Universal scaling and a novel quantum critical behavior of CeRhSb1-xSnx

We observe and explain a universal scaling rhochi = const for the electrical resistivity rho with the inverse magnetic susceptibility chi(-1) for the Kondo insulator CeRhSb(1-x)Snx. In the regime where the Kondo gap disappears (x > 0.12), the system forms a non-Fermi liquid (NFL), which transforms into a Fermi liquid at higher temperature. The NFL behavior is associated with the presence of a novel quantum critical point (QCP) at the Kondo insulator-correlated metal boundary. The divergent behavior of the resistivity, the susceptibility, and the specific heat has been observed when approaching the QCP from the metallic side and is interpreted as due to the competition between the Kondo and the intersite magnetic correlations.     

Magnetic-field control of quantum critical points of valence transition

We study the mechanism of how critical end points of first-order valence transitions are controlled by a magnetic field. We show that the critical temperature is suppressed to be a quantum critical point (QCP) by a magnetic field, and unexpectedly, the QCP exhibits nonmonotonic field dependence in the ground-state phase diagram, giving rise to the emergence of metamagnetism even in the intermediate valence-crossover regime. The driving force of the field-induced QCP is clarified to be cooperative phenomena of the Zeeman and Kondo effects, which create a distinct energy scale from the Kondo temperature. This mechanism explains the peculiar magnetic response in CeIrIn(5) and the metamagnetic transition in YbXCu(4) for X=In as well as the sharp contrast between X=Ag and Cd.     

Low temperature electron spin resonance of the Kondo ion in a heavy fermion metal: YbRh2Si2

We report an electron spin resonance (ESR) study on single crystals of the heavy fermion metal YbRh2Si2 which shows pronounced non-Fermi liquid behavior related to a close antiferromagnetic quantum critical point. It is shown that the observed ESR spectra can be ascribed to a bulk Yb3+ resonance. This is the first observation of ESR of the Kondo ion itself in a dense Kondo lattice system. The ESR signal occurs below the Kondo temperature (T(K)) which thus indicates the existence of large unscreened Yb3+ moments below T(K). We observe the spin dynamics as well as the static magnetic properties of the Yb3+ spins to be consistent with the results of nuclear magnetic resonance and magnetic susceptibility.     

Locally critical point in an anisotropic Kondo lattice

We report the first numerical identification of a locally quantum critical point at which the criticality of the local Kondo physics is embedded in that associated with a magnetic ordering. We are able to numerically access the quantum critical behavior by focusing on a Kondo-lattice model with Ising anisotropy. We also establish that the critical exponent for the q-dependent dynamical spin susceptibility is fractional and compares well with the experimental value for heavy fermions.     

The Kondo necklace model with planar anisotropy

We study the one-dimensional anisotropic Kondo necklace model at zero temperature through White's density matrix renormalization group technique. The ground state energy and the spin gap were calculated as a function of the exchange parameter for two anisotropy values. We found a finite critical point separating a Kondo singlet from an antiferromagnetic phase. The transition is highly congruent with a Kosterlitz–Thouless form. We observed that the critical point increases with the anisotropy.     

Magnetism, f-electron localization and superconductivity in 122-type heavy-fermion metals

Both CeCu2Si2 and YbRh2Si2 crystallize in the tetragonal ThCr2Si2 crystal structure. Recent neutron-scattering results on normal-state CeCu2Si2 reveal a slowing down of the quasielastic response which complies with the scaling expected for a quantum critical point (QCP) of itinerant, i.e., three-dimensional spin-density-wave (SDW), type. This interpretation is in full agreement with the non-Fermi-liquid behavior observed in transport and thermodynamic measurements. The momentum dependence of the magnetic excitation spectrum reveals two branches of an overdamped dispersive mode whose coupling to the heavy charge carriers is strongly retarded. These overdamped spin fluctuations are considered to be the driving force for superconductivity in CeCu2Si2 (Tc = 600 mK). The weak antiferromagnet YbRh2Si2 (TN = 70 mK) exhibits a magnetic-field-induced QCP at BN = 0.06 T (B⊥c). There is no indication of superconductivity down to T = 10 mK. The magnetic QCP appears to concur with a breakdown of the Kondo effect. Doping-induced variations of the average unit-cell volume result in a detachment of the magnetic and electronic instabilities. A comparison of the properties of these isostructural compounds suggests that 3D SDW QCPs are favorable for unconventional superconductivity. The question whether a Kondo-breakdown QCP may also give rise to superconductivity, however, remains to be clarified.     

Anderson impurity in a correlated conduction band

We investigate the physics of a magnetic impurity with spin 1/2 in a correlated metallic host. Describing the band by a Hubbard Hamiltonian, the problem is analyzed using dynamical mean-field theory in combination with Wilson's nonperturbative numerical renormalization group. We present results for the single-particle density of states and the dynamical spin susceptibility at zero temperature. New spectral features (side peaks) are found which should be observable experimentally. In addition, we find a general enhancement of the Kondo scale due to correlations. Nevertheless, in the metallic phase, the Kondo scale always vanishes exponentially in the limit of small hybridization.     

Competition between Kondo and RKKY correlations in the presence of strong randomness

We propose that competition between Kondo and magnetic correlations results in a novel universality class for heavy fermion quantum criticality in the presence of strong randomness. Starting from an Anderson lattice model with disorder, we derive an effective local field theory in the dynamical mean-field theory approximation, where randomness is introduced into both hybridization and Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions. Performing the saddle-point analysis in the U(1) slave-boson representation, we reveal its phase diagram which shows a quantum phase transition from a spin liquid state to a local Fermi liquid phase. In contrast with the clean limit case of the Anderson lattice model, the effective hybridization given by holon condensation turns out to vanish, resulting from the zero mean value of the hybridization coupling constant. However, we show that the holon density becomes finite when the variance of the hybridization is sufficiently larger than that of the RKKY coupling, giving rise to the Kondo effect. On the other hand, when the variance of the hybridization becomes smaller than that of the RKKY coupling, the Kondo effect disappears, resulting in a fully symmetric paramagnetic state, adiabatically connected to the spin liquid state of the disordered Heisenberg model. We investigate the quantum critical point beyond the mean-field approximation. Introducing quantum corrections fully self-consistently in the non-crossing approximation, we prove that the local charge susceptibility has exactly the same critical exponent as the local spin susceptibility, suggesting an enhanced symmetry at the local quantum critical point. This leads us to propose novel duality between the Kondo singlet phase and the critical local moment state beyond the Landau-Ginzburg-Wilson paradigm. The Landau-Ginzburg-Wilson forbidden duality serves the mechanism of electron fractionalization in critical impurity dynamics, where such fractionalized excitations are identified with topological excitations.     

Non-Kondo mechanism for resistivity minimum in spin ice conduction systems

We present a mechanism of resistivity minimum in conduction electron systems coupled with localized moments, which is distinguished from the Kondo effect. Instead of the spin-flip process in the Kondo effect, electrons are elastically scattered by local spin correlations which evolve in a particular way under geometrical frustration as decreasing temperature. This is demonstrated by the cellular dynamical mean-field theory for a spin-ice-type Kondo lattice model on a pyrochlore lattice. Peculiar temperature dependences of the resistivity, specific heat, and magnetic susceptibility in the non-Kondo mechanism are compared with the experimental data in metallic Ir pyrochlore oxides.     

Non-fermi-liquid scaling in Ce(Ru0.5Rh)2Si2

We study the temperature and field dependence of the magnetic and transport properties of the non-Fermi-liquid (NFL) compound Ce(Ru0.5Rh0.5)2Si2. For fields H less, similar0.1 T the results suggest that the observed NFL behavior is disorder driven. For higher fields, however, magnetic and transport properties are dominated by the coupling of the conduction electrons to critical spin fluctuations. The temperature dependence of the susceptibility as well as the scaling properties of the magnetoresistance are in very good agreement with the predictions of recent dynamical mean-field theories of Kondo alloys close to a spin-glass quantum critical point.     

Kondo effect in the presence of itinerant-electron ferromagnetism studied with the numerical renormalization group method

The Kondo effect in quantum dots (QDs)-artificial magnetic impurities-attached to ferromagnetic leads is studied with the numerical renormalization group method. It is shown that the QD level is spin split due to the presence of ferromagnetic electrodes, leading to a suppression of the Kondo effect. We find that the Kondo effect can be restored by compensating this splitting with a magnetic field. Although the resulting Kondo resonance then has an unusual spin asymmetry with a reduced Kondo temperature, the ground state is still a locally screened state, describable by Fermi liquid theory and a generalized Friedel sum rule, and transport at zero temperature is spin independent.     

Magnetic susceptibility of dilute magnetic alloys

The magnetic susceptibility of dilute magnetic alloys is calculated using the Nagoaka approximation to the Kondo problem. We use the exact solutions of the Nagaoka equations, or equivalently Suhl's dispersion relations, as obtained recently. Our result is represented by a universal function of a certain temperature parameter. In the case of ferromagnetic coupling no appreciable change of the free spin susceptibility is found over the whole temperature range. In the case of antiferromagnetic coupling we find that the free spin susceptibility is greatly reduced. In fact, for spin 1/2, the result indicates the breakdown of the expansion in terms of the impurity concentration and suggests the onset of impurity ferromagnetism.     

The entanglement dynamics of interacting qubits embedded in a spin environment with Dzyaloshinsky-Moriya term

We investigate the entanglement dynamics of two interacting qubits in a spin environment, which is described by an XY model with Dzyaloshinsky-Moriya (DM) interaction. The competing effects of environmental noise and interqubit coupling on entanglement generation for various system parameters are studied. We find that the entanglement generation is suppressed remarkably in weak-coupling region at quantum critical point (QCP). However, the suppression of the entanglement generation at QCP can be compensated both by increasing the DM interaction and by decreasing the anisotropy of the spin chain. Beyond the weak-coupling region, there exist resonance peaks of concurrence when the system-bath coupling equals to external magnetic field. We attribute the presence of resonance peaks to the flat band of the self-Hamiltonian. These peaks are highly sensitive to anisotropy parameter and DM interaction.     

Critical local-moment fluctuations,anomalous exponents,and omega/T scaling in the Kondo problem with a pseudogap

Experiments in heavy-fermion metals and related theoretical work suggest that critical local-moment fluctuations can play an important role near a zero-temperature phase transition. We study such fluctuations at the quantum critical point of a Kondo impurity model in which the density of band states vanishes as /epsilon/(r) at the Fermi energy (epsilon=0). The local spin response is described by a set of critical exponents that vary continuously with r. For 0相似文献   

Quantum critical Kondo quasiparticles probed by ESR in β-YbAlB₄

Electron spin resonance (ESR) can probe conduction electrons (CE) and local moment (LM) spin systems in different materials. A CE spin resonance (CESR) is observed in metallic systems based on light elements or with enhanced Pauli susceptibility. LM ESR can be seen in compounds with paramagnetic ions and localized d or f electrons. Here we report a remarkable and unprecedented ESR signal in the heavy-fermion superconductor β-YbAlB? [S. Nakatsuji et al., Nature Phys. 4, 603 (2008)] which behaves as a CESR at high temperatures and acquires characteristics of the Yb3? LM ESR at low temperature. This dual behavior strikes as an in situ unique observation of the Kondo quasiparticles in a quantum critical regime. The proximity to a quantum critical point may favor the appearance of this dual character of the ESR signal in β-YbAlB?.     

Impurity spin dynamics in the Kondo problem

We derive a system of coupled linear integral equations to determine the dynamical impurity spin susceptibility in the Kondo problem. It is shown that a high temperature solution of these equations is possible by an asymptotic expansion. This yields a Korringa width as one would expect from heuristic arguments.     

Pseudogap Fermi-Bose Kondo model

We consider a magnetic impurity coupled to both fermionic quasiparticles with a pseudogap density of states and bosonic spin fluctuations. Using renormalization group and large-N calculations we investigate the phase diagram of the resulting Fermi-Bose Kondo model. We show that the Kondo temperature is strongly reduced by low-energy spin fluctuations, and make connections to experiments in cuprate superconductors. Furthermore, we derive an exact exponent for the critical behavior of the conduction electron T matrix, and propose our findings to be relevant for certain scenarios of local quantum criticality in heavy-fermion metals.     

Low temperature behavior of the spin susceptibility in nearly magnetic dilute alloys

We show that the low temperature susceptibility of a dilute alloy containing nearly magnetic impurities varies like T² well below the characteristic temperature, (the spin fluctuation temperature or the Kondo one) in agreement with experiments.     

Zero-temperature magnetic transition in an easy-axis Kondo lattice model

We address the quantum transition of a spin-1/2 antiferromagnetic Kondo lattice model with an easy-axis anisotropy using the extended dynamical mean field theory. We derive results in real frequency by using the bosonic numerical renormalization group (BNRG) method and compare them with quantum Monte Carlo results in Matsubara frequency. The BNRG results show a logarithmic divergence in the critical local spin susceptibility, signaling a destruction of Kondo screening. The T=0 transition is consistent with being second order. The BNRG results also display some subtle features; we identify their origin and suggest means for further microscopic studies.