Chiral magnetic effect without chirality source in asymmetric Weyl semimetals

We describe a new type of the chiral magnetic effect (CME) that should occur in Weyl semimetals (WSMs) with an asymmetry in the dispersion relations of the left- and right-handed (LH and RH) chiral Weyl fermions. In such materials, time-dependent pumping of electrons from a non-chiral external source can generate a non-vanishing chiral chemical potential. This is due to the different capacities of the LH and RH chiral Weyl cones arising from the difference in the density of states in the LH and RH cones. The chiral chemical potential then generates, via the chiral anomaly, a current along the direction of an applied magnetic field even in the absence of an external electric field. The source of chirality imbalance in this new setup is thus due to the band structure of the system and the presence of (non-chiral) electron source, and not due to the parallel electric and magnetic fields. We illustrate the effect by an argument based on the effective field theory, and by the chiral kinetic theory calculation for a rotationally invariant WSM with different Fermi velocities in the left and right chiral Weyl cones; we also consider the case of a WSM with Weyl nodes at different energies. We argue that this effect is generically present in WSMs with different dispersion relations for LH and RH chiral Weyl cones, such as SrSi₂ recently predicted as a WSM with broken inversion and mirror symmetries, as long as the chiral relaxation time is much longer than the transport scattering time.     

Dynamics of a tunneling magnetic impurity: Kondo effect induced incoherence

We study how the formation of the Kondo compensation cloud influences the dynamical properties of a magnetic impurity that tunnels between two positions in a metal. The Kondo effect dynamically generates a strong tunneling impurity-conduction electron coupling, changes the temperature dependence of the tunneling rate, and may ultimately result in the destruction of the coherent motion of the particle at zero temperature. We find an interesting two-channel Kondo fixed point as well for a vanishing overlap between the electronic states that screen the magnetic impurity. We propose experiments where the predicted features could be observed.     

Chiral splitting of Kondo peak in triangular triple quantum dot

New characteristics of the Kondo effect, arising from spin chirality induced by the Berry phase in the equilibrium state, are investigated. The analysis is based on the hierarchical equations of motion (HEOM) approach in a triangular triple quantum-dot (TTQD) structure. In the absence of magnetic field, TTQD has four-fold degenerate chiral ground states with degenerate spin chirality. When a perpendicular magnetic field is applied, the chiral interaction is induced by the magnetic flux threading through TTQD and the four-fold degenerate states split into two chiral state pairs. The chiral excited states manifest as chiral splitting of the Kondo peak in the spectral function. The theoretical analysis is confirmed by the numerical computations. Furthermore, under a Zeeman magnetic field B, the chiral Kondo peak splits into four peaks, owing to the splitting of spin freedom. The influence of spin chirality on the Kondo effect signifies an important role of the phase factor. This work provides insight into the quantum transport of strongly correlated electronic systems.     

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.     

Large Seebeck coefficient resulting from chiral interactions in triangular triple quantum dots

We theoretically study thermoelectric transport properties through a triangular triple-quantum-dot (TTQD) structure in the linear response regime using the hierarchical equations of motion approach. It is demonstrated that large Seebeck coefficient can be obtained when properly matching the interdot tunneling strength and magnetic flux at the electron-hole symmetry point, as a result of spin chiral interactions in the TTQD system. We present a systematic investigation of the thermopower (the Seebeck coefficient) dependence on the tunneling strength, magnetic flux, and on-site energy. The Seebeck coefficient shows a clear breakdown of electron-hole symmetry in the vicinity of the Kondo regime, accompanied by the deviation from the semiclassical Mott relation in the Kondo and mixed-valence regimes, which result from the many-body effects of the Kondo correlated induced resonance together with spin chiral interactions.     

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.     

Unconventional anomalous Hall effect enhanced by a noncoplanar spin texture in the frustrated Kondo lattice Pr2Ir2O7

We have investigated the Hall effect in the geometrically frustrated Kondo lattice Pr2Ir2O7. In its spin-liquid-like paramagnetic regime, the Hall resistivity rho(xy) is found to increase logarithmically on cooling. Moreover, in this low temperature region, the field dependence of the Hall conductivity sigma(xy) shows a large enhancement up to 30 Omega(-1) cm(-1) as well as a nonmonotonic change with the magnetization. Our results are far different from the anomalous Hall effect due to the spin-orbit coupling observed in ordinary magnetic conductors. We discuss the possible spin-chirality effect in the Ir 5d conduction band due to the noncoplanar texture of Pr<111> Ising-like moments.     

Correlated electron effects in low energy Sr(+) ion scattering

Resonant charge transfer during low energy ion scattering reveals correlated-electron behavior at high temperature. The valence electron of a singly charged alkaline-earth ion is a magnetic impurity that interacts with the continuum of many-body excitations in the metal, leading to Kondo and mixed valence resonances near the Fermi energy. The occupation of these resonances is acutely sensitive to the surface temperature, which results in a marked temperature dependence of the ion neutralization. We report such a dependence for low energy Sr(+) scattered from polycrystalline gold.     

Why could electron spin resonance be observed in a heavy fermion Kondo lattice?

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancellation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh₂Si₂ in terms of their dependence on temperature and magnetic field.     

On the chiral magnetic effect in Weyl superfluid <Superscript>3</Superscript>He-A

In the theory of the chiral anomaly in relativistic quantum field theories (RQFTs), some results depend on a regularization scheme at ultraviolet. In the chiral superfluid ³He-A, which contains two Weyl points and also experiences the effects of chiral anomaly, the “trans-Planckian” physics is known and the results can be obtained without regularization. We discuss this on example of the chiral magnetic effect (CME), which has been observed in ³He-A in the 1990s [1]. There are two forms of the contribution of the CME to the Chern–Simons term in free energy, perturbative and non-perturbative. The perturbative term comes from the fermions living in the vicinity of the Weyl point, where the fermions are “relativistic” and obey the Weyl equation. The non-perturbative term originates from the deep vacuum, being determined by the separation of the two Weyl points in momentum space. Both terms are obtained using the Adler–Bell–Jackiw equation for chiral anomaly, and both agree with the results of the microscopic calculations in the “trans-Planckian” region. Existence of the two nonequivalent forms of the Chern–Simons term demonstrates that the results obtained within the RQFT depend on the specific properties of the underlying quantum vacuum and may reflect different physical phenomena in the same vacuum.     

Experimental test of the numerical renormalization-group theory for inelastic scattering from magnetic impurities

We present measurements of the phase coherence time taupsi in quasi-one-dimensional Au/Fe Kondo wires and compare the temperature dependence taupsi of with a recent theory of inelastic scattering from magnetic impurities [Phys. Rev. Lett. 93, 107204 (2004)10.1103/PhysRevLett.93.107204]. A very good agreement is obtained for temperatures down to 0.2T(K). Below the Kondo temperature T(K), the inverse of the phase coherence time varies linearly with temperature over almost one decade in temperature.     

Kondo-transport spectroscopy of single molecule magnets

We demonstrate that in a single molecule magnet strongly coupled to electrodes the Kondo effect involves all magnetic excitations. This Kondo effect is induced by the quantum tunneling of the magnetic moment. Importantly, the Kondo temperature TK can be much larger than the magnetic splittings. We find a strong modulation of the Kondo effect as a function of the transverse anisotropy parameter or a longitudinal magnetic field. Both for integer and half-integer spin this can be used for an accurate transport spectroscopy of the magnetic states in low magnetic fields on the order of the easy-axis anisotropy parameter. We set up a relationship between the Kondo effects for successive integer and half-integer spins.     

Kondo universal scaling for a quantum dot coupled to superconducting leads

We study competition between the Kondo effect and superconductivity in a single self-assembled InAs quantum dot contacted with Al lateral electrodes. Because of Kondo enhancement of Andreev reflections, the zero-bias anomaly develops side peaks, separated by the superconducting gap energy Delta. For ten valleys of different Kondo temperature T(K) we tune the gap Delta with an external magnetic field. We find that the zero-bias conductance in each case collapses onto a single curve with Delta/k(B)T(K) as the only relevant energy scale, providing experimental evidence for universal scaling in this system.     

外尔半金属及其输运特性

外尔半金属是继石墨烯以及拓扑绝缘体之后的又一个研究热点。相比于后两者,外尔半金 属独特的三维无能隙线性色散能带结构使得它有很多奇特的性质,如：手性反常、手性磁效应、 反弱局域化、手性朗道能级和负磁阻效应等。实际样品中无序总是不可避免的,所以考虑无序对 体系的影响是很有必要的。我们回顾了无序下第一类以及第二类外尔半金属的相变特性,并获得 了完整的相图,这些无序诱导的相变丰富了拓扑安德森绝缘体和安德森金属绝缘体相变的物理内 涵。我们同样回顾了长程短程无序影响下的第一类外尔半金属体系的输运,发现了一种不能采用 玻尔兹曼输运方程来描述的输运过程。我们介绍Imbert-Fedorov 位移这一光学中的效应在外尔 半金属中的实现,这为更好地应用外尔半金属提供了更多的可能性,随后采用波包散射,我们解 释了外尔半金属中的超高载流子迁移率问题的原因,最后我们给出一个简要的总结。     

Quantum transport in topological semimetals under magnetic fields

Topological semimetals are three-dimensional topological states of matter, in which the conduction and valence bands touch at a finite number of points, i.e., the Weyl nodes. Topological semimetals host paired monopoles and antimonopoles of Berry curvature at the Weyl nodes and topologically protected Fermi arcs at certain surfaces. We review our recent works on quantum transport in topological semimetals, according to the strength of the magnetic field. At weak magnetic fields, there are competitions between the positive magnetoresistivity induced by the weak anti-localization effect and negative magnetoresistivity related to the nontrivial Berry curvature. We propose a fitting formula for the magnetoconductivity of the weak anti-localization. We expect that the weak localization may be induced by inter-valley effects and interaction effect, and occur in double-Weyl semimetals. For the negative magnetoresistance induced by the nontrivial Berry curvature in topological semimetals, we show the dependence of the negative magnetoresistance on the carrier density. At strong magnetic fields, specifically, in the quantum limit, the magnetoconductivity depends on the type and range of the scattering potential of disorder. The high-field positive magnetoconductivity may not be a compelling signature of the chiral anomaly. For long-range Gaussian scattering potential and half filling, the magnetoconductivity can be linear in the quantum limit. A minimal conductivity is found at the Weyl nodes although the density of states vanishes there.     

Anomalous temperature dependence of the dephasing time in mesoscopic kondo wires

We present measurements of the magnetoconductance of long and narrow quasi-one-dimensional gold wires containing magnetic iron impurities in a temperature range extending from 15 mK to 4.2 K. The dephasing rate extracted from the weak antilocalization shows a pronounced plateau in a temperature region of 300-800 mK, associated with the phase breaking due to the Kondo effect. Below the Kondo temperature, the dephasing rate decreases linearly with temperature, in contradiction with standard Fermi-liquid theory. Our data suggest that the formation of a spin glass due to the interactions between the magnetic moments is responsible for the observed anomalous temperature dependence.     

Kondo resonances and anomalous gate dependence in the electrical conductivity of single-molecule transistors

We report Kondo resonances in the conduction of single-molecule transistors based on transition metal coordination complexes. We find Kondo temperatures in excess of 50 K, comparable to those in purely metallic systems. The observed gate dependence of the Kondo temperature is inconsistent with observations in semiconductor quantum dots and a simple single-dot-level model. We discuss possible explanations of this effect, in light of electronic structure calculations.     

Itinerant electron-driven chiral magnetic ordering and spontaneous quantum Hall effect in triangular lattice models

We study the Kondo Lattice and the Hubbard models on a triangular lattice. We find that at the mean-field level, these rotationally invariant models naturally support a noncoplanar chiral magnetic ordering. It appears as a weak-coupling instability at the band filling factor 3/4 due to the perfect nesting of the itinerant electron Fermi surface. This ordering is a triangular-lattice counterpart of the collinear Neel ordering that occurs on the half-filled square lattice. While the long-range magnetic ordering is destroyed by thermal fluctuations, the chirality can persist up to a finite temperature, causing a spontaneous quantum Hall effect in the absence of any externally applied magnetic field.     

Antiferromagnetism and the Kondo behaviour in Ce(Pt1−xPdx)Ga

We present the results of lattice parameters at room temperature, the static magnetic susceptibility and the magnetic resistivity between 1.8 and 300 K, and the low-temperature specific-heat measurements for the series Ce(Pt_1−xPd_x)Ga, (x=0.0, 0.2, 0.5, 0.8 and 1.0). Two maxima in the temperature dependence of the magnetic resistive curve for each sample are observed, one above 100 K, and another at around 4 K, which due to an interplay between crystal-field effect and the Kondo effect. As determined from the peak values of the temperature dependence of the specific heat data C(T), all samples exhibit antiferromagnetic ordering from 1.3 K for CePdGa to 3.4 K for CePtGa. The large reduction of entropy for each sample below T_N is associated with the Kondo effect.     

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.