arXiv subjects

A. V. Balatsky

Publications and source records attributed to A. V. Balatsky.

At least 19 recordsLinked to original sources

arXiv2025

Surface Local Impurity Scattering as a Probe for Topological Kondo Insulators

Shortly after the discovery of topological band insulators, topological Kondo insulators (TKIs) have also been theoretically predicted. The latter has ignited revival interest in the properties of Kondo insulators. Currently, the feasibility of topological nature in SmB$_6$ has been intensively analyzed by several complementary probes. Here by starting with a minimal-orbital Anderson lattice model, we explore the local electronic structure in a Kondo insulator. We show that the two strong topological regimes sandwiching the weak topological regime give rise to a single Dirac cone, which is located near the center or corner of the surface Brillouin zone. We further find that, when a single impurity is placed on the surface, low-energy resonance states are induced in the weak scattering limit for the strong TKI regimes and the resonance level moves monotonically across the hybridization gap with the strength of impurity scattering potential; while low energy states can only be induced in the unitary scattering limit for the weak TKI regime, where the resonance level moves universally toward the center of the hybridization gap. These impurity-induced low-energy quasiparticles will lead to characteristic signatures in scanning tunneling microscopy/spectroscopy, which has recently found success in probing into exotic properties in heavy fermion systems.

cond-mat.str-el↗

arXiv2022

Terahertz electric-field driven dynamical multiferroicity in SrTiO$_3$

The emergence of collective order in matter is among the most fundamental and intriguing phenomena in physics. In recent years, the ultrafast dynamical control and creation of novel ordered states of matter not accessible in thermodynamic equilibrium is receiving much attention. Among those, the theoretical concept of dynamical multiferroicity has been introduced to describe the emergence of magnetization by means of a time-dependent electric polarization in non-ferromagnetic materials. In simple terms, a large amplitude coherent rotating motion of the ions in a crystal induces a magnetic moment along the axis of rotation. However, the experimental verification of this effect is still lacking. Here, we provide evidence of room temperature magnetization in the archetypal paraelectric perovskite SrTiO$_3$ due to this mechanism. To achieve it, we resonantly drive the infrared-active soft phonon mode with intense circularly polarized terahertz electric field, and detect a large magneto-optical Kerr effect. A simple model, which includes two coupled nonlinear oscillators whose forces and couplings are derived with ab-initio calculations using self-consistent phonon theory at a finite temperature, reproduces qualitatively our experimental observations on the temporal and frequency domains. A quantitatively correct magnitude of the effect is obtained when one also considers the phonon analogue of the reciprocal of the Einsten - de Haas effect, also called the Barnett effect, where the total angular momentum from the phonon order is transferred to the electronic one. Our findings show a new path for designing ultrafast magnetic switches by means of coherent control of lattice vibrations with light.

cond-mat.str-el↗

arXiv2022

Vortex Excitations of Dirac Bose-Einstein Condensates

We explore vortices in non-equilibrium Dirac Bose-Einstein condensates (Dirac BEC) described by a stationary Dirac Gross-Pitaevskii equations (GPE). We find that the multi-component structure of Dirac equation enables the difference in phase winding of two condensates with respective phase winding number differing by one, $\ell_a - \ell_b = \pm 1$. We observe three classes of vortex states distinguished by their far-field behavior: A ring soliton on either of the two components in combination with a vortex on the other component, and, in the case of strong inter-component interactions, a vortex profile on both components. The latter are multiple core vortices due to the phase winding difference between the components. We also address the role of a Haldane gap on these vortices, which has a similar effect than inter-component by making the occupation on either sublattice more costly. We employ a numerical shooting method to reliably identify vortex solutions and use it to scan large parts of the phase space. We then use a classification algorithm on the integrated wavefunctions to establish a phase diagram of the different topological sectors.

cond-mat.quant-gas↗

arXiv2021

Axial Magnetoelectric Effect in Dirac semimetals

We propose a mechanism to generate a static magnetization via {\em axial magnetoelectric effect} (AMEE). Magnetization ${\bf M} \sim {\bf E}_5(ω)\times {\bf E}_5^{*}(ω)$ appears as a result of the transfer of the angular momentum of the axial electric field ${\bf E}_5(t)$ into the magnetic moment in Dirac and Weyl semimetals. We point out similarities and differences between the proposed AMEE and a conventional inverse Faraday effect (IFE). As an example, we estimated the AMEE generated by circularly polarized acoustic waves and find it to be on the scale of microgauss for gigahertz frequency sound. In contrast to a conventional IFE, magnetization rises linearly at small frequencies and fixed sound intensity as well as demonstrates a nonmonotonic peak behavior for the AMEE. The effect provides a way to investigate unusual axial electromagnetic fields via conventional magnetometry techniques.

cond-mat.mes-hall↗

arXiv2021

Understanding the onset of negative electronic compressibility in one- and two-band 2D electron gases: Application to LaAlO$_3$/SrTiO$_3$

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 LaAlO$_3$/SrTiO$_3$ (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.

cond-mat.str-el↗

arXiv2021

Bose-Einstein condensate of Dirac magnons: Pumping and collective modes

We explore the formation and collective modes of Bose-Einstein condensate of Dirac magnons (Dirac BEC). While we focus on two-dimensional Dirac magnons, an employed approach is general and could be used to describe Bose-Einstein condensates with linear quasiparticle spectrum in various systems. By using a phenomenological multicomponent model of pumped boson population together with bosons residing at Dirac nodes, the formation and time evolution of condensates of Dirac bosons is investigated. The condensate coherence and its multicomponent nature are manifested in the Rabi oscillations whose period is determined by the gap in the spin-wave spectrum. A Dirac nature of the condensates could be also probed by the spectrum of collective modes. It is shown that the Haldane gap provides an efficient means to tune between the gapped and gapless collective modes as well as controls their stability.

cond-mat.mes-hall↗

arXiv2020

Signatures of odd-frequency pairing in the Josephson junction current noise

Odd frequency (odd-$ω$) electron pair correlations naturally appear at the interface between BCS superconductors and other materials. The detection of odd-$ω$ pairs, which are necessarily non-local in time, is still an open problem. The main reason is that they do not contribute to static measurements described by time-local correlation functions. Therefore, dynamical measurements, which depend on non-local time correlations, are suitable for detecting these pairs. In this work, we study the signatures of odd-$ω$ pairs in the supercurrent noise through a weak link between two superconductors at different superconducting phases. We show that the finite frequency current noise can be decomposed into three different contributions coming from even frequency (even-$ω$), odd-$ω$ pair amplitudes, and electron-hole correlation functions. Odd-$ω$ pairing, which is inter-lead (between electrons at different sides of the junction), provides a positive contribution to the noise, becoming maximal at a superconducting phase difference of $π$. In contrast, intra-lead even-$ω$ pair amplitude tends to reduce the noise, except for a region close to $π$, controlled by the transmission of the junction.

cond-mat.mes-hall↗

arXiv2020

Interacting Dirac Materials

We investigate the extent to which the class of Dirac materials in two-dimensions provides general statements about the behavior of both fermionic and bosonic Dirac quasiparticles in the interacting regime. For both quasiparticle types, we find common features for the interaction induced renormalization of the conical Dirac spectrum. We perform the perturbative renormalization analysis and compute the self-energy for both quasiparticle types with different interactions and collate previous results from the literature whenever necessary. Guided by the systematic presentation of our results in Table~\ref{Summary}, we conclude that long-range interactions generically lead to an increase of the slope of the single-particle Dirac cone, whereas short-range interactions lead to a decrease. The quasiparticle statistics does not qualitatively impact the self-energy correction for long-range repulsion but does affect the behavior of short-range coupled systems, giving rise to different thermal power-law contributions. The possibility of a universal description of the Dirac materials based on these features is also mentioned.

cond-mat.str-el↗

arXiv2019

Non-Hermitian impurities in Dirac systems

Quasiparticle states in Dirac systems with complex impurity potentials are investigated. It is shown that an impurity site with loss leads to a nontrivial distribution of the local density of states (LDOS). While the real part of defect potential induces a well-pronounced peak in the density of states (DOS), the DOS is either weakly enhanced at small frequencies or even forms a peak at the zero frequency for a lattice in the case of non-Hermitian impurity. As for the spatial distribution of the LDOS, it is enhanced in the vicinity of impurity but shows a dip at a defect itself when the potential is sufficiently strong. The results for a two-dimensional hexagonal lattice demonstrate the characteristic trigonal-shaped profile for the LDOS. The latter acquires a double-trigonal pattern in the case of two defects placed at neighboring sites. The effects of non-Hermitian impurities could be tested both in photonic lattices and certain condensed matter setups.

cond-mat.dis-nn↗

arXiv2019

Quantum Pairing Time Orders

We propose the concept of the time-independent correlators for the even- and odd-frequency pairing states that can be defined for both bosonic and fermionic quasiparticles. These correlators explicitly capture the existence of two distinct classes of pairing states and provide a direct probe of the hidden Berezinskii order. This concept is illustrated in the cases of pairings for Majorana fermions and quasiparticles in Dirac semimetals. It is shown that the time-independent correlator is able to effectively capture the energy scale relevant for pairing.

cond-mat.supr-con↗

arXiv2019

Transient excitonic states in optically-pumped Dirac materials: overview of recent work

Driven and non-equilibrium quantum states of matter have attracted growing interest in both theoretical and experimental studies in condensed matter physics. We review recent progress in realizing transient collective states in driven or pumped Dirac materials (DMs). In particular, we focus on optically-pumped DMs which have been theoretically proposed as a promising platform for observation of a transient excitonic instability. Optical pumping combined with the linear (Dirac) dispersion of the electronic spectrum offers a knob for tuning the effective interaction between the photoexcited electrons and holes, and thus provides a way of reducing the critical coupling for excitonic instability. As a result, a transient excitonic condensate could be achieved in a pumped DM while it is not feasible in equilibrium. We provide a unifying theoretical framework for describing transient collective states in two- and three-dimensional DMs. We describe experimental signatures of the transient excitonic state and summarize numerical estimates of the magnitude of the effect, namely the size of the dynamically-induced excitonic gaps and the values of the critical temperatures for several specific systems. We also discuss general guidelines for identifying promising material candidates.Finally, we comment recent experimental efforts in realizing transient excitonic condensate in pumped DMs and outline outstanding issues and possible future directions.

cond-mat.mtrl-sci↗

arXiv2019

Spin channel induced directional dependent spin exchange interactions between divacantly substituted Fe atoms in graphene

In this study, we investigate the isolated magnetic interactions between two identical Fe atoms divacantly-substituted into graphene. Using density functional theory, we simulated the electronic and magnetic properties for a supercell of graphene with spatial variation of the Fe atoms along either the armchair or zig-zag directions. Overall, we find that the exchange interaction between the two Fe atoms fluctuates from ferromagnetic to antiferromagnetic as a function of the spatial distance in the armchair direction. Given the induced magnetic moment and increased density of states at the Fermi level by the surrounding carbon atoms, we conclude that an RKKY-like interaction may characterize the exchange interactions between the Fe atoms. Furthermore, we examined the same interactions for Fe atoms along the zig-zag direction in graphene and found no evidence for an RKKY interaction as this system shows standard superexchange between the transition-metal impurities. Therefore, we determine that Fe-substituted graphene produces a directional-dependent spin interaction, which may provide stability to spintronic and multifunctional devices and applications for graphene.

cond-mat.str-el↗

arXiv2019

Odd-frequency Berezinskii superconductivity in Dirac semimetals

We formulate a general framework for addressing both odd- and even-frequency superconductivity in Dirac semimetals and demonstrate that the odd-frequency or the Berezinskii pairing can naturally appear in these materials because of the chirality degree of freedom. We show that repulsive frequency-dependent interactions favor the Berezinskii pairing while an attractive electron-electron interaction allows for the BCS pairing. In the case of compensated Dirac and Weyl semimetals, both the conventional BCS and odd-frequency Berezinskii pairings require critical coupling. Since these pairings could originate from physically different mechanisms, our findings pave the way for controlling the realization of the Berezinskii superconductivity in topological semimetals. We also present the density of states with several cusp-like features that can serve as an experimentally verifiable signature of the odd-frequency gap.

cond-mat.supr-con↗

arXiv2019

Spectroscopic and optical response of odd-frequency superconductors

The optical response of superconductors with odd-frequency Berezinskii pairing is studied. By using a simple model with a parabolic dispersion law and a non-magnetic disorder, the spectral function, the electron density of states, and the optical conductivity are calculated for a few gap ansatzes. The spectral function and the electron density of states clearly reveal the gap for the Berezinskii pairing for the sufficiently strong frequency dependence of the order parameters. It is found that, similarly to the conventional BCS pairing, the odd-frequency gaps induce peaks in the real part of the conductivity, which, however, are sharper than in the BCS case. The magnitude and position of these peaks are determined by the frequency profile of the gap. The imaginary part of the optical conductivity for the Berezinskii pairing demonstrates sharp cusps that are absent in the case of the BCS superconductors. The corresponding results suggest that the Berezinskii pairing might allow for the optical transparency windows related to the onsets of the attenuation peaks in the real part of the conductivity. Thus, the study of the optical response not only provides an alternative way to probe the odd-frequency gaps but can reveal also additional features of the dynamic superconducting pairing.

cond-mat.supr-con↗

arXiv2019

Designing a path towards superconductivity through magnetic exchange in transition-metal intercalated bilayer graphene

This study examines the potential of superconductivity in transition metal (TM) intercalated bilayer graphene through a systematic study of the electronic and magnetic properties. We determine the electronic structure for all first row TM elements in the stable honeycomb configuration between two layers of graphene using density functional theory (DFT). Through an analysis of the electron density, we assess the induction of the magnetic moment in each case, where a comparison of the ferromagnetic and antiferromagnetic configurations allow us to ascertain an estimated exchange coupling between the transition-metal elements. By analyzing the electronic properties, we find that the carbon $p$-bands are degenerate with the TM $d$-bands and form an electron pocket below the Fermi energy at the $Γ-$point. These hybridized bands are analogous to the carbon $p$-band effect that produces superconductivity in intercalated graphite with alkali and alkaline-earth metals. Furthermore, since the bands are hybridized with the TM $d$-bands, their magnetic properties may provide bosonic modes from their spin-coupling to preserve the unique linear dispersion present in monolayer graphene. This study provides a designing route by using TMs for tuning magneto-electric Dirac materials and will encourage future experimental studies to further the fundamental knowledge of unconventional superconductivity.

cond-mat.mtrl-sci↗

arXiv2019

Dynamic multiferroicity of a ferroelectric quantum critical point

Quantum matter hosts a large variety of phases, some coexisting, some competing; when two or more orders occur together, they are often entangled and cannot be separated. Dynamical multiferroicity, where fluctuations of electric dipoles lead to magnetisation, is an example where the two orders are impossible to disentangle. Here we demonstrate elevated magnetic response of a ferroelectric near the ferroelectric quantum critical point (FE QCP) since magnetic fluctuations are entangled with ferroelectric fluctuations. We thus suggest that any ferroelectric quantum critical point is an \textit{inherent} multiferroic quantum critical point. We calculate the magnetic susceptibility near the FE QCP and find a region with enhanced magnetic signatures that appears near the FE QCP, and controlled by the tuning parameter of the ferroelectric phase. The effect is small but observable - we propose quantum paraelectric strontium titanate as a candidate material where the magnitude of the induced magnetic moments can be $\sim 5 \times 10^{-7} μ_{B}$ per unit cell near the FE QCP.

cond-mat.str-el↗

arXiv2018

Controlled healing of graphene nanopores

Nanopores of nanometer-size holes are very promising devices for many applications: DNA sequencing, sensory, biosensoring and molecular detectors, catalysis and water desalination. These applications require accurate control over nanopores size. We report computer simulation studies of regrowth and healing of graphene nanopores of different sizes ranging from 30 to 5 Å. We study mechanism, speed of nanopores regrowth and structure of healed areas in the wide range of temperatures. We report existence of at least two distinct healing mechanisms, one so called edge attachment where carbons are attached to the edges of graphene sheet and another mechanism that involves atom insertion directly into a sheet of graphene even in the absence of the edges. These findings point a significantly more complicated pathways for graphene annealing. They also provide an important enabling step in development of graphene based devices for numerous nanotechnology applications.

cond-mat.mtrl-sci↗

arXiv2017

Strain and ferroelectric soft mode induced superconductivity in strontium titanate

We investigate the effects of strain on superconductivity with particular reference to SrTiO$_3$. 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 (uniform) strain can be understood.

cond-mat.supr-con↗