arXiv subjects

Avraham Klein

Publications and source records attributed to Avraham Klein.

At least 19 recordsLinked to original sources

arXiv2026

Superconductivity from interband coupling to ferroelectric quantum critical fluctuations in two dimensions

Soft critical fluctuations associated with ferroelectric quantum phase transitions are typically transverse owing to their polar nature. This implies that the conventional density--density electron--phonon coupling to these modes is strongly suppressed, which is puzzling as a variety of materials exhibit enhanced superconductivity in the vicinity of ferroelectricity. An alternative coupling mechanism is an interband ``Stark''-like coupling that connects bands of opposite parity. In the limit where one of the bands is far in energy, these processes generate an effective quadratic (two-phonon) coupling. In contrast, when both bands lie close to the Fermi energy, the resulting interaction develops singular behavior due to the additional gapless electronic states, motivating a detailed study into the dynamics of this effective two-phonon coupling. To this end, we construct the quantum critical Eliashberg theory for a two-dimensional system across a wide range of interband gap magnitudes, near the quantum critical point. We find that the critical temperature $T_c$ is strongly enhanced relative to conventional BCS expectations. In the large-gap limit, the pairing kernel acquires higher-order logarithmic contributions, leading to a parametrically enhanced $T_c$ governed by cubic and quadratic logarithmic terms. In the small-gap regime, the pairing scale exhibits a modified BCS-like form with an enhanced dependence on the inverse square root of the dimensionless coupling constant. The enhancement is due to the dynamics of the two-phonon pairing whose infrared cutoff is set by $T_c$, resulting in a significant enhancement of superconductivity compared to three-dimensional systems, where it is set by the Fermi energy. Our results elucidate the unique dynamical properties of effective two-phonon interactions, and may be relevant to layered compounds like Td-MoTe$_2$ and doped SrTiO$_3$ membranes.

cond-mat.supr-con↗

arXiv2026

Interlayer Charge-Transfer Ferroelectric Fluctuations as a Pairing Mechanism in van der Waals Superconductors

Signatures of unconventional superconductivity have been reported in a wide range of van der Waals (vdW) materials. However, their microscopic origin remains unclear due to competing electronic orders, strong spin-orbit coupling, and structural instabilities in the normal state. Here we investigate the role of interlayer breathing and shear modes in superconducting vdW heterostructures. Contrary to conventional wisdom -- which assumes that weak interlayer bonding and large layer separation suppress electronic coupling to these modes -- we show that the associated charge transfer can generate a substantial pairing interaction. We develop a theory of superconductivity mediated by such interlayer modes and demonstrate that proximity to a ferroelectric or antiferroelectric quantum critical point provides a strong-coupling pairing channel. Within a two-dimensional model with SU(2) symmetry and in-plane isotropy, we find an accidental degeneracy between interlayer triplet states, which can occur even for an $s$-wave in-plane gap. We further show that Josephson coupling between layers, arising from either static magnetism or induced by paramagnetic correlations, can stabilize a time-reversal-symmetry-breaking superconducting state of the $s+i\,s$ type, which couples to magnetization when at least two mirror symmetries are absent. Our results are directly applicable to candidate chiral vdW superconductors such as 4Hb-TaS$_2$ and to sliding ferroelectric metals, exemplified by bilayer MoTe$_2$. More broadly, our work identifies ferroelectric fluctuations as a promising route to unconventional pairing in vdW systems and motivates experimental searches for chiral multicomponent superconductivity.

cond-mat.supr-con↗

arXiv2025

Microscopic origin of the nemato-elastic coupling and dynamics of hybridized collective nematic-phonon excitations

Electronically-driven nematic order breaks the rotational symmetry of a system, e.g., through a Pomeranchuk instability of the Fermi surface, with a concomitant distortion of the lattice. As a result, in a metal, the nematic collective mode interacts with two different sets of gapless excitations: the particle-hole excitations of the metal and the lattice fluctuations that become soft at the induced structural transition, namely, the transverse acoustic phonons. However, the \textit{dynamics} of these hybridized collective modes formed by the transverse acoustic phonons and the metallic electronic-nematic fluctuations has remained largely unexplored. Here we address this problem by developing a formalism in which the nemato-elastic coupling is obtained microscopically from the direct coupling between electrons and transverse acoustic phonons enabled by impurities present in the crystal. We then demonstrate the emergence of hybrid nemato-elastic modes that mix the characteristics of the transverse phonons and of the nematic fluctuations. Near the nematic quantum critical point in a metal, two massless modes emerge with intertwined dynamic behaviors, implying that neither mode dominates the response of the system. We systematically study the non-trivial dependence of these collective modes on the longitudinal and transverse momenta, revealing a rich landscape of underdamped and overdamped modes as the proximity to the quantum critical point and the strength of the electron-phonon coupling are changed. Since dynamics play an important role for determining superconducting instabilities, our results have implications for the study of pairing mediated by electronic nematic fluctuations.

cond-mat.str-el↗

arXiv2025

Intertwined geometries in collective modes of two dimensional Dirac fermions

It is well known that the time-dependent response of a correlated system can be inferred from its spectral correlation functions. As a textbook example, the zero sound collective modes of a Fermi liquid appear as poles of its particle-hole susceptibilities. However, the Fermi liquid's interactions endow these response functions with a complex analytic structure, so that this time/frequency relationship is no longer straightforward. We study how the geometry of this structure is modified by a nontrivial band geometry, via a calculation of the zero sound spectrum of a Dirac cone in two dimensions. We find that the chiral wavefunctions, that encode the band geometry, fundamentally change the analytic structure of the response functions, which encode its Riemannian geometry. As a result, isotropic interactions can give rise to a variety of unconventional zero sound modes, that, due to the geometry of the functions in frequency space, can only be identified via time-resolved probes. These modes are absent in a conventional Fermi liquid with similar interactions, so that these modes can be used as a sensitive probe for the existence of Dirac points in a band-structure.

cond-mat.str-el↗

arXiv2024

The Smearing of Quasi-Particles: Signatures in the Entanglement Entropy of Excited Many-Particle Systems

The entanglement spectrum serves as a powerful tool for probing the structure and dynamics of quantum many-body systems, revealing key information about symmetry, topology, and excitations. While the entanglement entropy (EE) of ground states typically follows an area law, highly excited states obey a volume law, leading to a striking contrast in their scaling behavior. In this paper, we investigate the crossover between these two regimes, focusing on the role of quasi-particles (QPs) in mediating this transition. By analyzing the energy dependence of EE in various many-body systems, we explore how the presence of long-lived QPs influences the entanglement structure of excited states. We present numerical results for spinless fermions, a spin chain near a many-body localization transition, and the Sachdev-Ye-Kitaev (SYK) model, which lacks a conventional QP description. Our findings are complemented by a theoretical model based on Fermi liquid theory, providing insight into the interaction-dependent scaling of EE and its consistency with numerical simulations. We find that a hallmark of QPs is a linear dependence of the eigenstate EE on energy, which breaks down at high energy and in the limit of strong interaction. The slope of this linear dependence reflects the QP weight, which reduces with interaction strength.

cond-mat.quant-gas↗

arXiv2024

Strong Coupling Theory of Superconductivity and Ferroelectric Quantum Criticality in metallic SrTiO$_3$

Superconductivity in doped SrTiO$_3$ has remained an enduring mystery for over 50 years. The material's status as a ``quantum" ferroelectric metal, characterized by a soft polar mode, suggests that quantum criticality could play a pivotal role in the emergence of its superconducting state. We show that the system is amenable to a strong coupling (Eliashberg) pairing analysis, with the dominant coupling to the soft mode being a ``dynamical'' Rashba coupling. We compute the expected $T_c$ for the entire phase diagram, all the way to the quantum critical point and beyond. We demonstrate that the linear coupling is sufficient to obtain a rough approximation of the experimentally measured phase diagram, but that nonlinear coupling terms are crucial in reproducing the finer features in the ordered phase. The primary role of nonlinear terms at the peak of the superconducting dome is to enhance the effective linear coupling induced by the broken order, shifting the dome's maximum into the ordered phase. Our theory quantitatively reproduces the three-dimensional experimental phase diagram in the space of carrier density, distance from the quantum critical point and temperature, and allows us to estimate microscopic parameters from the experimental data.

cond-mat.str-el↗

arXiv2024

Singlet, triplet, and mixed all-to-all pairing states emerging from incoherent fermions

The electron-electron and electron-phonon coupling in complex materials can be more complicated than simple density-density interactions, involving intertwined dynamics of spin, charge, and spatial symmetries. This motivates studying universal models with complex interactions, and studying whether in this case BCS-type singlet pairing is still the ``natural'' fate of the system. To this end, we construct a Yukawa-SYK model with nonlocal couplings in both spin and charge channels. Furthermore, we provide for time-reversal-symmetry breaking dynamics by averaging over the Gaussian Unitary ensemble rather than the Orthogonal ensemble. We find that the ground state of the system can be an orbitally nonlocal superconducting state arising from incoherent fermions with no BCS-like analog. The superconductivity has an equal tendency to triplet and singlet pairing states separated by a non-Fermi liquid phase. We further study the fate of the system within the superconducting phase and find that the expected ground state, away from the critical point, is a mixed singlet/triplet state. Finally, we find that while at $T_c$ the triplet and singlet transitions are dual to one another, below $T_c$ the duality is broken, with the triplet state more susceptible to orbital fluctuations just by virtue of its symmetry. Our results indicate that such fluctuation-induced mixed states may be an inherent feature of strongly correlated materials.

cond-mat.str-el↗

arXiv2023

Multiferroicity in plastically deformed SrTiO$_3$

A major challenge in the development of quantum technologies is to induce additional types of ferroic orders into materials that exhibit other useful quantum properties. Various techniques have been applied to this end, such as elastically straining, doping, or interfacing a compound with other materials. Plastic deformation introduces permanent topological defects and large local strains into a material, which can give rise to qualitatively new functionality. Here we show via local magnetic imaging that plastic deformation induces robust magnetism in the quantum paraelectric SrTiO3, in both conducting and insulating samples. Our analysis indicates that the magnetic order is localized along dislocation walls and coexists with polar order along the walls. The magnetic signals can be switched on and off in a controllable manner with external stress, which demonstrates that plastically deformed SrTiO3 is a quantum multiferroic. These results establish plastic deformation as a versatile platform for quantum materials engineering.

cond-mat.str-el↗

arXiv2022

A theory of criticality for quantum ferroelectric metals

A variety of compounds, for example doped paraelectrics and polar metals, exhibit both ferroelectricity and correlated electronic phenomena such as low-density superconductivity and anomalous transport. Characterizing such properties is tied to understanding the quantum dynamics of inversion symmetry breaking in the presence of itinerant electrons. Here, we present a comprehensive analysis of the normal state properties of a metal near a quantum critical transition to a ferroelectric state, in both two and three dimensions. Starting from a minimal model of electrons coupled to a \emph{transverse} polar phonon via a Rashba-type spin-orbit interaction, we compute the dynamical response of both electrons and phonons. We find that the system can evince both Fermi and non-Fermi liquid phases, as well as enhanced pairing in both singlet and triplet channels. Furthermore, we systematically compute corrections to one-loop theory and find a tendency to quantum order-by-disorder, leading to a phase diagram that can include second order, first order, and finite-momentum phase transitions. Finally, we show that the entire phase diagram can be controlled via application of external strain, either compressive or volume-preserving. Our results provide a map of the dynamical and thermodynamical phase space of quantum ferroelectic metals, which can serve in characterizing existing materials and in seeking applications for quantum technologies.

cond-mat.str-el↗

arXiv2022

How viscous bubbles collapse: topological and symmetry-breaking instabilities in curvature-driven hydrodynamics

The duality between deformations of elastic bodies and non-inertial flows in viscous liquids has been a guiding principle in decades of research. However, this duality is broken when a spheroidal or other doubly-curved liquid film is suddenly forced out of mechanical equilibrium, as occurs e.g. when the pressure inside a liquid bubble drops rapidly due to rupture or controlled evacuation. In such cases the film may evolve through a non-inertial yet geometrically-nonlinear surface dynamics, which has remained largely unexplored. We reveal the driver of such dynamics as temporal variations in the curvature of the evolving surface. Focusing on the prototypical example of a floating bubble that undergoes rapid depressurization, we show that the bubble surface evolves via a topological instability and a subsequent front propagation, whereby a small planar zone nucleates and expands in the spherically-shaped film, bringing about hoop compression and triggering another, symmetry-breaking instability and radial wrinkles that grow in amplitude and invade the flattening film. Our analysis reveals the dynamics as a non-equilibrium branch of "Jellium" physics, whereby a rate-of-change of surface curvature in a viscous film is akin to charge in an electrostatic medium that comprises polarizable and conducting domains. We explain key features underlying recent experiments and highlight a qualitative inconsistency between the prediction of linear stability analysis and the observed "wavelength" of surface wrinkles. Our analysis points to the existence of a nonlinear curvature-driven mechanism for pattern selection in viscous flows.

cond-mat.soft↗

arXiv2021

Defect-induced electronic smectic state at the surface of nematic materials

Due to the intertwining between electronic nematic and elastic degrees of freedom, lattice defects and structural inhomogeneities commonly found in crystals can have a significant impact on the electronic properties of nematic materials. Here, we show that defects commonly present at the surface of crystals generally shift the wave-vector of the nematic instability to a non-zero value, resulting in an incommensurate electronic smectic phase. Such a smectic state onsets above the bulk nematic transition temperature and is localized near the surface of the sample. We argue that this effect may explain not only recent observations of a modulated nematic phase in iron-based superconductors, but also several previous puzzling experiments that reported signatures consistent with nematic order before the onset of a bulk structural distortion.

cond-mat.supr-con↗

arXiv2021

Synergetic ferroelectricity and superconductivity in zero-density Dirac semimetals near quantum criticality

We study superconductivity in a three-dimensional zero-density Dirac semimetal in proximity to a ferroelectric quantum critical point. We find that the interplay of criticality, inversion-symmetry breaking, and Dirac dispersion gives rise to a robust superconducting state at the charge-neutrality point, where no Fermi surface is present. Using Eliashberg theory, we show that the ferroelectric quantum critical point is unstable against the formation of a ferroelectric density wave (FDW), whose fluctuations, in turn, lead to a first-order superconducting transition. Surprisingly, long-range superconducting and FDW orders are found to cooperate with each other, in contrast to the more usual scenario of phase competition. Therefore, we suggest that driving charge neutral Dirac materials, e.g., Pb$_x$Sn$_{1-x}$Te, through a ferroelectric quantum critical point may lead to superconductivity intertwined with FDW order.

cond-mat.supr-con↗

arXiv2021

The dynamical exponent of a quantum critical itinerant ferromagnet: a Monte Carlo study

We consider the effect of the coupling between 2D quantum rotors near an XY ferromagnetic quantum critical point and spins of itinerant fermions. We analyze how this coupling affects the dynamics of rotors and the self-energy of fermions.A common belief is that near a $q=0$ ferromagnetic transition, fermions induce an $\Omega/q$ Landau damping of rotors (i.e., the dynamical critical exponent is $z=3$) and Landau overdamped rotors give rise to non-Fermi liquid fermionic self-energy $\Sigma\propto \omega^{2/3}$. This behavior has been confirmed in previous quantum Monte Carlo (QMC) studies.Here we show that for the XY case the behavior is different.We report the results of large scale quantum Monte Carlo simulations,which show that at small frequencies $z=2$ and $\Sigma\propto \omega^{1/2}$. We argue that the new behavior is associated with the fact that a fermionic spin is by itself not a conserved quantity due to spin-spin coupling to rotors, and a combination of self-energy and vertex corrections replaces $1/q$ in the Landau damping by a constant. We discuss the implication of these results to experiments.

cond-mat.str-el↗

arXiv2021

Monte Carlo study of the pseudogap and superconductivity emerging from quantum magnetic fluctuations

The origin of the pseudogap behavior, found in many high-$T_c$ superconductors, remains one of the greatest puzzles in condensed matter physics. One possible mechanism is fermionic incoherence, which near a quantum critical point allows pair formation but suppresses superconductivity. Employing quantum Monte Carlo simulations of a model of itinerant fermions coupled to ferromagnetic spin fluctuations, represented by a quantum rotor, we report numerical evidence of pseudogap behavior, emerging from pairing fluctuations in a quantum-critical non-Fermi liquid. Specifically, we observe enhanced pairing fluctuations and a partial gap opening in the fermionic spectrum. However, the system remains non-superconducting until reaching a much lower temperature. In the pseudogap regime the system displays a "gap-filling" rather than "gap-closing" behavior, consistent with experimental observations. Our results provide the first unambiguous lattice model realization of a pseudogap state in a strongly correlated system, driven by superconducting fluctuations.

cond-mat.str-el↗

arXiv2020

Identification of non-Fermi liquid fermionic self-energy from quantum Monte Carlo data

Quantum Monte Carlo (QMC) simulations of correlated electron systems provide unbiased information about system behavior at a quantum critical point (QCP) and can verify or disprove the existing theories of non-Fermi liquid (NFL) behavior at a QCP. However, simulations are carried out at a finite temperature, where quantum-critical features are masked by finite temperature effects. Here we present a theoretical framework within which it is possible to separate thermal and quantum effects and extract the information about NFL physics at $T=0$. We demonstrate our method for a specific example of 2D fermions near a Ising-ferromagnetic QCP. We show that one can extract from QMC data the zero-temperature form of fermionic self-energy $\Sigma (\omega)$ even though the leading contribution to the self-energy comes from thermal effects. We find that the frequency dependence of $\Sigma (\omega)$ agrees well with the analytic form obtained within the Eliashberg theory of dynamical quantum criticality, and obeys $\omega^{2/3}$ scaling at low frequencies. Our results open up an avenue for QMC studies of quantum-critical metals.

cond-mat.str-el↗

arXiv2020

Normal state properties of quantum critical metals at finite temperature

We study the effects of finite temperature on normal state properties of a metal near a quantum critical point to an antiferromagnetic or Ising-nematic state. At $T = 0$ bosonic and fermionic self-energies are traditionally computed within Eliashberg theory and obey scaling relations with characteristic power-laws. Quantum Monte Carlo (QMC) simulations have shown strong systematic deviations from these predictions, casting doubt on the validity of the theoretical analysis. We extend Eliashberg theory to finite $T$ and argue that for the $T$ range accessible in the QMC simulations, the scaling forms for both fermionic and bosonic self energies are quite different from those at $T = 0$. We compare finite $T$ results with QMC data and find good agreement for both systems. This, we argue, resolves the key apparent contradiction between the theory and the QMC simulations.

cond-mat.str-el↗

arXiv2019

Hidden and mirage collective modes in two dimensional Fermi liquids

We show that a two-dimensional (2D) isotropic Fermi liquid harbors two new types of collective modes, driven by quantum fluctuations, in addition to conventional zero sound: "hidden" and "mirage" modes. The hidden modes occur for relatively weak attractive interaction both in the charge and spin channels with any angular momentum $l$. Instead of being conventional damped resonances within the particle-hole continuum, the hidden modes propagate at velocities larger than the Fermi velocity and have infinitesimally small damping in the clean limit, but are invisible to spectroscopic probes. The mirage modes are also propagating modes outside the particle-hole continuum that occur for sufficiently strong repulsion interaction in channels with $l\geq 1$. They do give rise to peaks in spectroscopic probes, but are not true poles of the dynamical susceptibility. We argue that both hidden and mirage modes occur due to a non-trivial topological structure of the Riemann surface, defined by the dynamical susceptibility. The hidden modes reside below a branch cut that glues two sheets of the Riemann surface, while the mirage modes reside on an unphysical sheet of the Riemann surface. We show that both types of modes give rise to distinct features in time dynamics of a 2D Fermi liquid that can be measured in pump-probe experiments.

cond-mat.str-el↗

arXiv2019

Collective modes near a Pomeranchuk instability

We consider collective excitations of a Fermi liquid. For each value of the angular momentum $l$, we study the evolution of longitudinal and transverse collective modes in the charge (c) and spin (s) channels with the Landau parameter $F_l^{c(s)}$, starting from positive $F_l^{c(s)}$ and all the way to the Pomeranchuk transition at $F_l^{c(s)} = -1$. In each case, we identify a critical zero-sound mode, whose velocity vanishes at the Pomeranchuk instability. For $F_l^{c(s)} < -1$, this mode is located in the upper frequency half-plane, which signals an instability of the ground state. In a clean Fermi liquid the critical mode may be either purely relaxational or almost propagating, depending on the parity of $l$ and on whether the response function is longitudinal or transverse. These differences lead to qualitatively different types of time evolution of the order parameter following an initial perturbation. A special situation occurs for the $l = 1$ order parameter that coincides with the spin or charge current. In this case the residue of the critical mode vanishes at the Pomeranchuk transition. However, the critical mode can be identified at any distance from the transition, and is still located in the upper frequency half-plane for $F_1^{c(s)} < -1$. The only peculiarity of the charge/spin current order parameter is that its time evolution occurs on longer scales than for other order parameters. We also analyze collective modes away from the critical point, and find that the modes evolve with $F_l^{c(s)}$ on a multi-sheet Riemann surface. For certain intervals of $F_l^{c(s)}$, the modes either move to an unphysical Riemann sheet or stay on the physical sheet but away from the real frequency axis. In that case, the modes do not give rise to peaks in the imaginary parts of the corresponding susceptiblities.

cond-mat.str-el↗