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Vasily R. Shaginyan
Professor, Leading Researcher , Petersburg Nuclear Physics Institute
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Phys. Rev. B 2011 Aug; 84
Thermodynamic properties of the kagome lattice in herbertsmithite
Strongly correlated Fermi systems are among the most intriguing and fundamental systems in physics, whose realization in some compounds is still to be discovered. We show that ZnCu3(OH)6Cl2 can be viewed as a strongly correlated Fermi system whose lo... expand abstractw-temperature thermodynamics in magnetic fields is defined by a Fermi quantum spin liquid. Our calculations of its thermodynamic properties are in good agreement with recent experimental facts and allow us to reveal their scaling behavior which strongly resembles that observed in heavy-fermion metals and two-dimensional 3He. collapse abstract
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Thermodynamic Properties of Kagome Lattice in ZnCu_3(OH)_6Cl_2 Herbertsmithite
Strongly correlated Fermi systems are among the most intriguing and fundamental systems in physics, whose realization in some compounds is still to be discovered. We show that herbertsmithite ZnCu_3(OH)_6Cl_2 can be viewed as a strongly correlated ... expand abstractFermi system whose low temperature thermodynamic in magnetic fields is defined by a quantum critical spin liquid. Our calculations of its thermodynamic properties are in good agreement with recent experimental facts and allow us to reveal their scaling behavior which strongly resembles that observed in HF metals and 2D 3He. collapse abstract
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EPL 93 (2011) 17008
High magnetic fields thermodynamics of heavy fermion metal YbRh2Si2
We perform comprehensive theoretical analysis of high magnetic field behavior of the heavy-fermion (HF) compound YbRh2Si2. At low magnetic fields B, YbRh2Si2 has a quantum critical point related to the suppression of antiferromagnetic ordering at a... expand abstract critical magnetic field. Our calculations of the thermodynamic properties of YbRh2Si2 in wide magnetic field range allow us to straddle a possible metamagnetic transition region and probe the properties of both low-field HF liquid and high-field fully polarized one. Namely, high magnetic fields B\sim B^*\sim 10 T fully polarize corresponding quasiparticle band generating Landau Fermi liquid (LFL) state and suppressing HF (actually NFL) one, while at elevating temperatures both HF state and corresponding NFL properties are restored. Our calculations are in good agreement with experimental facts and show that the fermion condensation quantum phase transition is indeed responsible for the observed NFL behavior and quasiparticles survive both high temperatures and high magnetic fields. collapse abstract
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JETP Letters 2010 Dec; 92(8)
Second wind of the Dulong-Petit law at a quantum critical point
Renewed interest in 3He physics has been stimulated by experimental observation of non-Fermi-liquid behavior of dense 3He films at low temperatures. Abnormal behavior of the specific heat C(T) of two-dimensional liquid 3He is demonstrated in the occu... expand abstractrrence of a T-independent term in C(T). To uncover the origin of this phenomenon, we have considered the group velocity of transverse zero sound propagating in a strongly correlated Fermi liquid. For the first time, it is shown that if two-dimensional liquid 3He is located in the vicinity of the quantum critical point associated with a divergent quasiparticle effective mass, the group velocity depends strongly on temperature and vanishes as T is lowered toward zero. The predicted vigorous dependence of the group velocity can be detected in experimental measurements on liquid 3He films. We have demonstrated that the contribution to the specific heat coming from the boson part of the free energy due to the transverse zero-sound mode follows the Dulong-Petit Law. In the case of two-dimensional liquid 3He, the specific heat becomes independent of temperature at some characteristic temperature of a few millikelvins. collapse abstract
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Baryon Asymmetry of Universe as Manifestation of Quantum Phase Transition
A novel mechanism for explaining the matter-antimatter asymmetry of the universe is considered. The mechanism does not require the baryon number violating interactions or CP violation at a microscopic level. Our analysis of the matter-antimatter as... expand abstractymmetry is in the context of salient experimental results obtained in the condensed matter physics. collapse abstract
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Physics Reports 492, 31 (2010)
Scaling Behavior of Heavy Fermion Metals
Strongly correlated Fermi systems are fundamental systems in physics that are best studied experimentally, which until very recently have lacked theoretical explanations. This review discusses the construction of a theory and the analysis of phenom... expand abstractena occurring in strongly correlated Fermi systems such as heavy-fermion (HF) metals and two-dimensional (2D) Fermi systems. It is shown that the basic properties and the scaling behavior of HF metals can be described within the framework of a fermion condensation quantum phase transition (FCQPT) and extended quasiparticle paradigm that allow us to explain the non-Fermi liquid behavior observed in strongly correlated Fermi systems. In contrast to the Landau paradigm stating that the quasiparticle effective mass is a constant, the effective mass of new quasiparticles strongly depends on temperature, magnetic field, pressure, and other parameters. Having analyzed collected facts on strongly correlated Fermi systems with quite different microscopic nature, we find these to exhibit the same non-Fermi liquid behavior at FCQPT. We show both analytically and using arguments based entirely on the experimental grounds that the data collected on very different strongly correlated Fermi systems have a universal scaling behavior, and materials with strongly correlated fermions can unexpectedly be uniform in their diversity. Our analysis of strongly correlated systems such as HF metals and 2D Fermi systems is in the context of salient experimental results. Our calculations of the non-Fermi liquid behavior, the scales and thermodynamic, relaxation and transport properties are in good agreement with experimental facts. collapse abstract
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JETP Lett. 90, 47 (2009)
Energy scales and the non-Fermi liquid behavior in YbRh2Si2
Multiple energy scales are detected in measurements of the thermodynamic and transport properties in heavy fermion metals. We demonstrate that the experimental data on the energy scales can be well described by the scaling behavior of the effective... expand abstract mass at the fermion condensation quantum phase transition, and show that the dependence of the effective mass on temperature and applied magnetic fields gives rise to the non-Fermi liquid behavior. Our analysis is placed in the context of recent salient experimental results. Our calculations of the non-Fermi liquid behavior, of the scales and thermodynamic and transport properties are in good agreement with the heat capacity, magnetization, longitudinal magnetoresistance and magnetic entropy obtained in remarkable measurements on the heavy fermion metal YbRh2Si2. collapse abstract
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Ukr. J. Phys. 55, 65 (2010)
Fermion condensation: a strange idea successfully explaining behavior of numerous objects in Nature
Strongly correlated Fermi systems are among the most intriguing, best experimentally studied and fundamental systems in physics. These are, however, in defiance of theoretical understanding. The ideas based on the concepts like Kondo lattice and in... expand abstractvolving quantum and thermal fluctuations at a quantum critical point have been used to explain the unusual physics. Alas, being suggested to describe one property, these approaches fail to explain the others. This means a real crisis in theory suggesting that there is a hidden fundamental law of nature, which remains to be recognized. A theory of fermion condensation quantum phase transition, preserving the extended quasiparticles paradigm and intimately related to unlimited growth of the effective mass as a function of temperature, magnetic field etc, is capable to resolve the problem. We discuss the construction of the theory and show that it delivers theoretical explanations of vast majority of experimental results in strongly correlated systems such as heavy-fermion metals and quasi-two-dimensional Fermi systems. collapse abstract
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Phys. Lett. A 374 (2010) 659
Behavior of the antiferromagnetic phase transition near the fermion condensation quantum phase transition in YbRh2Si2
Low-temperature specific-heat measurements on YbRh2Si2 at the second order antiferromagnetic (AF) phase transition reveal a sharp peak at T_N=72 mK. The corresponding critical exponent alpha turns out to be alpha=0.38, which differs significantly f... expand abstractrom that obtained within the framework of the fluctuation theory of second order phase transitions based on the scale invariance, where alpha=0.1. We show that under the application of magnetic field the curve of the second order AF phase transitions passes into a curve of the first order ones at the tricritical point leading to a violation of the critical universality of the fluctuation theory. This change of the phase transition is generated by the fermion condensation quantum phase transition. Near the tricritical point the Landau theory of second order phase transitions is applicable and gives alpha=1/2. We demonstrate that this value of alpha is in good agreement with the specific-heat measurements. collapse abstract
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Phys. Lett. A 338, 393 (2005).
Quasiparticles and order parameter near quantum phase transition in heavy fermion metals
It is shown that the Landau paradigm based upon both the quasiparticle concept and the notion of the order parameter is valid and can be used to explain the anomalous behavior of the heavy fermion metals near quantum critical points. The understand... expand abstracting of this phenomenon has been problematic largely because of the absence of theoretical guidance. Exploiting this paradigm and the fermion condensation quantum phase transition, we investigate the anomalous behavior of the heavy electron liquid near its critical point at different temperatures and applied magnetic fields. We show that this anomalous behavior is universal and can be used to capture the essential aspects of recent experiments on heavy-fermion metals at low temperatures. collapse abstract
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JETP Lett. 81, 222 (2005).
Dissymmetrical tunnelling in heavy fermion metals
A tunnelling conductivity between a heavy fermion metal and a simple metallic point is considered. We show that at low temperatures this conductivity can be noticeably dissymmetrical with respect to the change of voltage bias. The dissymmetry can b... expand abstracte observed in experiments on the heavy fermion metals whose electronic system has undergone the fermion condensation quantum phase transition. collapse abstract
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Physica B 378-380, 127 (2006)
Quasiparticles near quantum phase transition in heavy fermion metals
We have shown that the Landau paradigm based upon both the quasiparticle concept and the notion of the order parameter can be used to explain the anomalous behavior of heavy fermion metals. Exploiting this paradigm and the fermion condensation quan... expand abstracttum phase transition (FCQPT) we show that this anomalous behavior is universal and can be used to capture the essential aspects of recent experiments on the heavy-fermion metals at low temperatures. Behind FCQPT a tunneling conductivity between a heavy fermion metal and a simple metallic point can be noticeably dissymmetrical with respect to the change of voltage bias. We show that at T=0 and beyond FCQPT the Hall coefficient undergoes a jump upon magnetic-field tuning HF metals. collapse abstract
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JETP Letters 82, 215 (2005).
Hall coefficient in heavy fermion metals
Experimental studies of the antiferromagnetic (AF) heavy fermion metal $\rm YbRh_2Si_2$ in a magnetic field $B$ indicate the presence of a jump in the Hall coefficient at a magnetic-field tuned quantum state in the zero temperature limit. This quan... expand abstracttum state occurs at $B\geq B_{c0}$ and induces the jump even though the change of the magnetic field at $B=B_{c0}$ is infinitesimal. We investigate this by using the model of heavy electron liquid with the fermion condensate. Within this model the jump takes place when the magnetic field reaches the critical value $B_{c0}$ at which the ordering temperature $T_N(B=B_{c0})$ of the AF transition vanishes. We show that at $B\to B_{c0}$, this second order AF phase transition becomes the first order one, making the corresponding quantum and thermal critical fluctuations vanish at the jump. At $T\to0$ and $B=B_{c0}$, the Gr\"uneisen ratio as a function of temperature $T$ diverges. We demonstrate that both the divergence and the jump are determined by the specific low temperature behavior of the entropy $S(T)\propto S_0+a\sqrt{T}+bT$ with $S_0$, $a$ and $b$ are temperature independent constants. collapse abstract
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JETP Lett. 77, 178 2003.
Magnetoresistance of Highly Correlated Electron Liquid
The behavior in magnetic fields of a highly correlated electron liquid approaching the fermion condensation quantum phase transition from the disordered phase is considered. We show that at sufficiently high temperatures $T\geq T^*(x)$ the effectiv... expand abstracte mass starts to depend on $T$, $M^*\propto T^{-1/2}$. This $T^{-1/2}$ dependence of the effective mass at elevated temperatures leads to the non-Fermi liquid behavior of the resistivity, $\rho(T)\propto T$ and at higher temperatures $\rho(T)\propto T^{3/2}$. The application of a magnetic field $B$ restores the common $T^2$ behavior of the resistivity. The effective mass depends on the magnetic field, $M^*(B)\propto B^{-2/3}$, being approximately independent of the temperature at $T\leq T^*(B)\propto B^{4/3}$. At $T\geq T^*(B)$, the $T^{-1/2}$ dependence of the effective mass is re-established. We demonstrate that this $B-T$ phase diagram has a strong impact on the magnetoresistance (MR) of the highly correlated electron liquid. The MR as a function of the temperature exhibits a transition from the negative values of MR at $T\to 0$ to the positive values at $T\propto B^{4/3}$. Thus, at $T\geq T^*(B)$, MR as a function of the temperature possesses a node at $T\propto B^{4/3}$. collapse abstract
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Physics of Atomic Nuclei 66, 1802 (2003).
From the Bose-Einstein to Fermion Condensation
The appearance of the fermion condensation, which can be compared to the Bose-Einstein condensation, in different liquids is considered, its properties are discussed, and a large number of experimental evidences in favor of the existence of the fer... expand abstractmion condensate is presented. collapse abstract
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Phys. Lett. A315, 288 (2003).
Magnetic field-induced Landau Fermi Liquid in high-T_c metals
We consider the behavior of strongly correlated electron liquid in high-temperature superconductors within the framework of the fermion condensation model. We show that at low temperatures the normal state recovered by the application of a magnetic... expand abstract field larger than the critical field can be viewed as the Landau Fermi liquid induced by the magnetic field. In this state, the Wiedemann-Franz law and the Korringa law are held and the elementary excitations are the Landau Fermi Liquid quasiparticles. Contrary to what might be expected from the Landau theory, the effective mass of quasiparticles depends on the magnetic field. The recent experimental verifications of the Wiedemann-Franz law in heavily hole-overdoped, overdoped and optimally doped cuprates and the verification of the Korringa law in the electron-doped copper-oxide superconductor strongly support the existence of fermion condensate in high-T_c metals. collapse abstract
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JETP Lett. 77, 671 (2003).
Quasiparticles in the Superconducting State of High-T_c Metals
We consider the behavior of quasiparticles in the superconducting state of high-T_c metals within the framework of the theory of superconducting state based on the fermion condensation quantum phase transition. We show that the behavior coincides w... expand abstractith the behavior of Bogoliubov quasiparticles, whereas the maximum value of the superconducting gap and other exotic properties are determined by the presence of the fermion condensate. If at low temperatures the normal state is recovered by the application of a magnetic field suppressing the superconductivity, the induced state can be viewed as Landau Fermi liquid. These observations are in good agreement with recent experimental facts. collapse abstract
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JETP Letters 79, 286 (2004).
Universal Behavior of Heavy-Fermion Metals Near a Quantum Critical Point
The behavior of the electronic system of heavy fermion metals is considered. We show that there exist at least two main types of the behavior when the system is nearby a quantum critical point which can be identified as the fermion condensation qua... expand abstractntum phase transition (FCQPT). We show that the first type is represented by the behavior of a highly correlated Fermi-liquid, while the second type is depicted by the behavior of a strongly correlated Fermi-liquid. If the system approaches FCQPT from the disordered phase, it can be viewed as a highly correlated Fermi-liquid which at low temperatures exhibits the behavior of Landau Fermi liquid (LFL). At higher temperatures $T$, it demonstrates the non-Fermi liquid (NFL) behavior which can be converted into the LFL behavior by the application of magnetic fields $B$. If the system has undergone FCQPT, it can be considered as a strongly correlated Fermi-liquid which demonstrates the NFL behavior even at low temperatures. It can be turned into LFL by applying magnetic fields $B$. We show that the effective mass $M^*$ diverges at the very point that the N\'eel temperature goes to zero. The $B-T$ phase diagrams of both liquids are studied. We demonstrate that these $B-T$ phase diagrams have a strong impact on the main properties of heavy-fermion metals such as the magnetoresistance, resistivity, specific heat, magnetization, volume thermal expansion, etc. collapse abstract
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Physics Letters A 330, 10 (2004).
On the relation between the Hartree-Fock and Kohn-Sham approaches
We show that the Hartree-Fock (HF) results cannot be reproduced within the framework of Kohn-Sham (KS) theory because the single-particle densities of finite systems obtained within the HF calculations are not $v$-representable, i.e., do not corres... expand abstractpond to any ground state of a $N$ non-interacting electron systems in a local external potential. For this reason, the KS theory, which finds a minimum on a different subset of all densities, can overestimate the ground state energy, as compared to the HF result. The discrepancy between the two approaches provides no grounds to assume that either the KS theory or the density functional theory suffers from internal contradictions. collapse abstract
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Phys. Lett. A 329, 108 (2004).
Fermion Condensation Quantum Phase Transition versus Conventional Quantum Phase Transitions
The main features of fermion condensation quantum phase transition (FCQPT), which are distinctive in several aspects from that of conventional quantum phase transition (CQPT), are considered. We show that in contrast to CQPT, whose physics in quant... expand abstractum critical region is dominated by thermal and quantum fluctuations and characterized by the absence of quasiparticles, the physics of a Fermi system near FCQPT or undergone FCQPT is controlled by the system of quasiparticles resembling the Landau quasiparticles. Contrary to the Landau quasiparticles, the effective mass of these quasiparticles strongly depends on the temperature, magnetic fields, density, etc. This system of quasiparticles having general properties determines the universal behavior of the Fermi system in question. As a result, the universal behavior persists up to relatively high temperatures comparatively to the case when such a behavior is determined by CQPT. We analyze striking recent measurements of specific heat, charge and heat transport used to study the nature of magnetic field-induced QCP in heavy-fermion metal CeCoIn$_5$ and show that the observed facts are in good agreement with our scenario based on FCQPT and certainly seem to rule out the critical fluctuations related with CQPT. Our general consideration suggests that FCQPT and the emergence of novel quasiparticles near and behind FCQPT and resembling the Landau quasiparticles are distinctive features intrinsic to strongly correlated substances. collapse abstract
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JETP Lett. 80, 263 (2004).
Investigation of the Field-Tuned Quantum Critical Point in CeCoIn_5
The main properties and the type of the field-tuned quantum critical point in the heavy-fermion metal CeCoIn$_5$ arisen upon applying magnetic fields $B$ are considered within the scenario based on the fermion condensation quantum phase transition.... expand abstract We analyze the behavior of the effective mass, resistivity, specific heat, charge and heat transport as functions of applied magnetic fields $B$ and show that in the Landau Fermi liquid regime these quantities demonstrate the critical behavior which is scaled by the critical behavior of the effective mass. We show that in the high-field non-Fermi liquid regime, the effective mass exhibits very specific behavior, $M^*\sim T^{-2/3}$, and the resistivity demonstrates the $T^{2/3}$ dependence. Finally, at elevated temperatures, it changes to $M^*\sim T^{-1/2}$, while the resistivity becomes linear in $T$. In zero magnetic field, the effective mass is controlled by temperature $T$, and the resistivity is also linear in $T$. The obtained results are in good agreement with recent experimental facts. collapse abstract
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Phys. Rev. B 63, 224507 (2001).
Quasiparticle picture of high temperature superconductors in the frame of Fermi liquid with the fermion condensate
A model of a Fermi liquid with the fermion condensate (FC) is applied to the consideration of quasiparticle excitations in high temperature superconductors, in their superconducting and normal states. Within our model the appearance of the fermion ... expand abstractcondensate presents a quantum phase transition, that separates the regions of normal and strongly correlated electron liquids. Beyond the phase transition point the quasiparticle system is divided into two subsystems, one containing normal quasiparticles and the other --- fermion condensate localized at the Fermi surface and characterized by almost dispersionless single-particle excitations. In the superconducting state the quasiparticle dispersion in systems with FC can be presented by two straight lines, characterized by effective masses $M^*_{FC}$ and $M^*_L$, respectively, and intersecting near the binding energy which is of the order of the superconducting gap. This same quasiparticle picture persists in the normal state, thus manifesting itself over a wide range of temperatures as new energy scales. Arguments are presented that fermion systems with FC have features of a quantum protectorate. collapse abstract
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JETP Letters 73, 232 (2001)
Fermion-Condensation Quantum Phase Transition in High-T_c Superconductors
The effect of a quantum phase transition associated with the appearance of the fermion condensate in an electron liquid on the properties of superconductors is considered. It is shown that the electron system in both superconducting and normal stat... expand abstractes exhibits characteristic features of a quantum protectorate after the point of this fermion-condensation quantum phase transition. The single-particle spectrum of a superconductor can be represented by two straight lines corresponding to two effective masses $M^*_{FC}$ and $M^*_{L}$. The $M^*_{FC}$ mass characterizes the spectrum up to the binding energy $E_0$, which is of the order of the superconducting gap in magnitude, and $M^*_{L}$ determines the spectrum at higher binding energies. Both effective masses are retained in the normal state; however, $E_0=4T$. These results are used to explain the lineshape of single-particle excitations and other remarkable properties of high-$T_c$ superconductors and are in a good agreement with recent experimental data. collapse abstract
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Physica B 312-313C, 413 (2002).
Fermion-condensation quantum phase transition in high temperature superconductors
The effect of a quantum phase transition associated with the appearance of the fermion condensate in an electron liquid on the properties of superconductors is considered. It is shown that the electron system in both superconducting and normal stat... expand abstractes exhibits characteristic features of a quantum protectorate after the point of this fermion-condensation quantum phase transition. The single-particle spectrum of a superconductor can be represented by two straight lines corresponding to two effective masses $M^*_{FC}$ and $M^*_{L}$. The $M^*_{FC}$ mass characterizes the spectrum up to the binding energy $E_0$ and $M^*_L$ determines the spectrum at higher binding energies. Both effective masses are retained in the normal state. These results are used to explain the lineshape of single-particle excitations and other properties of high-$T_c$ superconductors and are in a good agreement with recent experimental data. collapse abstract
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JETP Lett. 74, 396 (2001)
Theory of high-T_c superconductivity based on the fermion-condensation quantum phase transition
A theory of high temperature superconductivity based on the combination of the fermion-condensation quantum phase transition and the conventional theory of superconductivity is presented. This theory describes maximum values of the superconducting ... expand abstractgap which can be as big as $\Delta_1\sim 0.1\epsilon_F$, with $\epsilon_F$ being the Fermi level. We show that the critical temperature $2T_c\simeq\Delta_1$. If there exists the pseudogap above $T_c$ then $2T^*\simeq\Delta_1$, and $T^*$ is the temperature at which the pseudogap vanishes. A discontinuity in the specific heat at $T_c$ is calculated. The transition from conventional superconductors to high-$T_c$ ones as a function of the doping level is investigated. The single-particle excitations and their lineshape are also considered. collapse abstract
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