We report measurements of the thermal conductivity $κ$ on single crystalline CeRu$_4$Sn$_6$ in the temperature range between 80mK and 80K, along the main crystallographic directions. $κ$ is phonon-dominated in the whole temperature range and is found to be essentially isotropic. At low temperatures the data are well approximated by $κ\propto T^2$, which is attributed to a predominant scattering of phonons on electrons. We describe the data with a Callaway fit in the whole temperature range giving good agreement at low and high temperatures.
We report the temperature-pressure-magnetic field phase diagram made from electrical resistivity measurements for the ferromagnetic (FM) Kondo lattice CeRuPO. The ground state at zero field changes from the FM state to another state, which is suggested to be an antiferromagnetic (AFM) state, above ~0.7 GPa, and the magnetically ordered state is completely suppressed at ~2.8 GPa. In addition to the collapse of the AFM state under pressure and a magnetic field, a metamagnetic (MM) transition from a paramagnetic state to a polarized paramagnetic state appears. CeRuPO will give us a rich playgroun…
Kondo insulators and in particular their non-cubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu$_4$Sn$_6$ employing optical reflectivity measurements in broad frequency and temperature ranges, and local density approximation plus dynamical mean field theory calculations. The calculations provide evidence for a Kondo insulator-like response within the $a-a$ plane and a more metallic response along the c axis and qualitatively reproduce the experimental observations, helping to identify their…
Bulk sensitive hard x-ray photoelectron spectroscopy data of the Ce 3$p$ core level of CeRu$_4$Sn$_6$ are presented. Using a combination of full multiplet and configuration iteration model we were able to obtain an accurate lineshape analysis of the data, thereby taking into account correlations for the strong plasmon intensities. We conclude that CeRu$_4$Sn$_6$ is a moderately mixed valence compound with a weight of 8% for the Ce $f^0$ configuration in the ground state.
Spin dynamics in the new Kondo insulator compound CeRu2Al10 has been studied using unpolarized and polarized neutron scattering on single crystals. In the unconventional ordered phase forming below T0 = 27.3 K, two excitation branches are observed with significant intensities, the lower one of which has a gap of 4.8 +/- 0.3 meV and a pronounced dispersion up to about 8.5 meV. Comparison with RPA magnon calculations assuming crystal-field and anisotropic exchange couplings captures major aspects of the data, but leaves unexplained discrepancies, pointing to a key role of direction-specific hybr…
Magnetic ground state of Rh-doped Kondo semiconductor CeRu$_2$Al$_{10}$ [Ce(Ru$_{1-x}$Rh$_x$)$_2$Al$_{10}$] is investigated by muon-spin relaxation method. Muon-spin precession with two frequencies is observed in the $x$ = 0 sample, while only one frequency is present in the $x$ = 0.05 and 0.1 samples, which is attributed to the broad static field distribution at the muon site. The internal field at the muon site is enhanced from about 180 G in $x$ = 0 sample to about 800 G in the Rh-doped samples, supporting the spin-flop transition as suggested by macroscopic measurements, and the boundary o…
We investigate the thermal and transport properties of CexLa1-xRu2Al10 to clarify the origin of the recently discovered mysterious phase below T0=27 K in CeRu2Al10 where a large magnetic entropy is released, however, the existence of an internal magnetic field is ruled out by 27Al-NQR measurement. We find that T0 decreases with decreasing x and disappears at x~0.45. T0 of CeRu2Al10 is suppressed down to 26 K under H=14.5 T along the a-axis. These results clearly indicate that the transition has a magnetic origin and is ascribed to the interaction between Ce ions. Considering the results of spe…
The Kondo system CeRu$_4$Sn$_6$ shows a strong anisotropy in its electric, optic and magnetic properties. We employ density functional theory plus dynamical mean field theory and show that the predominant Ce-$f$ state has total angular moment $J=5/2$ and $z$-component $m_J=\pm 1/2$ in agreement with recent X-ray absorption experiments. Even though CeRu$_4$Sn$_6$ has the direct gap of a Kondo insulator through most of the Brillouin zone it remains weakly metallic. This is because of (i) a band crossing in the $z$-direction and (ii) a negative indirect gap.
The structural, electronic and magnetic properties of the Kondo-lattice system CeRuSi2 are experimentally investigated and analyzed in the series of other ternary cerium compounds. This system is shown to be an excellent model system demonstrating coexistence of the Kondo effect and anomalous ferromagnetism with a small magnetic moment which is confirmed by magnetic and μSR measurements. Data on specific heat, resistivity and Seebeck coefficient are presented. Being deduced from the resistivity and specific heat data, the non-Fermi-liquid behavior is observed at low temperatures, which is unus…
We have investigated the local low-energy excitations in CeRu$_4$Sn$_6$, a material discussed recently in the framework of strongly correlated Weyl semimetals, by means of Ce $M_5$ resonant inelastic x-ray scattering (RIXS). The availability of both $^2$F$_\frac{5}{2}$ and $^2$F$_\frac{7}{2}$ excitations of the Ce $4f^1$ configuration in the spectra allows for the determination of the crystal-electric field parameters that explain quantitatively the temperature dependence and anisotropy of the magnetic susceptibility. The absence of an azimuthal dependence in the spectra indicates that all cry…
The filled-skutterudite compound CeRu$_4$Sb$_{12}$ shows a pseudo-gap structure in the optical conductivity spectra similar to the Kondo insulators, but metallic behavior below 80 K. The resistivity shows a large peak at 80 K, and the Seebeck coefficient is positive and also shows a large peak at nearly the same temperature. In order to explain all these features, a simplified tight-binding model, which captures the essential features of the band calculation, is proposed. Using this model and introducing the correlation effect within the framework of the dynamical mean field approximation and …
Optical conductivity [s(w)] of Ce-filled skutterudite CeRu4Sb12 has been measured at high pressure to 8 GPa and at low temperature, to probe the pressure evolution of its electronic structures. At ambient pressure, a mid-infrared peak at 0.1 eV was formed in s(w) at low temperature, and the spectral weight below 0.1 eV was strongly suppressed, due to a hybridization of the f electron and conduction electron states. With increasing external pressure, the mid-infrared peak shifts to higher energy, and the spectral weight below the peak was further depleted. The obtained spectral data are analyze…
The temperature dependence of the thermoelectric power, S(T), and the electrical resistivity of the magnetically ordered CeRu_2Ge_2 (T_N=8.55 K and T_C=7.40 K) were measured for pressures p < 16 GPa in the temperature range 1.2 K < T < 300 K. Long-range magnetic order is suppressed at a p_c of approximately 6.4 GPa. Pressure drives S(T) through a sequence of temperature dependences, ranging from a behaviour characteristic for magnetically ordered heavy fermion compounds to a typical behaviour of intermediate-valent systems. At intermediate pressures a large positive maximum develops above 10 K…
The combination of low-temperature specific-heat and nuclear-magnetic-resonance (NMR) measurements reveals important information of the ground-state properties of CeRu$_4$Sn$_6$, which has been proposed as a rare example of a tetragonal Kondo-insulator (KI). The NMR spin-latticerelaxation rate $1/T_1$ deviates from the Korringa law below 100 K signaling the onset of an energy gap $ΔE_g1/k_B \simeq 30$K. This gap is stable against magnetic fields up to 10 T. Below 10 K, however, unusual low-energy excitations of in-gap states are observed, which depend strongly on the field H. The specific heat…
We report temperature-dependent polarized optical conductivity [$σ(ω)$] spectra of CeFe$_2$Al$_{10}$, which is a reference material for CeRu$_2$Al$_{10}$ and CeOs$_2$Al$_{10}$ with an anomalous magnetic transition at 28 K. The $σ(ω)$ spectrum along the b-axis differs greatly from that in the $ac$-plane, indicating that this material has an anisotropic electronic structure. At low temperatures, in all axes, a shoulder structure due to the optical transition across the hybridization gap between the conduction band and the localized $4f$ states, namely $c$-$f$ hybridization, appears at 55 meV. Ho…
We studied the physical properties of two Kondo-lattice compounds, CeRu$_2$As$_2$ and CeIr$_2$As$_2$, by a combination of electric transport, magnetic and thermodynamic measurements. They are of ThCr$_2$Si$_2$-type and CaBe$_2$Ge$_2$-type crystalline structures, respectively. CeRu$_2$As$_2$ shows localized long-range antiferromagnetic ordering below $T_N$=4.3 K, with a moderate electronic Sommerfeld coefficient $γ_0$=35 mJ/mol$\cdot$K$^2$. A field-induced metamagnetic transition is observed near 2 T below $T_N$. Magnetic susceptibility measurements on aligned CeRu$_2$As$_2$ powders suggest tha…
Systems that have been prepared to undergo a second-order phase transition at zero Kelvin, the so-called quantum critical systems, appear to fall into two categories: (chemically) heavily-doped systems where the unusual properties can be related to a disorder-induced distribution of Kondo shielding temperatures, and (almost) stoichiometric systems where the departures from Fermi-liquid theory have been attributed to intrinsic instabilities. Here we show that this distinction is not as clear cut and that magnetic clusters associated with a distribution of Kondo shielding temperatures are also p…
Electrical resistivity $ρ$, specific heat C, and magnetic susceptibility $χ$ measurements made on the filled skutterudite CeRu_4As_{12} reveal non-Fermi liquid (NFL) T - dependences at low T, i.e., $ρ$(T) $\sim$ T^{1.4} and weak power law or logarithmic divergences in C(T)/T and $χ$(T). Measurements also show that the T - dependence of the thermoelectric power S(T) deviates from that seen in other Ce systems. The NFL behavior appears to be associated with fluctuations of the Ce valence between 3^+ and 4^+ rather than a typical Kondo lattice scenario that would be appropriate for an integral Ce…
Inelastic neutron scattering (INS) and specific heat measurements have been performed on the intermediate valence compound CeRuSi$_3$, which is isostructural to the noncentrosymmetric pressure-induced superconductors CeRhSi$_3$, CeIrSi$_3$ and CeCoGe$_3$. INS measurements at 7~K reveal a broad peak at (58.5~$\pm$~1.4)~meV, while at 300~K, broad quasielastic scattering is observed. This indicates a large Kondo temperature of $T_{\rm K}~\sim~680$~K. The magnetic contribution to the specific heat ($C_{\rm mag}$) has a value of $γ~=~62.5(1)$~mJ/mol~K$^2$ at low temperatures and above about 100~K c…
A Kondo semiconductor CeRu$_2$Al$_{10}$ with an orthorhombic crystal structure shows an unusual antiferromagnetic ordering at rather high temperature $T_0$ of 27.3 K, which is lower than the Kondo temperature $T_{\rm K}\sim$ 60 K. In optical conductivity [$σ(ω)$] spectra that directly reflect electronic structure, the $c$-$f$ hybridization gap between the conduction and $4f$ states is observed at around 40 meV along the three principal axes. However, an additional peak at around 20 meV appears only along the $b$ axis. With increasing $x$ to 0.05 in Ce(Ru$_{1-x}$Rh$_x$)$_2$Al$_{10}$, the $T_0$ …
Despite intense research in the field of strongly correlated electron behavior for the past few decades, there has been very little effort to understand this phenomenon in nano particles of the Kondo lattices. In this article, we review the results of our investigation on the fine particles (less than 1 micron) of some of the alloys obtained by high-energy ball-milling to bring out that this synthetic method paves a way to study strong electron correlations in nanocrystals of such alloys. We primarily focus on the alloys of the series, CeRu(2-x)Rh(x)Si2, lying at different positions in Doniach…
We have investigated the elastic properties of the Ce-based filled skutterudite antimonides CeRu$_{4}$Sb$_{12}$ and CeOs$_{4}$Sb$_{12}$ by means of ultrasonic measurements. CeRu$_{4}$Sb$_{12}$ shows a slight increase around 130 K in the temperature dependence of the elastic constants $C$$_{11}$, ($C$$_{11}$-$C$$_{12}$)/2 and $C$$_{44}$. No apparent softening toward low temperature due to a quadrupolar response of the 4$f$-electronic ground state of the Ce ion was observed at low temperatures. In contrast CeOs$_{4}$Sb$_{12}$ shows a pronounced elastic softening toward low temperature in the lon…
The phonon dynamics of filled skutterudite CeRu4Sb12 have been studied at room temperature by inelastic neutron scattering. Optical phonons associated with a large vibration of Ce atoms are observed at a relatively low energy of E = 6 meV, and show anticrossing behavior with acoustic phonons. We propose that the origin of the low lattice thermal conductivity in filled skutterudites can be attributed to intensive Umklapp scattering originating from low-lying optical phonons. By an analysis based on a Born-von Karman force model, the longitudinal force constants of the nearest Ce-Sb and Ce-Ru pa…
The optical conductivity in a Kondo lattice system is presented in terms of the memory function formalism. I use Kondo-lattice Hamiltonian for explicit calculations. I compute the frequency dependent imaginary part of the memory function ($M^{\p\p}(\om)$), and the real part of the memory function $M^{\p}(\om)$ by using the Kramers-Kronig transformation. Optical conductivity is computed using the generalized Drude formula. I find that high frequency tail of the optical conductivity scales as $σ(\om) \sim \frac{1}{\om}$ instead of the Drude $\frac{1}{\om^2}$ law. Such a behaviour is seen in stra…
The CeT2Al10 family of orthorhombic compounds exhibits a very peculiar evolution from a Kondo-insulator (T: Fe) to an unconventional long-range magnetic order (T: Ru, Os). Inelastic neutron scattering experiments performed on single-crystal CeFe2Al10 reveal that this material develops a spin-gap in its magnetic spectral response below ~ 50 K, with a magnetic excitation dispersing from $E = 10.2 \pm 0.5$ meV at the Y zone-boundary point [q = (0,1,0)] to $\approx 12$ meV at the top of the branch. The excitation shows a pronounced polarization of the magnetic fluctuations along a, the easy anisot…
The observation of quantum criticality in diverse classes of strongly correlated electron systems has been instrumental in establishing ordering principles, discovering new phases, and identifying the relevant degrees of freedom and interactions. At focus so far have been insulators and metals. Semimetals, which are of great current interest as candidate phases with nontrivial topology, are much less explored in experiments. Here we study the Kondo semimetal CeRu$_4$Sn$_6$ by magnetic susceptibility, specific heat, and inelastic neutron scattering experiments. The power-law divergence of the m…
The occupancy of the 4f^n contributions in the Kondo semiconductors CeM2Al10(M = Ru, Os and Fe) has been quantitatively determined by means of bulk-sensitive hard x-ray photoelectron spectroscopy (HAXPES) on the Ce 3d core levels. Combining a configuration interaction scheme with full multiplet calculations allowed to accurately describe the HAXPES data despite the presence of strong plasmon excitations in the spectra. The configuration interaction parameters obtained from this analysis -- in particular the hybridization strength V_eff and the effective f binding energy Delta_f -- indicate a s…
We study the finite-frequency inter-band transition peak in the optical conductivity of a heavy fermion system close to a Kondo breakdown quantum critical point, where the lattice Kondo temperature vanishes. As the system approaches the phase transition from the heavy Fermi liquid side, we find a new cross-over regime where the peak position is related to, but is not directly proportional to, the lattice Kondo scale. In particular, the position of the peak moves to lower energies, but remains finite at the critical point. On the other hand, the peak value changes non-monotonically and eventual…
Using real-space dynamical mean field quantum Monte Carlo simulations, we study the optical conductivity of two-dimensional topological Kondo insulating states. We consider model parameters which allow us to consider mixed valence and local moment regimes. The real space resolution inherent to our approach reveals a renormalization of the hybridization gap as one approaches the edge. Low energy transport is dominated by the helical edge state and the corresponding Drude weight scales as the coherence scale of the heavy fermion state. The concomitant renormalization of the edge state velocity l…
Recently the Ce-based caged-type compounds having general formula CeT2Al10 (T=Fe, Ru and Os) have generated considerable interest due to the Kondo semiconducting paramagnetic ground state (down to 40 mK) observed in CeFe2Al10 and anomalously high magnetic ordering temperature with a spin gap formation at low temperatures in Kondo semimetals CeRu2Al10 and CeOs2Al10. The formation of long-range magnetic ordering out of the Kondo semiconducting/semimetallic state itself is extraordinary and these are the first examples of this enigmatic coexistence of electronic ground states. These compounds als…
We present a high-resolution photoemission study on the strongly correlated Ce-compounds CeCu_6, CeCu_2Si_2, CeRu_2Si_2, CeNi_2Ge_2, and CeSi_2. Using a normalization procedure based on a division by the Fermi-Dirac distribution we get access to the spectral density of states up to an energy of 5 k_BT above the Fermi energy E_F. Thus we can resolve the Kondo resonance and the crystal field (CF) fine-structure for different temperatures above and around the Kondo temperature T_K. The CF peaks are identified with multiple Kondo resonances within the multiorbital Anderson impurity model. Our theo…
We measured the polarized optical conductivity of URu$_2$Si$_2$ from room temperature down to 5 K, covering the Kondo state, the coherent Kondo liquid regime, and the hidden-order phase. The normal state is characterized by an anisotropic behavior between the ab plane and c axis responses. The ab plane optical conductivity is strongly influenced by the formation of the coherent Kondo liquid: a sharp Drude peak develops and a hybridization gap at 12 meV leads to a spectral weight transfer to mid-infrared energies. The c axis conductivity has a different behavior: the Drude peak already exists a…
We study the density of states and the optical conductivity of a Kondo lattice which is immersed in a massless Dirac Fermi sea, as characterized by a linear dispersion relation. As a result of the hybridization $V$ with the $f$-electron levels, the pseudo-gap in the conduction band becomes duplicated and is shifted both into the upper and the lower quasiparticle band. We find that due to the linear dispersion of the Dirac fermions, the Kondo insulator gap is observable in the optical conductivity in contrast to the Kondo lattice system in a conventional conduction band, and the resulting gap\,…
The optical conductivity of the double exchange and the orbital degenerate Kondo lattice Hamiltonian (orbital t-J model) are invetigated by means of finite temperature diagonalization for different doping concentrations. The temperature dependence of sum rules, kinetic energy and Drude weight are studied. In particular it is shown that the double exchange model cannot explain the anomalous absorption in the uniform ferromagnetic phase of manganites discovered Okimoto et al. [Phys. Rev. Lett. 75, 109 (1995)]. The orbital t-J model explains the increase of the anomalous absorption with decreasin…
Optical reflectivity experiments have been conducted on single crystals of the Kondo insulator YbB_12 in order to obtain its optical conductivity, σ(ω). Upon cooling below 70 K, a strong supression of σ(ω) is seen in the far-infrared region, indicating the opening of an energy gap of ~ 25 meV. This gap development is coincident with a rapid decrease in the magnetic susceptibility, which shows that the gap opening has significant influence on magnetic properties. A narrow, asymmetric peak is observed at ~40 meV in σ(ω), which is attributed to optical transitions between the Yb 4f-derived states…
Recently, nonlinear responses have been actively studied in both experiments and theory. Particularly interesting are inversion-symmetry broken systems, where an even-order nonlinear electrical conductivity can be nonzero, resulting in nonreciprocity. Second-order nonlinear conductivities attract much attention because of their sensitivity to detect inversion-symmetry breaking in materials and their functionalities. However, while the nonlinear response has been actively studied in noninteracting systems for a long time, the nonlinear response in strongly correlated materials is still poorly u…
We utilize terahertz time domain spectroscopy to investigate thin films of the heavy fermion compound SmB6, a prototype Kondo insulator. Temperature dependent terahertz (THz) conductivity measurements reveal a rapid decrease in the Drude weight and carrier scattering rate at ~T*=20 K, well below the hybridization gap onset temperature (100 K). Moreover, a low-temperature conductivity plateau (below 20K) indicates the emergence of a surface state with an effective electron mass of 0.1me. Conductivity dynamics following optical excitation are also measured and interpreted using Rothwarf-Taylor (…
We report on the observation of large, step-like anomalies in the magnetization ($ΔM = 0.74$\,$μ_{\rm B}$/Ce), in the magnetostriction ($Δl/l_{0} = 2.0 \cdot 10^{-3}$), and in the magnetoresistance in polycrystals of the paramagnetic heavy-fermion system CeTiGe at a critical magnetic field $μ_0 H_c \approx $ 12.5\,T at low temperatures. The size of these anomalies is much larger than those reported for the prototypical heavy-fermion metamagnet CeRu$_2$Si$_2$. Furthermore, hysteresis between increasing and decreasing field data indicate a real thermodynamic, first-order type of phase transition…
Samarium hexaboride (SmB6) has recently been considered to be a topological Kondo insulator (TKI), the first strongly correlated electron system to exhibit topological surface conduction states. In this contribution, results of electrical resistivity measurements between 80 K and 0.08 K of various SmB6 single crystalline samples are presented, analyzed and discussed. The received results imply that the residual conductivity of SmB6 below about 4 K is of non-activated (metallic-like) nature. It is shown that this metallic-like behavior can be attributed both to surface (2D) conduction states, a…
We study the quantum correction to conductivity on the surface of cubic topological Kondo insulators with multiple Dirac bands. We consider the model of time-reversal invariant disorder which induces the scattering of the electrons within the Dirac bands as well as between the bands. When only intraband scattering is present we find three long-range diffusion modes which lead to weak antilocalization correction to conductivity, which remains independent of the microscopic details such as Fermi velocities and relaxation times. Interband scattering gaps out two diffusion modes leaving only one l…
We study both the electrical and thermal transport near the heavy-fermion quantum critical point (QCP), identified with the breakdown of the Kondo effect as an orbital selective Mott transition. We show that the contribution to the electrical conductivity comes mainly from conduction electrons while the thermal conductivity is given by both conduction electrons and localized fermions (spinons), scattered with dynamical exponent $z = 3$. This scattering mechanism gives rise to a quasi-linear temperature dependence of the electrical and thermal resistivity. The characteristic feature of the Kond…
We have observed the metal-insulator transition in the strongly correlated insulator FeSi with the chemical substitution of Al at the Si site. The magnetic susceptibility, heat capacity, and field dependent conductivity are measured for Al concentrations ranging from 0 to 0.08. For concentrations greater than 0.01 we find metallic properties quantitatively similar to those measured in Si:P with the exception of a greatly enhanced quasiparticle mass. Below 2 K the temperature and field dependent conductivity can be completely described by the theory of disordered Fermi Liquids.
We study the antiferromagnetic phase of the Kondo lattice model on bipartite lattices at half-filling using the dynamical mean-field theory with numerical renormalization group as the impurity solver, focusing on the detailed structure of the spectral function, self-energy, and optical conductivity. We discuss the deviations from the simple hybridization picture, which adequately describes the overall band structure of the system (four quasiparticle branches in the reduced Brillouin zone), but neglects all effects of the inelastic-scattering processes. These lead to additional structure inside…
Recent experiments on Kondo insulator SmB6 have revealed bulk excitations that could be Fermions without charge. This would lead to a residual linear term in the thermal conductivity that was not observed universally in experiments. To solve this mystery, we introduce a novel symmetric geometry for performing thermal transport measurements under strains. Magnetic-field-independent residual linear terms were found in flux-grown SmB6 at nominal zero strain, and they grew under a tensile strain, which is known to modify Sm valence. A small growth-dependent difference in the Sm valence might expla…
We present low-temperature heat and charge transport as well as caloric properties of a ThAsSe single crystal. An extra -AT1/2 term in the electrical resistivity, independent of magnetic fields as high as 14 T, provides evidence for an unusual scattering of conduction electrons. Additionally, both the thermal conductivity and specific heat show a glass-type temperature dependence which signifies the presence of tunneling states. These observations apparently point to an experimantal realization of a two-channel Kondo effect derived from structural two-level systems.
The Kondo insulator SmB6 has long been known to exhibit low temperature (T < 10K) transport anomaly and has recently attracted attention as a new topological insulator candidate. By combining low-temperature and high energy-momentum resolution of the laser-based ARPES technique, for the first time, we probe the surface electronic structure of the anomalous conductivity regime. We observe that the bulk bands exhibit a Kondo gap of 14 meV and identify in-gap low-lying states within a 4 meV window of the Fermi level on the (001)-surface of this material. The low-lying states are found to form ele…
In this paper we use the semiclassical Boltzmann equation to investigate the transport properties of Dirac fermion on the surface of topological insulator with magnetic impurities. The results obtained show that there is also a minimal conductivity in this system as in graphene. We also argue the low temperature transport property, and find that there is no low temperature anomaly known as Kondo effect when the temperature is T>10^{-6}K.
The Kondo insulator SmB$_6$ has long been known to display anomalous transport behavior at low temperatures, T$<5$ K. In this temperatures range, a plateau is observed in the dc resistivity, contrary to the exponential divergence expected for a gapped system. Recent theoretical calculations suggest that SmB$_6$ may be the first topological Kondo insulator (TKI) and propose that the residual conductivity is due to topological surface states which reside within the Kondo gap. Since the TKI prediction many experiments have claimed to observe high mobility surface states within a perfectly insulat…
The microscopic theory of interaction between the heavy fermions and the crystal field excitations in Kondo lattices is presented. It is shown that the heavy-fermion spectrum scaled by the Kondo temperature $T_K$ can be modified by the crystal field excitations with the energy $Δ_{CF}$ provided the inequality $Δ_{CF}<T_K$ is realized. On the base of general description of excitation spectrum the detailed qualitative and quantitative explanation of anisotropic inelastic neutron scattering spectra and low-temperature specific heat of orthorhombic CeNiSn is given. The theory resolves the apparent…
The interaction between strong correlation and Berry curvature is an open territory of in the field of quantum materials. Here we report large anomalous Hall conductivity in a Kondo lattice ferromagnet USbTe which is dominated by intrinsic Berry curvature at low temperatures. However, the Berry curvature induced anomalous Hall effect does not follow the scaling relation derived from Fermi liquid theory. The onset of the Berry curvature contribution coincides with the Kondo coherent temperature. Combined with ARPES measurement and DMFT calculations, this strongly indicates that Berry curvature …
The Drude model describes the free-electron conduction in simple metals, governed by the freedom that the mobile electrons have within the material. In strongly correlated systems, however, a significant deviation of the optical conductivity from the simple metallic Drude behavior is observed. Here, we investigate the optical conductivity of the heavy-fermion system CeCu$_{\mathrm{6-x}}$Au$_{\mathrm{x}}$, using time-resolved, phase-sensitive terahertz spectroscopy. Terahertz electric field creates two types of excitations in heavy-fermion materials: First, the intraband excitations that leave …
We report the anisotropic changes in the electronic structure of a Kondo semiconductor CeOs$_2$Al$_{10}$ across an anomalous antiferromagnetic ordering temperature ($T_0$) of 29 K, using optical conductivity spectra. The spectra along the $a$- and $c$-axes indicate that a $c$-$f$ hybridization gap emerges from a higher temperature continuously across $T_0$. Along the b-axis, on the other hand, a different energy gap with a peak at 20 meV appears below 39 K, which is higher temperature than $T_0$, because of structural distortion. The onset of the energy gap becomes visible below $T_0$. Our obs…
We present time-domain THz spectroscopy data of a thin film of the Kondo-lattice antiferromagnet CeCu$_2$Ge$_2$. The low frequency complex conductivity has been obtained down to temperatures below the onset of magnetic order. At low temperatures a narrow Drude-like peak forms, which is similar to ones found in other heavy fermion compounds that do not exhibit magnetic order. Using this data in conjunction with DC resistivity measurements, we obtain the frequency dependence of the scattering rate and effective mass through an extended Drude model analysis. The zero frequency limit of this analy…
The interplay of Kondo screening and magnetic ordering in strongly correlated materials containing local moments is a subtle problem.[1] Usually the number of conduction electrons matches or exceeds the number of moments, and a Kondo-screened heavy Fermi liquid develops at low temperatures.[2] Changing the pressure, magnetic field, or chemical doping can displace this heavy Fermi liquid in favor of a magnetically ordered state.[3,4] Here we report the discovery of a version of such a `Kondo lattice' material, YbIr$_3$Si$_7$, in which the number of free charge carriers is much less than the num…
We present a study of the resonant inelastic scattering response of \ybin\ excited at the tender Yb $M_5$ X-ray edge. In the high-temperature, paramagnetic phase, we observe a multiplet structure which can be understood at an ionic level. Upon cooling through the valence transition at $T_v\sim$ 40$K$, we observe a strong renormalization of the low-energy spectra, indicating a sensitivity to the formation of an intermediate valence phase at low temperatures. Similar spectrum renormalization has been observed in the optical conductivity, which suggests that the low-energy electronic structure po…
Kondo-based semimetals and semiconductors are of extensive current interest as a viable platform for strongly correlated states. It is thus important to understand the routes towards such dilute-carrier correlated states. One established pathway is through Kondo effect in metallic non-magnetic analogues. Here we advance a new mechanism, through which Kondo-based semimetals develop out of conduction electrons with a low carrier-density in the presence of an even number of rare-earth sites. We demonstrate this effect by studying the Kondo material Yb3Ir4Ge13 along with its closed-f-shell counter…
Phase diagram of an Ising-spin Kondo lattice model on a kagome lattice is investigated by a Monte Carlo simulation. We find that the system exhibits a peculiar ferrimagnetic state at a finite temperature, in which each triangle is in a two-up one-down spin configuration but the spin correlation does not develop any superstructure. We call this state the loop liquid, as it is characterized by the emergent degree of freedom, self-avoiding up-spin loops. We elucidate that the system shows phase transitions from the loop liquid to ferrimagnetically ordered states and a crossover to a partially fer…
Optical conductivity [$σ(ω)$] of the Kondo semiconductor YbB$_{12}$ has been measured over wide ranges of temperature ($T$=8$-$690 K) and photon energy ($\hbar ω\geq$ 1.3 meV). The $σ(ω)$ data reveal the entire crossover of YbB$_{12}$ from a metallic electronic structure at high $T$ into a semiconducting one at low $T$. Associated with the gap development in $σ(ω)$, a clear onset is newly found at $\hbarω$=15 meV for $T \leq$ 20 K. The onset energy is identified as the gap width of YbB$_{12}$ appearing in $σ(ω)$. This gap in σ(ω)$ is interpreted as the indirect gap, which has been predicted in…
