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G. S. Japaridze

Publications and source records attributed to G. S. Japaridze.

At least 19 recordsLinked to original sources
arXiv2025

Comment on paper: Evidence for Dirac flat band superconductivity enabled by quantum geometry, Nature 614, 440 (2023)

We demonstrate that an absolutely flat band retains the superconducting state at $T_c\to 0$. When $T_c>0$ the flat band disappears, since it must be modified by the superconducting state. Thus, a number of the results on ultra-strong coupling superconductivity in flat band considered in the article ("Evidence for Dirac flat band superconductivity enabled by quantum geometry", Nature 614, 440 (2023)) were predicted and explained many years ago. One has to take into account that at $T_c>0$ the flat band distorts, becoming tilted. As a result, the charge carriers' velocity $v_F\propto T_c$ becomes finite, rather than being extremely slow, as it is stated in the article. Thus, the statement "the charge carriers' group velocity $v_ F$ is extremely slow" is incorrect and leads the authors to the conceptional misunderstanding, confusing the reader.

cond-mat.supr-con↗
arXiv2023

Comment on "Shot noise in a strange metal"

The recent paper (Science 382, 907 (2023)) is devoted to measurements of shot noise to probe excitations in nanowires of the heavy fermion (HF) metal $\rm YbRh_2Si_2$. The authors observed that shot noise is strongly suppressed, and claim that the suppression cannot be attributed to either electron-phonon or electron-electron interactions in a Fermi liquid. Their observation suggests that the current is not carried by well-defined quasiparticles in the $\rm YbRh_2Si_2$, and calls for similar research into other strange metals. In this comment, we show that it is unlikely that the affected carriers in bulk $\rm YbRh_2Si_2$ would have undergone any fragmentation.

cond-mat.str-el↗
arXiv2022

Comment on "Stranger than metals"

P. W. Phillips, N. E. Hussey, P. Abbamonte (Review Article, 8 July 2022, eabh4273) consider heavy fermion (HF) metals and high-$T_c$ superconductors naming them strange metals. They analyze such features of strange metals as quantum criticality, Planckian dissipation and recently observed fundamental link between the high-$T_c$ superconductivity and strange metals, and conclude that these problems can be possibly resolved within the framework of theories based on gravity, etc. In this comment we discuss that this claim is not correct and the successful description of the quantum criticality, Planckian dissipation and recently observed fundamental link between the high-$T_c$ superconductivity and strange metals has been given within the framework of the fermion condensation theory.

cond-mat.str-el↗
arXiv2021

Effect of superconductivity on the shape of flat bands

For the first time, basing both on experimental facts and our theoretical consideration, we show that Fermi systems with flat bands should be tuned with the superconducting state. Experimental measurements on magic-angle twisted bilayer graphene of the Fermi velocity $V_F$ as a function of the temperature $T_c$ of superconduction phase transition have revealed $V_F\propto T_c\propto 1/N_s(0)$, where $N_s(0)$ is the density of states at the Fermi level. We show that the high-$T_c$ compounds $\rm Bi_2Sr_2CaCu_2O_{8+x}$ exhibit the same behavior. Such observation is a challenge to theories of high-$T_c$ superconductivity, since $V_F$ is negatively correlated with $T_c$, for $T_c\propto 1/V_F\propto N_s(0)$. We show that the theoretical idea of forming flat bands in strongly correlated Fermi systems can explain this behavior and other experimental data collected on both $\rm Bi_2Sr_2CaCu_2O_{8+x}$ and twisted bilayer graphene. Our findings place stringent constraints on theories describing the nature of high-$T_c$ superconductivity and the deformation of flat band by the superconducting phase transition.

cond-mat.supr-con↗
arXiv2021

Ultra spin liquid in $\rm Lu_3Cu_2Sb_3O_{14}$

We analyze measurements of $C_{\rm mag}/T$, specific heat $C_{\rm mag}$ divided by temperature $T$, of the recently observed ultra spin liquid. The measurements are carried out in magnetic fields on the triangular lattice compound $\rm Lu_3Cu_2Sb_3O_{14}$. We show that the obtained heat capacity $C_{\rm mag}/T$ formed by ultra spin liquid as a function of temperature $T$ versus magnetic field $B$ behaves very similar to the electronic specific heat $C_{el}/T$ of the heavy fermion (HF) metal $\rm YbRh_2Si_2$ and that of the quantum magnet $\rm ZnCu_3(OH)_6Cl_2$. We further demonstrate that the spinon effective mass $M^*\propto C_{\rm mag}/T$ exhibits the universal scaling coinciding with that observed in HF metals and in $\rm ZnCu_3(OH)_6Cl_2$. Based on these observations we conclude that a strongly correlated spin liquid determines the thermodynamic properties of the ultra spin liquid of $\rm Lu_3Cu_2Sb_3O_{14}$.

cond-mat.str-el↗
arXiv2020

Universal $T/B$ scaling behavior of heavy fermion compounds

In our mini-review, we address manifestations of $T/B$ scaling behavior of heavy-fermion (HF) compounds, where $T$ and $B$ are respectively temperature and magnetic field. Using experimental data and the fermion condensation theory, we show that this scaling behavior is typical of HF compounds including HF metals, quasicrystals, and quantum spin liquids. We demonstrate that such scaling behavior holds down to the lowest temperature and field values, so that $T/B$ varies in a wide range, provided the HF compound is located near the topological fermion condensation quantum phase transition (FCQPT). Due to the topological properties of FCQPT, the effective mass $M^*$ exhibits a universal behavior, and diverges as $T$ goes to zero. Such a behavior of $M^*$ has important technological applications. We also explain how to extract the universal scaling behavior from experimental data collected on different heavy-fermion compounds. As an example, we consider the HF metal $\rm YbCo_2Ge_4$, and show that its scaling behavior is violated at low temperatures. Our results obtained show good agreement with experimental facts.

cond-mat.str-el↗
arXiv2019

Fermion condensation, $T$-linear resistivity and Planckian limit

We explain recent challenging experimental observations of universal scattering rate related to the linear-temperature resistivity exhibited by a large corps of both strongly correlated Fermi systems and conventional metals. We show that the observed scattering rate in strongly correlated Fermi systems like heavy fermion metals and high-$T_c$ superconductors stems from phonon contribution that induce the linear temperature dependence of a resistivity. The above phonons are formed by the presence of flat band, resulting from the topological fermion condensation quantum phase transition (FCQPT). We emphasize that so - called Planckian limit, widely used to explain the above universal scattering rate, may occur accidentally as in conventional metals its experimental manifestations (e.g. scattering rate at room and higher temperatures) are indistinguishable from those generated by the well-know phonons being the classic lattice excitations. Our results are in good agreement with experimental data and show convincingly that the topological FCQPT can be viewed as the universal agent explaining the very unusual physics of strongly correlated Fermi systems.

cond-mat.str-el↗
arXiv2018

Revealing quantum spin liquid in the herbertsmithite $\rm ZnCu_{3}(OH)_6Cl_{2}$

Based on experimental data and our theoretical analysis, we provide a strategy for unambiguous establishing of gapless quantum spin liquid state (QSL) in herbertsmithite and other materials. To clarify the nature of QSL, we recommend measurements of heat transport, low-energy inelastic neutron scattering and optical conductivity under the application of external magnetic field at low temperatures. We also suggest that artificially introduced inhomogeneity into $\rm ZnCu_{3}(OH)_6Cl_2$ can stabilize QSL, and serves as a test elucidating the contribution coming from impurities. We predict the results of these measurements in the case of gapless QSL.

cond-mat.str-el↗
arXiv2018

Universal behavior of quantum spin liquid and optical conductivity in the insulator herbertsmithite

We analyze optical conductivity with the goal to demonstrate experimental manifestation of a new state of matter, the so-called fermion condensate. Fermion condensates are realized in quantum spin liquids, exhibiting typical behavior of heavy fermion metals. Measurements of the low-frequency optical conductivity collected on the geometrically frustrated insulator herbertsmithite provide important experimental evidence of the nature of its quantum spin liquid composed of spinons. To analyze recent measurements of the herbertsmithite optical conductivity at different temperatures, we employ a model of strongly correlated quantum spin liquid located near the fermion condensation phase transition. Our theoretical analysis of the optical conductivity allows us to expose the physical mechanism of its temperature dependence. We also predict a dependence of the optical conductivity on a magnetic field. We consider an experimental manifestation (optical conductivity) of a new state of matter (so-called fermion condensate) realized in quantum spin liquids, for, in many ways, they exhibit typical behavior of heavy-fermion metals. Measurements of the low-frequency optical conductivity collected on the geometrically frustrated insulator herbertsmithite produce important experimental evidence of the nature of its quantum spin liquid composed of spinons. To analyze recent measurements of the herbertsmithite optical conductivity at different temperatures, we employ a model of strongly correlated quantum spin liquid located near the fermion condensation phase transition. Our theoretical analysis of the optical conductivity allows us to reveal the physical mechanism of its temperature dependence. We also predict a dependence of the optical conductivity on a magnetic field.

cond-mat.str-el↗
arXiv2018

New state of matter: heavy-fermion systems, quantum spin liquids, quasicrystals, cold gases, and high temperature superconductors

We report on a new state of matter manifested by strongly correlated Fermi systems including various heavy-fermion (HF) metals, two-dimensional quantum liquids such as $\rm ^3He$ films, certain quasicrystals, and systems behaving as quantum spin liquids. Generically, these systems can be viewed as HF systems or HF compounds, in that they exhibit typical behavior of HF metals. At zero temperature, such systems can experience a so-called fermion-condensation quantum phase transition (FCQPT). Combining analytical considerations with arguments based entirely on experimental grounds we argue and demonstrate that the class of HF systems is characterized by universal scaling behavior of their thermodynamic, transport, and relaxation properties. That is, the quantum physics of different HF compounds is found to be universal, emerging irrespective of the individual details of their symmetries, interactions, and microscopic structure. This observed universal behavior reveals the existence of a new state of matter manifest in HF compounds. We propose a simple, realistic model to study the appearance of flat bands in two-dimensional ensembles of ultracold fermionic atoms, interacting with coherent resonant light. It is shown that signatures of these flat bands may be found in peculiarities in their thermodynamic and spectroscopic properties. We also show that the FCQPT, in generating flat bands and altering Fermi surface topology, is an essential progenitor of the exotic behavior of the overdoped high-temperature superconductors represented by $\rm La_{2-x}Sr_xCuO_4$, whose superconductivity differs from that predicted by the classical Bardeen-Cooper-Schrieffer theory. The theoretical results presented are in good agreement with recent experimental observations, closing the colossal gap between these empirical findings and Bardeen-Cooper-Schrieffer-like theories.

cond-mat.str-el↗
arXiv2018

Flat bands and the physics of strongly correlated Fermi systems

Some materials can have the dispersionless parts in their electronic spectra. These parts are usually called flat bands and generate the corps of unusual physical properties of such materials. These flat bands are induced by the condensation of fermionic quasiparticles, being very similar to the Bose condensation. The difference is that fermions to condense, the Fermi surface should change its topology, leading to violation of time-reversal (T) and particle-hole (C) symmetries. Thus, the famous Landau theory of Fermi liquids does not work for the systems with fermion condensate (FC) so that several experimentally observable anomalies have not been explained so far. Here we use FC approach to explain recent observations of the asymmetric tunneling conductivity in heavy-fermion compounds and graphene and its restoration in magnetic fields, as well as the violation of Leggett theorem, recently observed experimentally in overdoped cuprates, and recent observation of the challenging universal scaling connecting linear-$T$-dependent resistivity to the superconducting superfluid density.

cond-mat.str-el↗
arXiv2017

The influence of topological phase transition on the superfluid density of overdoped copper oxides

We show that a topological quantum phase transition, generating flat bands and altering Fermi surface topology, is a primary reason for the exotic behavior of the overdoped high-temperature superconductors represented by $\rm La_{2-x}Sr_xCuO_4$, whose superconductivity features differ from what is described by the classical Bardeen-Cooper-Schrieffer theory [J.I. Bozović, X. He, J. Wu, and A. T. Bollinger, Nature 536, 309 (2016)]. We demonstrate that 1) at temperature $T=0$, the superfluid density $n_s$ turns out to be considerably smaller than the total electron density; 2) the critical temperature $T_c$ is controlled by $n_s$ rather than by doping, and is a linear function of the $n_s$; 3) at $T>T_c$ the resistivity $ρ(T)$ varies linearly with temperature, $ρ(T)\propto αT$, where $α$ diminishes with $T_c\to 0$, while in the normal overdoped (non superconducting) region with $T_c=0$, the resistivity becomes $ρ(T)\propto T^2$. The theoretical results presented are in good agreement with recent experimental observations, closing the colossal gap between these empirical findings and Bardeen-Cooper-Schrieffer-like theories.

cond-mat.supr-con↗
arXiv2016

Strongly correlated Fermi systems as a new state of matter

The aim of this review paper is to expose a new state of matter exhibited by strongly correlated Fermi systems represented by various heavy-fermion (HF) metals, two-dimensional liquids like $\rm ^3He$, compounds with quantum spin liquids, quasicrystals, and systems with one-dimensional quantum spin liquid. We name these various systems HF compounds, since they exhibit the behavior typical of HF metals. In HF compounds at zero temperature the unique phase transition, dubbed throughout as the fermion condensation quantum phase transition (FCQPT) can occur; this FCQPT creates flat bands which in turn lead to the specific state, known as the fermion condensate. Unlimited increase of the effective mass of quasiparticles signifies FCQPT; these quasiparticles determine the thermodynamic, transport and relaxation properties of HF compounds. Our discussion of numerous salient experimental data within the framework of FCQPT resolves the mystery of the new state of matter. Thus, FCQPT and the fermion condensation can be considered as the universal reason for the non-Fermi liquid behavior observed in various HF compounds. We show analytically and using arguments based completely on the experimental grounds that these systems exhibit universal scaling behavior of their thermodynamic, transport and relaxation properties. Therefore, the quantum physics of different HF compounds is universal, and emerges regardless of the microscopic structure of the compounds. This uniform behavior allows us to view it as the main characteristic of a new state of matter exhibited by HF compounds.

cond-mat.str-el↗
arXiv2016

Comment on "Correlated impurities and intrinsic spin liquid physics in the kagome material Herbertsmithite" (T. H. Han et al., Phys. Rev. B 94, 060409(R) (2016))

Recently Han et al. have provided an analysis of the observed behavior of $\rm ZnCu_{3}(OH)_6Cl_2$ Herbertsmithite based on a separation of the contributions to its thermodynamic properties due to impurities from those due to the kagome lattice. The authors developed an impurity model to account for the experimental data and claimed that it is compatible with the presence of a small spin gap in the kagome layers. We argue that the model they advocate is problematic, conflicting with the intrinsic properties of $\rm ZnCu_{3}(OH)_6Cl_2$ as observed and explained in recent experimental and theoretical investigations. We show that the existence of the gap in the kagome layers is not in itself of a vital importance, for it does not govern the thermodynamic and transport properties of $\rm ZnCu_3(OH)_6Cl_2$. Measurements of heat transport in magnetic fields could clarify the quantum-critical features of spin-liquid physics of $\rm ZnCu_{3}(OH)_6Cl_2$.

cond-mat.str-el↗
arXiv2016

Scaling behavior of the thermopower of the archetypical heavy-fermion metal $\rm{YbRh_2Si_2}$

We reveal and explain a scaling behavior of the thermopower $S/T$ exhibiting by the archetypical heavy-fermion (HF) metal $\rm{YbRh_2Si_2}$ under the application of magnetic field $B$ at temperatures $T$. We show that the same scaling is demonstrated by such different HF compounds as $β$-${\rm YbAlB_4}$ and the strongly correlated layered cobalt oxide $\rm [BiBa_{0.66}K_{0.36}O_{2}]CoO_{2}$. Using $\rm{YbRh_2Si_2}$ as an example, we demonstrate that the scaling behavior of $S/T$ is violated at the antiferromagnetic phase transition, while both the residual resistivity $ρ_0$ and the density of states $N$ experience jumps at the phase transition, making the thermopower experience two jumps and change its sign. Our elucidation is based on flattening of the single-particle spectrum that profoundly affects $ρ_0$ and $N$. To depict the main features of the $S/T$ behavior, we construct the $T-B$ schematic phase diagram of $\rm{YbRh_2Si_2}$. Our calculated $S/T$ for the HF compounds are in good agreement with experimental facts and support our observations.

cond-mat.str-el↗
arXiv2014

Comment on "Topological excitations and the dynamic structure factor of spin liquids on the kagome lattice" (Punk, M., Chowdhury, D. & Sachdev, S. Nature Physics 10, 289-293 (2014))

The authors of a recent paper evidently take the view that the whole of progress made toward a theoretical understanding of the physics of quantum spin liquids (QSL) is associated with models of the kind proposed and applied in their present work. As motivation for this work, they observe that in contrast to existing theoretical models of both gapped and gapless spin liquids, which give rise to sharp dispersive features in the dynamic structure factor, the measured dynamic structure factor reveals an excitation continuum that is remarkably flat as a function of frequency. They go on to assert that "so far, the only theoretical model for a spin liquid state on the kagome lattice which naturally gives rise to a flat excitation band at low energies consists of the $Z_2$ spin liquids". Here we point out that there already exists a different and demonstrably successful approach to the QSL problem that does naturally feature a flat band.

cond-mat.str-el↗
arXiv2014

General properties of phase diagrams of heavy-fermion metals

We study the temperature-magnetic field T-B phase diagrams of heavy fermion (HF) metals, and show that at sufficiently high temperatures outside the ordered phase the crossover temperature T*(B), regarded as the energy scale, follows a linear B-dependence, crossing the origin of the T-B phase diagram. This behavior of T*(B) constitutes the general property, and is formed by the presence of fermion condensation quantum phase transition hidden within the ordered phase. Our result is in good agreement with the experimental T-B phase diagram of the HF metals YbRh2Si2, Yb(Rh{0.93}Co{0.07})2Si2, and Yb(Rh{0.94}Ir{0.06})2Si2. To support our observations, we analyze the isothermal magnetization M, and demonstrate that dM/dT exhibits a universal temperature behavior over magnetic field scaling. The obtained results are in good agreement with the corresponding data collected on YbRh2Si2 as a function of magnetic field at different temperatures under hydrostatic pressure.

cond-mat.str-el↗