a distinct contribution to the isotope exponent, due to the proximity to a QCP, which can be used as an experimental signature for the relation between SC and QCP. The relation is demonstrated in a scenario where the SC pairing is due to modes related to a structural instability. Within this model the isotope exponent is derived in terms of microscopic parameters.

cond-mat.supr-con↗

arXiv2015

Currents induced by magnetic impurities in superconductors with spin-orbit coupling

We show that superconducting currents are generated around magnetic impurities and ferromagnetic islands proximity-coupled to superconductors with finite spin-orbit coupling. Using the Ginzburg-Landau theory, T-matrix calculation, as well as self-consistent numerical simulation on a lattice, we find a strong dependence of the current on the direction and magnitude of the magnetic moment. We establish that in the case of point magnetic impurities, the current is carried by the induced Yu-Shiba-Rusinov (YSR) subgap states. In the vicinity of the phase transition, where the YSR states cross at zero energy, the current increases dramatically. Furthermore, we show that the currents are orthogonal to the local spin polarization and, thus, can be probed by measuring the spin-polarized local density of states.

cond-mat.str-el↗

arXiv2015

U.S. stock market interaction network as learned by the Boltzmann Machine

We study historical dynamics of joint equilibrium distribution of stock returns in the U.S. stock market using the Boltzmann distribution model being parametrized by external fields and pairwise couplings. Within Boltzmann learning framework for statistical inference, we analyze historical behavior of the parameters inferred using exact and approximate learning algorithms. Since the model and inference methods require use of binary variables, effect of this mapping of continuous returns to the discrete domain is studied. The presented analysis shows that binarization preserves market correlation structure. Properties of distributions of external fields and couplings as well as industry sector clustering structure are studied for different historical dates and moving window sizes. We found that a heavy positive tail in the distribution of couplings is responsible for the sparse market clustering structure. We also show that discrepancies between the model parameters might be used as a precursor of financial instabilities.

q-fin.ST↗

arXiv2015

Many-body instabilities and mass generation in slow Dirac materials

Some Kondo insulators are expected to possess topologically protected surface states with linear Dirac spectrum, the topological Kondo insulators. Because the bulk states of these systems typically have heavy effective electron masses, the surface states may exhibit extraordinarily small Fermi velocities that could force the effective fine structure constant of the surface states into the strong coupling regime. Using a tight-binding model we study the many-body instabilities of these systems and identify regions of parameter space in which the system exhibits spin density wave, and charge density wave order.

cond-mat.mes-hall↗

arXiv2015

Quantum critical origin of the superconducting dome in SrTiO$_3$

We investigate the origin of superconductivity in doped SrTiO$_3$ (STO) using a combination of density functional and strong coupling theories within the framework of quantum criticality. Our density functional calculations of the ferroelectric soft mode frequency as a function of doping reveal a crossover from quantum paraelectric to ferroelectric behavior at a doping level coincident with the experimentally observed top of the superconducting dome. Based on this finding, we explore a model in which the superconductivity in STO is enabled by its proximity to the ferroelectric quantum critical point and the soft mode fluctuations provide the pairing interaction on introduction of carriers. Within our model, the low doping limit of the superconducting dome is explained by the emergence of the Fermi surface, and the high doping limit by departure from the quantum critical regime. We predict that the highest critical temperature will increase and shift to lower carrier doping with increasing $^{18}$O isotope substitution, a scenario that is experimentally verifiable.

cond-mat.supr-con↗