The layered perovskite Sr$_2$RuO$_4$ is a most intensively studied superconductor, but its pairing mechanism, which is often coupled intimately with magnetic fluctuations in correlated materials, is still an open question. Here we present a systematic evolution of the Seebeck coefficient in Co- and Mn-substituted Sr$_2$RuO$_4$ single crystals, in which ferromagnetic and antiferromagnetic glassy states respectively emerge in proximity to the superconducting phase of the parent compound. We find that the Seebeck coefficient $S$ divided by temperature $T$, $S/T$, shows a maximum near characteristic temperatures seen in the irreversible magnetization $M_{\rm ir}$ in both of the Co- and Mn-substituted crystals, demonstrating both of the ferromagnetic and antiferromagnetic fluctuations to enhance the Seebeck coefficient. Interestingly, $S/T$ increases with lowering temperature in the parent compound, reminiscent of non-Fermi-liquid behavior, indicating an essential role of coexisting ferromagnetic and antiferromagnetic fluctuations for the itinerant electrons in Sr$_2$RuO$_4$.
We study the role of charge density-wave fluctuations on the temperature dependence of Seebeck coefficient in quasi-one dimensional conductors with a Peierls instability. The description of low-dimensional incommensurate charge density-wave fluctuations as obtained by a generalized Ginzburg-Landau approach for arrays of weakly coupled chains is embodied in the numerical solution of the semi-classical Boltzmann transport equation. The energy and temperature dependence of the scattering time of electrons on fluctuations can then be extracted and its influence on the Seebeck coefficient calculate…
We have systematically measured the transport properties in the layered rhodium oxide K$_{x}$RhO$_{2}$ single crystals ($0.5\lesssim x \lesssim 0.67$), which is isostructural to the thermoelectric oxide Na$_{x}$CoO$_{2}$. We find that below $x = 0.64$ the Seebeck coefficient is anomalously enhanced at low temperatures with increasing $x$, while it is proportional to the temperature like a conventional metal above $x=0.65$, suggesting an existence of a critical content $x^{*} \simeq 0.65$. For the origin of this anomalous behavior, we discuss a filling-induced Lifshitz transition, which is char…
We present a comprehensive transport investigation of the itinerant antiferromagnet Mn3Si which undergoes a spin density wave (SDW) order below T_N~21.3K. The electrical resistivity, the Hall-, Seebeck and Nernst effects exhibit pronounced anomalies at the SDW transition, while the heat conductivity is phonon dominated and therefore is insensitive to the intrinsic electronic ordering in this compound. At temperatures higher than T_N our data provide strong evidence for a large fluctuation regime which extends up to ~200K in the resistivity, the Seebeck effect and the Nernst effect. From the co…
It has been proposed that paramagnetic materials exhibit a unique thermoelectric effect near the ferromagnetic transition point due to spin fluctuations. This phenomenon is often referred to as paramagnon drag. We calculate the contribution of this paramagnon drag to the Seebeck coefficient microscopically on the basis of the linear response theory. This leed to a general formula for the contribution to the Seebeck coefficient due to the paramagnon drag, and we clarify the conditions in which the Seebeck coefficient enhances near the ferromagnetic transition point for a single-band and isotrop…
We study the influence of inelastic electron-electron scattering on the temperature variation of the Seebeck coefficient in the normal phase of quasi-one-dimensional organic superconductors. The theory is based on the numerical solution of the semi-classical Boltzmann equation for which the collision integral equation is solved with the aid of the electronic umklapp scattering vertex calculated by the renormalization group method. We show that the one-loop renormalization group flow of momentum and temperature dependent umklapp scattering, in the presence of nesting alterations of the Fermi su…
We investigate theoretically the Seebeck effect in materials close to a ferromagnetic quantum critical point to explain anomalous behaviour at low temperatures. It is found that the main effect of spin fluctuations is to enhance the coefficient of the leading $T$-linear term, and a quantum critical behaviour characterized by a spin-fluctuation temperature appears in the temperature dependence of correction terms as in the specific heat.
We report the electronic structure, electric and thermal transport properties of Ru$_{1-x}$Ir$_{x}$Se$_2$ ($x \leq 0.2$). RuSe$_2$ is a semiconductor that crystallizes in a cubic pyrite unit cell. The Seebeck coefficient of RuSe$_2$ exceeds -200 $μ$V/K around 730 K. Ir substitution results in the suppression of the resistivity and the Seebeck coefficient, suggesting the removal of the peaks in density of states near the Fermi level. Ru$_{0.8}$Ir$_{0.2}$Se$_{2}$ shows a semiconductor-metal crossover at about 30 K. The magnetic field restores the semiconducting behavior. Our results indicate the…
We study the response of the thermopower of a quantum dot in the Kondo regime to sinusoidal displacement of the dot energy level via a gate voltage using time dependent non-crossing approximation and linear response Onsager relations. Instantaneous thermopower begins to exhibit complex fluctuations when the driving amplitude is increased at constant driving frequency. We also find that the time averaged thermopower decreases steadily until it saturates at constant driving amplitude as a function of inverse driving frequency. On the other hand, time averaged thermopower is found to be quite sen…
The spin Seebeck effect (SSE) refers to the generation of a spin current as a result of a temperature gradient in a magnetic material, which can be detected electrically via the inverse spin Hall effect in a metallic contact. Since the discovery of SSE in 2008, intensive studies on SSE have been conducted to elucidate its origin. SSEs appear in a wide range of magnetic materials including ferro-, ferri-, and antiferro-magnets and also paramagnets with classical or quantum spin fluctuation. SSE voltage reflects fundamental properties of a magnet, such as elementary excitation, static magnetic o…
We investigated the effect of pressure on the magnetic and thermoelectric properties of Sr$_{3.1}$Y$_{0.9}$Co$_{4}$O$_{10+δ}$. The magnetization is reduced with the application of pressure, reflecting the spin-state modification of the Co$^{3+}$ ions into the nonmagnetic low-spin state. Accordingly, with increasing pressure, the Seebeck coefficient is enhanced, especially at low temperatures, at which the effect of pressure on the spin state becomes significant. These results indicate that the spin-orbital entropy is a key valuable for the thermoelectric properties of the strongly correlated c…
Enhanced Anomalous Nernst Effect in the Ferromagnetic Kondo Lattice CeCo2As2
International Center for Quantum Materials, School of Physics, Peking University（北京大学物理学院量子材料国际中心）；State Key Laboratory of Quantum Functional Materials, Department of Physics, and Guangdong Basic Research Center of Excellence for Quantum Science, Southern University of Science and Technology（南方科技大学物理学院量子功能材料国家重点实验室及广东省量子科学基础研究中心）；Department of Chemistry, Michigan State University（密歇根州立大学化学系）
We investigate f-electron charge susceptibility in a two-impurity Anderson model on the basis of Wilson's numerical renormalization group method. The f-electron charge susceptibility diverges logarithmically at the critical point of this model when conduction-electron bands exhibit particle-hole symmetry. Although the critical point disappears without the particle-hole symmetry, the f-electron charge fluctuation is much more enhanced near the crossover regime between the Kondo-Yosida singlet and intersite spin-singlet states than that in the single-impurity case. This result shows that charge …
It is shown on the basis of the lowest order perturbation expansion with respect to critical fluctuations that the critical fluctuations give rise to an enhancement of the potential scattering of non-magnetic impurities. This qualitatively accounts for the enhancement of the resistivity due to impurities which has been observed in variety of systems near the quantum critical point, while the higher order processes happen to give much larger enhancement as seen from the Ward identity arguments. The cases with dynamical critical exponent $z$=2 and $z$=3 are discussed explicitly.
The appearance of unconventional superconductivity near heavy-fermion quantum critical points (QCPs) motivates investigation of pairing correlations close to a ``beyond Landau'' Kondo-destruction QCP. We focus on a two-Anderson-impurity cluster in which Kondo destruction is induced by a pseudogap in the conduction-electron density of states. Analysis via continuous-time quantum Monte-Carlo and the numerical renormalization group reveals a previously unstudied QCP that both displays the critical-local moment fluctuations characteristic of Kondo destruction and leads to a strongly enhanced singl…
The Nernst coefficient for the quasi-one-dimensional metal, Li(0.9)Mo(6)O(17), is found to be among the largest known for metals (~500 microV/KT at T~20K), and is enhanced in a broad range of temperature by orders of magnitude over the value expected from Boltzmann theory for carrier diffusion. A comparatively small Seebeck coefficient implies that Li(0.9)Mo(6)O(17) is bipolar with large, partial Seebeck coefficients of opposite sign. A very large thermomagnetic figure of merit, ZT~0.5, is found at high field in the range T~35-50K.
Spin liquid is a state of electron spins in which quantum fluctuation breaks magnetic ordering while maintaining spin correlation. It has been a central topic in magnetism because of its relevance to high-Tc superconductivity and topological states. However, utilizing spin liquid has been quite difficult. Typical spin liquid states are realized in one-dimensional spin systems, called quantum spin chains. Here, we show that a spin liquid in a spin-1/2 quantum chain generates and carries spin current via its long-range spin fluctuation. This is demonstrated by observing an anisotropic negative s…
We present initial Raman spectroscopy experiments on exfoliated flakes of $α$-RuCl$_3$, from tens of nm thick down to single layers. Besides unexpectedly finding this material to be air stable, in the thinnest layers we observe the appearance with decreasing temperature of a symmetry-forbidden mode in crossed polarization, along with an anomalous broadening of a mode at 164 cm$^{-1}$ that is known to couple to a continuum of magnetic excitations. This may be due to an enhancement of magnetic fluctuations and evidence for a distorted honeycomb lattice in single- and bi-layer samples.
Efficient thermoelectric materials should present large Seebeck coefficient, high electrical conductivity and low thermal conductivity. An enhanced Seebeck coefficient can be obtained from materials where the Fermi level can be aligned with a large and narrow peak of the density of states, particularly when a substantial band valley degeneracy occurs. A high electrical conductivity comes as a consequence of large conductive hopping paths between the atoms of the material. Both physical quantities can be decoupled and optimized independently if their origins can be ascribed to different sets of…
Kondo insulator FeSb$_2$ with large Seebeck coefficient would have potential in thermoelectric applications in cryogenic temperature range if it had not been for large thermal conductivity $κ$. Here we studied the influence of different chemical substitutions at Fe and Sb site on thermal conductivity and thermoelectric effect in high quality single crystals. At $5\%$ of Te doping at Sb site thermal conductivity is suppressed from $\sim 250$ W/Km in undoped sample to about 8 W/Km. However, Cr and Co doping at Fe site suppresses thermal conductivity more slowly than Te doping, and even at 20$\%$…
Unconventional superconductivity typically occurs in materials in which a small change of a parameter such as bandwidth or doping leads to antiferromagnetic or Mott insulating phases. As such competing phases are approached, the properties of the superconductor often become increasingly exotic. For example, in organic superconductors and underdoped high-$T_\mathrm{c}$ cuprate superconductors a fluctuating superconducting state persists to temperatures significantly above $T_\mathrm{c}$. By studying alloys of quasi-two-dimensional organic molecular metals in the $κ$-(BEDT-TTF)$_2$X family, we r…
We report Raman scattering experiments on the strongly correlated electron system NaxCoO2 with x= 0.71 and ordered Na vacancies. In this doping regime, NaxCoO2 exhibits a large and unusual thermopower and becomes superconducting upon hydration. Our Raman scattering data reveal pronounced low energy fluctuations that diverge in intensity at low temperatures. Related to these fluctuations is a drastic decrease of an electronic scattering rate from 50 to 3 cm-1. This observation is quite different from the behavior of Na disordered samples that have a larger and temperature independent scattering…
Time-domain techniques have shown the potential of photo-manipulating existing orders and inducing new states of matter in strongly correlated materials. Using time-resolved exact diagonalization, we perform numerical studies of pump dynamics in a Mott-Peierls system with competing charge and spin density waves. A light-enhanced $d$-wave superconductivity is observed when the system resides near a quantum phase boundary. By examining the evolution of spin, charge and superconducting susceptibilities, we show that a sub-dominant state in equilibrium can be stabilized by photomanipulating charge…
We have studied NpPdSn by means of the heat capacity, electrical resistivity, Seebeck and Hall effect, $^{237}$Np Mössbauer spectroscopy, and neutron diffraction measurements in the temperature range 2-300 K and under magnetic fields up to 14 T. NpPdSn orders antiferromagnetically below the Néel temperature $T_N$ = 19 K and shows localized magnetism of Np$^{3+}$ ion with a a doubly degenerate ground state. In the magnetic state the electrical resistivity and heat capacity are characterized by electron-magnon scattering with spin-waves spectrum typical of anisotropic antiferromagnets. An enhanc…
Thermoelectric materials can be used to convert heat to electric power through the Seebeck effect. We study magneto-thermoelectric figure of merit (ZT) in three-dimensional Dirac semimetal Cd$_3$As$_2$ crystal. It is found that enhancement of power factor and reduction of thermal conductivity can be realized at the same time through magnetic field although magnetoresistivity is greatly increased. ZT can be highly enhanced from 0.17 to 1.1 by more than six times around 350 K under a perpendicular magnetic field of 7 Tesla. The huge enhancement of ZT by magnetic field arises from the linear Dira…
We have studied the magnetic excitations in Ca$_{2-x}$Sr$_x$RuO$_4$, x=0.52 and 0.62, which exhibit an anomalous high susceptibility and heavy mass Fermi liquid behavior. Our inelastic neutron scattering experiments reveal strongly enhanced magnetic fluctuations around an incommensurate wave vector (0.22,0,0) pointing to a magnetic instability. The magnetic fluctuations show no correlation in c-direction and also along the RuO$_2$-planes the signal is extremely broad, $Δq =0.45$ Å$^{-1}$. These fluctuations can quantitatively account for the high specific heat coefficient and relate to the hig…
The pairing mechanism in different classes of correlated materials, including iron based superconductors, is still under debate. For FeSe monolayers, uniform nematic fluctuations have been shown in a lattice Monte Carlo study to play a potentially important role. Here, using dynamical mean field theory calculations for the same model system, we obtain a similar phase diagram and provide an alternative interpretation of the superconductivity in terms of local orbital fluctuations and phase rigidity. Our study clarifies the relation between the superconducting order parameter, superfluid stiffne…
An electron-phonon interaction arising from the modulation of the superexchange interaction by phonons is studied within the theoretical framework of Kondo lattices. It is relevant in strongly correlated electron liquids in cuprate oxide superconductors, which lie in the vicinity of the Mott-Hubbard metal-insulator transition. It is enhanced by antiferromagnetic and superconducting fluctuations, which are developed mainly because of the superexchange interaction. When the enhancement of the electron-phonon interaction is large enough, it can explain the softening of phonons and kinks in the qu…
Motivated by the widespread experimental observations of nematicity in strongly underdoped cuprate superconductors, we investigate the possibility of enhanced nematic fluctuations in the vicinity of a Mott insulator that displays Néel-type antiferromagnetic order. By performing a strong-coupling expansion of an effective model that contains both Cu-$d$ and O-$p$ orbitals on the square lattice, we demonstrate that quadrupolar fluctuations in the $p$-orbitals inevitably generate a biquadratic coupling between the spins of the $d$-orbitals. The key point revealed by our classical Monte Carlo simu…
We show theoretically that the nuclear spin-lattice relaxation rate (T_1T)^{-1} and the spin susceptibility exhibit divergent behaviors toward zero temperature at the quantum critical point (QCP) of the first-order valence transition. Remarkable enhancement in (T_1T)^{-1} and the spin susceptibility is induced by valence fluctuations even at the valence-crossover temperature far away from the QCP. This mechanism well explains peculiar behaviors observed recently in YbAuCu_4 and also gives a systematic explanation for YbXCu_4 for X=In, Au, Ag, Tl, and Pd from the viewpoint of the closeness to t…
We study the spin-fluctuation-mediated $s\pm$-wave superconductivity in the bilayer Hubbard model with vertical and diagonal interlayer hoppings. As in the two-leg ladder model with diagonal hoppings, studied previously by the present authors, superconductivity is strongly enhanced when one of the bands lies just below (or touches) the Fermi level, that is, when the band is incipient. The strong enhancement of superconductivity is because large weight of the spin fluctuations lies in an appropriate energy range, whereas the low energy, pair-breaking spin fluctuations are suppressed. The optimi…
We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly against the common lore, instead of enhancing the ($π$,0) order, itinerant carriers with well nested Fermi surfaces is found to induce significant amount of \textit{spatial} and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be intra-pocket nesting-associated long-rang…
