Kondo Temperature Research Articles

Crossover from Kondo to Fermi-liquid behavior induced by high magnetic field in 1T−VTe2 single crystals

The magnetic and magnetotransport properties of metallic $1T\text{\ensuremath{-}}{\mathrm{VTe}}_{2}$ single crystals were investigated at temperatures from 1.3 to 300 K and in magnetic fields up to 35 T. Upon applying a high magnetic field, it is found that the electrical resistivity displays a crossover from the logarithmic divergence of the single-impurity Kondo effect to the Fermi liquid behavior at low temperatures. The Brillouin scale of the negative magnetoresistivity above the Kondo temperature ${T}_{\mathrm{K}}$ = 12 K indicates that the Kondo features originate from intercalated V ions, with $S$ = 1/2. Both magnetic susceptibility and Hall effect show an anomaly around ${T}_{\mathrm{K}}$. By using the modified Hamann expression we successfully describe the temperature-dependent resistivity under various magnetic fields, which shows the characteristic peak below ${T}_{\mathrm{K}}$ due to the splitting of the Kondo resonance.

Universality and thermoelectric transport properties of quantum dot systems

We discuss the temperature-dependent thermoelectric transport properties of semiconductor nanostructures comprising a quantum dot coupled to quantum wires: the thermal dependence of the electrical conductance, thermal conductance, and thermopower. We explore the universality of the thermoelectric properties in the temperature range associated with the Kondo crossover. In this thermal range, general arguments indicate that any equilibrium property's temperature dependence should be a universal function of the ratio ${T}^{*}=T/{T}_{K}$, where ${T}_{K}$ is the Kondo temperature. Considering the particle-hole symmetric, spin-degenerate Anderson model, the zero-bias electrical conductance has already been shown to map linearly onto a universal conductance through a quantum dot embedded or side-coupled to a quantum wire. Employing rigorous renormalization-group arguments, we calculate universal thermoelectric transport coefficients that allow us to extend this result to the thermopower and the thermal conductance. We present numerical renormalization-group results to illustrate the physics in our findings. Applying the universal thermoelectric coefficients to recent experimental results of the electrical conductance and thermovoltages versus ${V}_{\mathrm{gate}}$, at different temperatures in the Kondo regime, we calculate all the thermoelectric properties and obtain simple analytical fitting functions that can be used to predict the experimental results of these properties. However, we cannot check all of them, due to the lack of available experimental results over a broad temperature range.

Fingerprints of the Local Moment Formation and its Kondo Screening in the Generalized Susceptibilities of Many-Electron Problems.

We identify the precise hallmarks of the local magnetic moment formation and its Kondo screening in the frequency structure of the generalized charge susceptibility. The sharpness of our identification even pinpoints an alternative criterion to determine the Kondo temperature of strongly correlated systems on the two-particle level, which only requires calculations at the lowest Matsubara frequency. We showcase its strength by applying it to the single impurity and the periodic Anderson model as well as to the Hubbard model. Our results represent a significant progress for the general understanding of quantum field theory at the two-particle level and allow for tracing the limits of the physics captured by perturbative approaches for correlated systems.

Kondo screening regimes in multi-Dirac and Weyl systems

We have investigated the Kondo physics of a single magnetic impurity embedded in multi-Dirac (Weyl) node fermionic systems. By using a generic effective model for the host material and employing a numerical renormalization group approach we access the low temperature behavior of the system, identifying the existence of Kondo screening in single-, double-, and triple-Dirac (Weyl) node models. We find that in any multi-Dirac node systems the low-energy regime lies within one of the known classes of pseudogap Kondo problem, extensively studied in the literature. Kondo screening is also observed for time reversal symmetry broken Weyl systems. This is, however, possible only in the particle-hole symmetry broken regime obtained for finite chemical potential $\mu$. Although weakly, breaking time-reversal symmetry suppresses the Kondo resonance, especially in the single-node Weyl semimetals. More interesting Kondo screening regimes are obtained for inversion symmetry broken multi-Weyl fermions. In these systems the Kondo regimes of double- and triple-Weyl node models are much richer than in the single-Weyl node model. While in the single-Weyl node model the Kondo temperature increases monotonically with $|\mu|$ regardless the value of the inversion symmetry breaking parameter $Q_0$, in double- and triple-Weyl node models there are two distinct regimes: (i) For $Q_0< |\mu|$ the Kondo temperature depends strongly on $\mu$, while (ii) for $Q_0 > |\mu|$ the Kondo temperature depends very weakly on $\mu$, resembling the flat-band single impurity Anderson model.

Evolution of lattice coherence in the intermediate-valence heavy-fermion compound EuNi2P2 studied by point contact spectroscopy

We performed a point contact spectroscopy study in an intermediate-valence heavy-fermion (HF) compound $\mathrm{Eu}{\mathrm{Ni}}_{2}{\mathrm{P}}_{2}$. Above the Kondo temperature ${T}_{\mathrm{K}}$, the differential conductance spectra show a broad peak due to the Kondo resonance at the Eu site. Below ${T}_{\text{K}}$, the broad peak splits into two peaks that can be reproduced by the summation of two Fano curves with the same parameters, indicating the emergence of the indirect hybridization gap in an Anderson lattice. With decreasing temperature, the separation between the two peaks increases and saturates at very low temperatures. The results reveal the evolution of the electronic structure in the HF state due to the development of the lattice coherence.

Spectral and transport properties of a half-filled Anderson impurity coupled to phase-biased superconducting and metallic leads

We derive and apply a general scheme for mapping a setup consisting of a half-filled single-level quantum dot coupled to one normal metallic and two superconducting phase-biased leads onto an ordinary half-filled single impurity Anderson model with single modified tunneling density of states. The theory allows for the otherwise unfeasible application of the standard numerical renormalization group and enables us to obtain phase-dependent local spectral properties as well as phase-dependent induced pairing and Josephson current. The resulting transport properties match well with the numerically exact continuous-time hybridization-expansion quantum Monte Carlo. For weakly coupled normal electrode, the spectral properties can be interpreted in terms of normal-electrode-broadened Andreev bound states with phase-dependent position analogous to the superconducting Anderson model, which coexist in the $\ensuremath{\pi}$-like phase with a Kondo peak whose phase-dependent Kondo temperature is extracted.

Temperature-dependent angle-resolved photoemission spectroscopy study of the Ce 4f states in a possible topological Kondo insulator CeRhAs

The electronic structure of a possible topological Kondo insulator of CeRhAs has been investigated by employing temperature $(T)$-dependent angle-resolved photoemission spectroscopy (ARPES). Fermi surfaces (FSs) and band structures are successfully measured for three orthogonal crystallographic planes. The measured Fermi-edge states are found to have a three-dimensional (3D) character, contradictory to the proposed topological surface states of the $2\text{D}$ character. The measured FSs due to the Ce $4f$ electrons agree well with those from the density functional theory band structures unfolded into the reduced Ce-only unit cell. The theoretically predicted hourglass-type bulk bands along $\overline{X}\overline{S}\overline{X}$ are not clearly resolved, in spite of the evident existence of those band features. $T$-dependent ARPES measurements reveal that the coherent Ce $4f$ states, having two pseudogap structures of ${\mathrm{\ensuremath{\Delta}}}_{1}\ensuremath{\sim}80$ meV and ${\mathrm{\ensuremath{\Delta}}}_{2}\ensuremath{\sim}30$ meV, persist to remain above 200 K, in agreement with the high Kondo temperature of CeRhAs.

Possible multiorbital ground state in CeCu2Si2

The crystal-field ground state wave function of CeCu$_2$Si$_2$ has been investigated with linear polarized $M$-edge x-ray absorption spectroscopy from 250mK to 250K, thus covering the superconducting ($T_{\text{c}}$=0.6K), the Kondo ($T_{\text{K}}$$\approx$20K) as well as the Curie-Weiss regime. The comparison with full-multiplet calculations shows that the temperature dependence of the experimental linear dichroism is well explained with a $\Gamma_7^{(1)}$ crystal-field ground-state and the thermal population of excited states at around 30meV. The crystal-field scheme does not change throughout the entire temperature range thus making the scenario of orbital switching unlikely. Spectroscopic evidence for the presence of the Ce 4$f^0$ configuration in the ground state is consistent with the possibility for a multi-orbital character of the ground state. We estimate from the Kondo temperature and crystal-field splitting energies that several percents of the higher lying $\Gamma_6$ state and $\Gamma_7^{(2)}$ crystal-field states are mixed into the primarily $\Gamma_7^{(1)}$ ground state. This estimate is also supported by re-normalized band-structure calculations that uses the experimentally determined crystal-field scheme.

Magnetic Moments Induced by Atomic Vacancies in Transition Metal Dichalcogenide Flakes.

2D magnetism plays a key role in both fundamental physics and potential device applications. However, the instability of the discovered 2D magnetic materials has been one main obstacle in deep research and potential application of 2D magnetism. Here, a localized magnetic moment induced by Pt vacancies in air-stable type-II Dirac semimetal PtSe2 flakes is reported. The localized magnetic moments give rise to the Kondo effect, evidenced by logarithmic increment of resistance with decreasing temperature and isotropic negative longitudinal magnetoresistance. Additionally, the induced magnetic moment and Kondo temperature appear to depend on thickness in the thinner samples (<10nm). The small magnetocrystalline anisotropy revealed by first-principles calculation indicates that the magnetic moments are randomly localized instead of long-range ordered. The findings demonstrate a new means to induce magnetism in 2D non-magnetic materials.

Thermally Reduced Soft Magnetic CuFe Nanoparticles for High-Performance Electrical Devices

Developing economically soft magnetic materials for high-performance electrical devices is indispensable. Here, we present the structural and magnetic properties of thermally reduced soft CuFe nanoparticles. The fcc cubic structure of iron-rich Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">37</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">63</sub> and their composition was confirmed by Rietveld refinement. Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">37</sub> Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">63</sub> nanoparticles exhibited high saturation magnetization and coercivity of 127 emu/g (142 emu/g) and 4.3 Oe (31 Oe), respectively, at 300 K (5 K). They showed transitions at ~34 and ~249 K due to the Kondo temperature of CuFe and minor fraction of CuFe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sub> , respectively. The exchange coupling between Cu and Fe was not significant, as demonstrated by field-cooled magnetization curves at 5 K. The magnetocaloric effect (MCE) in the range of fields and temperatures was estimated whereas the maximum MCE of -8.71 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> J.kg <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> .K <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> was achieved at 222 K. These soft magnetic materials, which exhibited stable high saturation magnetization with less heating effect during magnetization and demagnetization cycles, would be suitable candidates for magnetic applications.

Phonons, Q -dependent Kondo spin fluctuations, and 4f phonon resonance in YbAl3

The intermediate valence (IV) compound YbAl$_3$ exhibits nonintegral valence (Yb 4$f^{14-n_f}$ (5d6s)$^z$ where z = 2+n$_f$ = 2.75) in a moderately heavy (m* = 20-30me) ground state with a large Kondo temperature (T$_K$ ~ 500-600K). We have measured the magnetic fluctuations and the phonon spectra on single crystals of this material by time-of-flight inelastic neutron scattering (INS) and inelastic x-ray scattering (IXS). We find that at low temperature, the Kondo-scale spin fluctuations have a momentum (Q) dependence similar to that seen recently in the IV compound CePd$_3$ and which can be attributed to particle-hole excitations in a coherent itinerant 4$f$ correlated ground state. The Q-dependence disappears as the temperature is raised towards room temperature and the 4$f$ electron band states become increasingly incoherent. The measured phonons can be described adequately by a calculation based on standard DFT+$U$ density functional theory, without recourse to considering 4$f$ correlations dynamically. A low temperature magnetic peak observed in the neutron scattering at ~ 30meV shows dispersion identical to an optic phonon branch. This 4$f$/phonon resonance disappears for T > 150K. The phonons appear to remain unaffected by the resonance. We discuss several possibilities for the origin of this unusual excitation, including the idea that it arises from the large amplitude beating of the light Al atoms against the heavy Yb atoms, resulting in a dynamic 4$f$/3$p$ hybridization.

Heat flow enhancement in a nanoscale plasmonic junction induced by Kondo resonances and electron-phonon coupling

Recently, we showed that plasmon-exciton coupling can increase entropy current through a bridge coupled to plasmonic metal nanoparticles. Here we show that electron-phonon coupling can also be used to control the entropy current in similar systems. Entropy current tends to decrease due to electron-phonon coupling and to exhibit a monotonous decrease upon temperature ramping. However, an anomaly affecting the current where it is enhanced by electron-phonon coupling is indicated at around 42 times the system's Kondo temperature. We therefore report means to control heat flow by tuning the Kondo resonance through the electron-phonon coupling. We analyze the conditions that bring about these trends due to electron-phonon coupling by employing non-equilibrium Green's function formulation addressing the entropy current and the derived heat flow.

Tunable Magnetic Scattering Effects at the LaAlO3/SrTiO3 Interface by Ionic Liquid Gating

The gating effect achieved by an ionic liquid and its electric double layer allows for charge transfer, which can be an order of magnitude larger than that with conventional dielectrics. However, large charged ions also cause inevitable Coulomb scattering in the conducting channel formed at the interface, which can limit the carrier mobility enhancement. In this work, we study the effect of LaAlO3 thickness on the transport properties in LaAlO3/SrTiO3 heterostructures by ionic liquid gating. We find that the transport properties of the LaAlO3/SrTiO3 interface are dominated by intrinsic interactions rather than LaAlO3 thickness and possible effects of ions in the liquid. We observe a Kondo effect, which is enhanced while increasing the gate voltage. For the smallest thickness of the LAO layer, we also observe a gate-tunable and temperature-dependent anomalous Hall effect, which always emerges near the Kondo temperature. Our experiments pave the way to manipulate the various magnetic interactions at the LaAlO3/SrTiO3 interface.

Curie-Weiss susceptibility in strongly correlated electron systems

The genesis of the Curie-Weiss magnetic response observed in most transition metals that are Fermi liquids at low temperatures has been an enigma for decades and has not yet been fully explained from microscopic principles. We show on the single-impurity Anderson model how the quantum dynamics of strong electron correlations leads to the Curie-Weiss magnetic susceptibility sufficiently above the Kondo temperature. Such behavior has not yet been demonstrated and can be observed only when the bare interaction is substantially screened (renormalized) and a balance between quantum and thermal fluctuations is kept. We set quantitative criteria for the existence of the Curie-Weiss law.

Observation of tunable single-atom Yu-Shiba-Rusinov states

The coupling of a spin to an underlying substrate is the basis for a plethora of phenomena. In the case of a metallic substrate, Kondo screening of the adatom magnetic moment can occur. As the substrate turns superconducting, an intriguing situation emerges where the pair breaking due to the adatom spins leads to Yu-Shiba-Rusinov bound states, but also intertwines with Kondo phenomena. Through scanning tunneling spectroscopy, we analyze the interdependence of Kondo screening and superconductivity. Our data obtained on single Fe adatoms on Nb(110) show that the coupling and the resulting YSR states are strongly adsorption site-dependent and reveal a quantum phase transition at a Kondo temperature comparable to the superconducting gap. The experimental signatures are rationalized by combined density functional theory and continuous-time quantum Monte-Carlo calculations to rigorously treat magnetic and hybridization effects on equal footing.

Vibron quasi-bound state stability for tetragonal CeCuxAg1-xAl3

The vibron quasi-bound state stability was analyzed for the series of compounds CeCuxAg1-xAl3, 0.2 < x ≤ 1 (tetragonal phase) by using heat capacity and inelastic neutron scattering. The measurements of heat capacity show a significative reduction of ordering temperatures from 2.3 K at CeCu0.85Ag0.15Al3 down to 1.6 K for CeCu0.7Ag0.3Al3, increasing again up to 2.8 K for CeCu0.3Ag0.7Al3, while Kondo temperatures change from 9 K to 7 K. The electronic contribution of heat capacity, γ, slightly increases with unit-cell volume from 200 mJ/mol-K2 (x = 0.85) to 210 mJ/mol-K2 (x = 0.3), indicating small changes in the density of electronic states at Fermi level. The measurements of inelastic neutron scattering indicate the existence of two underlaying crystal-field excitations for cerium ions in paramagnetic regime well described according to Vegard’s law while the volume of the series CeCuxAg1-xAl3 compounds is increased. Additionally, a new magnetic excitation is found only in CeCu0.85Ag0.15Al3 and CeCu0.7Ag0.3Al3 compounds that comes from the coupling between a crystal field excitation and a single “orthorhombic” phonon mode, via single ion magnetoelastic interaction. The intensity of this coupling increases from 0.4 meV/ion for CeCuAl3 to 0.7 meV/ion for CeCu0.7Ag0.3Al3, but after, it fades away for CeCu0.3Ag0.7Al3. The intensity of this coupling is highly dependent on the energy resonance between phonon mode and crystal-field excitation, but also on the strength of the oscillator that vanishes when the volume of the unit-cell increases sufficiently. Furthermore, the existence of this coupling suggests the presence of an effective attraction between pairs of 4f-electrons, which would explain the reduction of ordering temperatures for compounds with copper composition around 50%.

Kondo effect at ultimate atomic-scale two-dimensional limit: Au/Si(111) 3×3 reconstruction with embedded Cr atoms

The occurrence of the Kondo effect in the ultimate two-dimensional (2D) case, when the thickness of the conducting film is reduced down to the atomic-scale limit, was explored taking the Tl-adsorbed Au/Si(111)$\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$ reconstruction with embedded Cr atoms as a prototype system. The reconstruction is essentially an atomic-layer film, which resides on a semiconductor Si(111) substrate and demonstrates conductivity of a metallic type, at least down to 1.8 K. The tight binding of Cr atoms in the interstitial sites surrounded by six Si atoms prevents them from clustering. The Kondo effect in this system was investigated experimentally using low-temperature scanning tunneling spectroscopy and electronic transport measurements. Both techniques yield a Kondo temperature of $\ensuremath{\sim}70--80\phantom{\rule{0.28em}{0ex}}\mathrm{K}$. A substantial increase of the sample resistivity caused by adding Cr atoms is believed to be a specific feature of the extremely thin metallic films, where small amounts of magnetic impurities can effectively block off the narrow conduction channel.

Protracted Kondo screening and kagome bands in the heavy-fermion metal Ce3Al

Ce$_{3}$Al is an archetypal heavy-fermion compound with multiple crystalline phases. Here, we try to investigate its electronic structures in the hexagonal phase ($\alpha$-Ce$_{3}$Al) and cubic phase ($\beta$-Ce$_{3}$Al) by means of a combination of density functional theory and single-site dynamical mean-field theory. We confirm that the 4$f$ valence electrons in both phases are itinerant, accompanied with strong valence state fluctuations. Their 4$f$ band structures are heavily renormalized by electronic correlations, resulting in large effective electron masses. The Kondo screening in Ce$_{3}$Al would be protracted over a wide range of temperature since the single-impurity Kondo temperature $T_{K}$ is much higher than the coherent Kondo temperature $T^{*}_{K}$. Especially, the crystal structure of $\alpha$-Ce$_{3}$Al forms a layered kagome lattice. We observe conspicuous kagome-derived flat bands and Dirac cones (or gaps) in its quasiparticle band structure. Therefore, it is concluded that the hexagonal phase of Ce$_{3}$Al will be a promising candidate of heavy-fermion kagome metal.

Tunable Kondo Resonance at a Pristine Two-Dimensional Dirac Semimetal on a Kondo Insulator.

The proximity of two different materials leads to an intricate coupling of quasiparticles so that an unprecedented electronic state is often realized at the interface. Here, we demonstrate a resonance-type many-body ground state in graphene, a nonmagnetic two-dimensional Dirac semimetal, when grown on SmB6, a Kondo insulator, via thermal decomposition of fullerene molecules. This ground state is typically observed in three-dimensional magnetic materials with correlated electrons. Above the characteristic Kondo temperature of the substrate, the electron band structure of pristine graphene remains almost intact. As temperature decreases, however, the Dirac Fermions of graphene become hybridized with the Sm 4f states. Remarkable enhancement of the hybridization and Kondo resonance is observed with further cooling and increasing charge-carrier density of graphene, evidencing the Kondo screening of the Sm 4f local magnetic moment by the conduction electrons of graphene at the interface. These findings manifest the realization of the Kondo effect in graphene by the proximity of SmB6 that is tuned by the temperature and charge-carrier density of graphene.

Field-induced SU(4) to SU(2) Kondo crossover in a half-filling nanotube dot: Spectral and finite-temperature properties

We study finite-temperature properties of the Kondo effect in a carbon nanotube (CNT) quantum dot using the Wilson numerical renormalization group (NRG). In the absence of magnetic fields, four degenerate energy levels of the CNT consisting of spin and orbital degrees of freedom give rise to the SU(4) Kondo effect. We revisit the universal scaling behavior of the SU(4) conductance for quarter- and half-filling in a wide temperature range. We find that the filling dependence of the universal scaling behavior at low temperatures $T$ can be explained clearly with an extended Fermi-liquid theory. This theory clarifies that a $T^{2}$ coefficient of conductance becomes zero at quarter-filling whereas the coefficient at half-filling is finite. We also study a field-induced crossover from the SU(4) to SU(2) Kondo state observed at the half-filled CNT dot. The crossover is caused by the matching of the spin and orbital Zeeman splittings, which lock two levels among the four at the Fermi level even in magnetic fields $B$. We find that the conductance shows the SU($4$) scaling behavior at $\mu_{B}B<k_{B}T_{K}^{\mathrm{SU(4)}}$ and it exhibits the SU($2$) universality at $\mu_{B}B\gg k_{B}T_{K}^{\mathrm{SU(4)}}$, where $T_{K}^{\mathrm{SU(4)}}$ is the SU($4$) Kondo temperature. To clarify how the excited states evolve along the SU(4) to SU(2) crossover, we also calculate the spectral function. The results show that the Kondo resonance width of the two states locked at the Fermi level becomes sharper with increasing fields. The spectral peaks of the other two levels moving away from the Fermi level merge with atomic limit peaks for $\mu_{B}B \gtrsim k_{B}T_{K}^{\mathrm{SU(4)}}$.