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H. Kontani

Publications and source records attributed to H. Kontani.

At least 37 records · Page 2Linked to original sources
arXiv2011

Indication of intrinsic spin Hall effect in 4d and 5d transition metals

We have investigated spin Hall effects in 4$d$ and 5$d$ transition metals, Nb, Ta, Mo, Pd and Pt, by incorporating the spin absorption method in the lateral spin valve structure; where large spin current preferably relaxes into the transition metals, exhibiting strong spin-orbit interactions. Thereby nonlocal spin valve measurements enable us to evaluate their spin Hall conductivities. The sign of the spin Hall conductivity changes systematically depending on the number of $d$ electrons. This tendency is in good agreement with the recent theoretical calculation based on the intrinsic spin Hall effect.

cond-mat.mes-hall↗
arXiv2010

Terahertz Hall Measurements On Optimally Doped Single Crystal Bi-2212

The infrared Hall angle in optimally doped single crystal $\rm Bi_2 Sr_2 Ca Cu_2 O_{8+x}$ was measured from 3.05 to 21.75 meV as a continuous function of temperature from 25 to 300\,K. In the normal state, the temperature dependence of the real part of the cotangent of the infrared Hall angle obeys the same power law as dc measurements. The measured Hall frequency $\rm ω_H$ is significantly larger than the expected value based upon ARPES data analyzed in terms of the relaxation time approximation. This discrepancy as well as the temperature dependence of $\rm Re(\cot{θ_H})$ and $ω_H$ is well described by a Fermi liquid theory in which current vertex corrections produced by electron-magnon scattering are included.

cond-mat.supr-con↗
arXiv2009

Origin of the anomalous Hall Effect in overdoped n-type cuprates: current vertex corrections due to antiferromagnetic fluctuations

The anomalous magneto-transport properties in electron doped (n-type) cuprates were investigated using Hall measurements at THz frequencies. The complex Hall angle was measured in overdoped Pr$_{\rm 2-x}$Ce$_{\rm x}$CuO$_{\rm 4}$ samples (x=0.17 and 0.18) as a continuous function of temperature above $T_c$ at excitation energies 5.24 and 10.5 meV. The results, extrapolated to low temperatures, show that inelastic scattering introduces electron-like contributions to the Hall response. First principle calculations of the Hall angle that include current vertex corrections (CVC) induced by electron interactions mediated by magnetic fluctuations in the Hall conductivity reproduce the temperature, frequency, and doping dependence of the experimental data. These results show that CVC effects are the source of the anomalous Hall transport properties in overdoped n$\text{-}$type cuprates.

cond-mat.supr-con↗
arXiv2009

Intrinsic Spin Hall Effect in s-wave Superconducting State: Analysis of Rashba Model

A general expression for the spin Hall conductivity (SHC) in the s-wave superconducting state at finite temperatures is derived. Based on the expression, we study the SHC in a two-dimensional electron gas model in the presence of Rashba spin-orbit interaction (SOI). The SHC is zero in the normal state, whereas it takes a large negative value as soon as the superconductivity occurs, due to the change in the quasiparticle contributions. Since this remarkable behavior is independent of the strength of the SOI, it will be widely observed in thin films of superconductors with surface-induced Rashba SOI, or in various non-centrosymmetric superconductors.

cond-mat.mes-hall↗
arXiv2008

Giant Extrinsic Spin Hall Effect due to Rare-Earth Impurities

We investigate the extrinsic spin Hall effect in the electron gas model due to magnetic impurities, by focusing on Ce- and Yb-impurities. In the dilute limit, the skew scattering term dominates the side jump term. For Ce-impurities, the spin Hall angle $α_{\rm SH}$ due to skew scattering is given by $-8π\sinδ_2/7$, where $δ_2 (\ll 1)$ is the phase shift for$d (l=2)$ partial wave. Since $α_{\rm SH}$ reaches $O(10^{-1})$ if $δ_2 \simge 0.03$, the spin Hall effect is anticipated to be considerable in metals with rare-earth impurities. The giant extrinsic SHE originates from the large orbital angular momentum, which is also significant for the intrinsic SHE.

cond-mat.mes-hall↗
arXiv2008

Giant Orbital Hall Effect in Transition Metals: Origin of Large Spin and Anomalous Hall Effects

In transition metals and their compounds, the orbital degrees of freedom gives rise to an orbital current, in addition to the ordinary spin and charge currents. We reveal that considerably large spin and anomalous Hall effects (SHE and AHE) observed in transition metals originate from an orbital Hall effect (OHE). To elucidate the origin of these novel Hall effects, a simple periodic s-d hybridization model is proposed as a generic model. The giant positive OHE originates from the orbital Aharonov-Bohm phase factor, and induces spin Hall conductivity that is proportional to the spin-orbit polarization at the Fermi level, which is positive (negative) in metals with more than (less than) half-filling.

cond-mat.mes-hall↗
arXiv2008

Impurity Effects in Sign Reversing Fully-Gapped Superconductors: Analysis of FeAs Superconductors

To understand the impurity effect on Tc in FeAs superconductors, we analyze a simple two-band BCS model with repulsive interband interaction. The realized fully-gapped superconducting state with sign reversal, which is predicted by spin fluctuation theories in this compound, is suppressed by impurities due to the interband hopping of Cooper pairs, if the interband impurity scattering $I'$ is equal to the intraband one $I$. When $|I'/I| < 1$, in highly contrast, Tc is almost unchanged by strong impurity scattering since the interband scattering is almost prohibited by the multiple scattering effect. Since $|I'/I| \sim 0.5$ is expected, the robustness of superconductivity against impurities in FeAs superconductors is naturally understood in term of the sign reversing fully-gapped state.

cond-mat.supr-con↗
arXiv2008

Magneto-transport properties governed by the antiferromagnetic fluctuations in heavy fermion superconductor CeIrIn$_{5}$

In quasi-two dimensional Ce(Ir,Rh)In$_5$ system, it has been suggested that the phase diagram contains two distinct domes with different heavy fermion superconducting states. We here report the systematic pressure dependence of the electron transport properties in the normal state of CeRh$_{0.2}$Ir$_{0.8}$In$_{5}$ and CeIrIn$_{5}$, which locates in first and second superconducting dome, respectively. We observed non-Fermi liquid behavior at low temperatures in both compounds, including non-quadratic $T-$dependence of the resistivity, large enhancement of the Hall coefficient, and the violation of the Kohler's rule in the magnetoresistance. We show that the cotangent of Hall angle $\cot Θ_H$ varies as $T^2$, and the magnetoresistance is quite well scaled by the Hall angle as $Δρ_{xx}/ρ_{xx}\propto \tan^2Θ_H$. The observed transport anomalies are common features of Ce$M$In$_{5}$ ($M$=Co, Rh, and Ir) and high-$T_c$ cuprates, suggesting that the anomalous transport properties observed in CeIrIn$_{5}$ are mainly governed by the antiferromagnetic spin fluctuations, not by the Ce-valence fluctuations which has been proposed to be the possible origin for the second superconducting dome.

cond-mat.str-el↗
arXiv2008

Study of Intrinsic Spin Hall Effect and Orbital Hall Effect in 4d- and 5d- Transition Metals

We study the intrinsic spin Hall conductivity (SHC) in various $5d$-transition metals (Ta, W, Re, Os, Ir, Pt, and Au) and 4d-transition metals (Nb, Mo, Tc, Ru, Rh, Pd, and Ag) based on the Naval Research Laboratory tight-binding model, which enables us to perform quantitatively reliable analysis. In each metal, the obtained intrinsic SHC is independent of resistivity in the low resistive regime ($ρ< 50 μΩ\text{cm}$) whereas it decreases in proportion to $ρ^{-2}$ in the high resistive regime. In the low resistive regime, the SHC takes a large positive value in Pt and Pd, both of which have approximately nine $d$-electrons per ion ($n_d=9$). On the other hand, the SHC takes a large negative value in Ta, Nb, W, and Mo where $n_d<5$. In transition metals, a conduction electron acquires the trajectory-dependent phase factor that originates from the atomic wavefunction. This phase factor, which is reminiscent of the Aharonov-Bohm phase, is the origin of the SHC in paramagnetic metals and that of the anomalous Hall conductivity in ferromagnetic metals. Furthermore, each transition metal shows huge and positive $d$-orbital Hall conductivity (OHC), independently of the strength of the spin-orbit interaction (SOI). Since the OHC is much larger than the SHC, it will be possible to realize a {\it orbitronics device} made of transition metals.

cond-mat.str-el↗
arXiv2008

Giant Intrinsic Spin and Orbital Hall Effects in Sr2MO4 (M=Ru,Rh,Mo)

We investigate the intrinsic spin Hall conductivity (SHC) and the d-orbital Hall conductivity (OHC) in metallic d-electron systems, by focusing on the t_{2g}-orbital tight-binding model for Sr2MO4 (M=Ru,Rh,Mo). The conductivities obtained are one or two orders of magnitude larger than predicted values for p-type semiconductors with 5% hole doping. The origin of these giant Hall effects is the ``effective Aharonov-Bohm phase'' that is induced by the d-atomic angular momentum in connection with the spin-orbit interaction and the inter-orbital hopping integrals. The huge SHC and OHC generated by this mechanism are expected to be ubiquitous in multiorbital transition metal complexes, which pens the possibility of realizing spintronics as well as orbitronics devices.

cond-mat.str-el↗
arXiv2008

Study of intrinsic spin and orbital Hall effects in Pt based on a (6s, 6p, 5d) tight-binding model

We study the origin of the intrinsic spin Hall conductivity (SHC) and the d-orbital Hall conductivity (OHC) in Pt based on a multiorbital tight-binding model with spin-orbit interaction. We find that the SHC reaches 1000 \hbar/eΩcm when the resistivity ρis smaller than ~10 μΩcm, whereas it decreases to 300 \hbar/eΩcm when ρ~ 100 μΩcm. In addition, the OHC is still larger than the SHC. The origin of huge SHE and OHE in Pt is the large ``effective magnetic flux'' that is induced by the interorbital transition between d_{xy}- and d_{x2-y2}-orbitals with the aid of the strong spin-orbit interaction.

cond-mat.mes-hall↗
arXiv2007

Spin Hall effect in Sr2RuO4 and transition metals (Nb,Ta)

We study the intrinsic spin Hall conductivity (SHC) and the $d$-orbital Hall conductivity (OHC) in metallic $d$-electron systems based on the multiorbital tight-binding model. The obtained Hall conductivities are much larger than that in $p$-type semiconductors. The origin of these huge Hall effects is the "effective Aharonov-Bohm phase" induced by the signs of inter-orbital hopping integrals as well as atomic spin-orbit interaction. Huge SHC and OHC due to this mecahnism is ubiquitous in multiorbital transition metals.

cond-mat.str-el↗
arXiv2007

Intrinsic anomalous Hall effect in ferromagnetic metals studied by the multi d-orbital tight-binding model

To elucidate the origin of anomalous Hall effect (AHE) in ferromagnetic transition metals, we study the intrinsic AHE based on a multi-orbital (xz,yz)-tight-binding model. We find that a large anomalous velocity comes from the off-diagonal (inter-orbital) hopping. By this reason, the present model shows a large intrinsic anomalous Hall conductivity (AHC) which is compatible with typical experimental values in ferromagnets [100-1000 [1/Ωcm]], without necessity to assume a special band structure at the Fermi level. In good metals where resistivity ρis small, the intrinsic AHC is constant (dissipation-less) as found by Karplus and Luttinger. In bad metals, however, we find that the AHC is proportional to ρ^{-2} when \hbar/2τis larger than the minimum band-splitting measured from the Fermi level. This crossover behavior of the intrinsic AHE, which was first derived in J. Phys. Soc. Jpn. 63 (1994) 2627, is recently observed in various ferromagnetic metals universally by A. Asamitsu et al. We also stress that the present (xz,yz)-tight binding model shows a huge spin Hall effect in a paramagnetic state.

cond-mat.str-el↗
arXiv2006

Non-Fermi liquid behavior in the magnetotransport of CeMIn5 (M: Co and Rh): Striking similarity between quasi 2D heavy fermion and high-Tc cuprates

We present a systematic study of the dc-resistivity, Hall effect, and magnetoresistance in the normal state of quasi 2D heavy fermion superconductors CeMIn5 (M: Rh and Co) under pressure. Here the electronic system evolves with pressure from an antiferromagnetic (AF) metal, through a highly unconventional non-Fermi liquid, and finally into a Fermi-liquid state. The amplitude of the Hall coefficient increases dramatically with decreasing T, reaching at low temperatures a value significantly larger than 1/ne. Furthermore, the magnetoresistance is characterized by T- and H-dependence which clearly violate Kohler's rule. We found that the Hall angle cotΘvaries as T^2, and the magnetoresistance is well scaled by the Hall angle as Δρ_{xx}/ρ_{xx}\propto \tan^2Θ. These non-Fermi liquid properties in the electron transport are remarkably pronounced when the AF fluctuations are enhanced in the vicinity of the QCP. We lay particular emphasis on the striking resemblance of these anomalous magnetotransport with those of the high-Tc cuprates. We argue that features commonly observed in quasi 2D heavy fermion and cuprates very likely capture universal features of strongly correlated electron systems.

cond-mat.str-el↗
arXiv2006

Evolution of Hall coefficient in two-dimensional heavy fermion CeCoIn$_5$

We report on the pressure dependence of the Hall coefficient $R_H$ in quasi-2D heavy fermion CeCoIn$_5$. At ambient pressure, below a temperature associated with the emergence of non-Fermi liquid properties, $R_H$ is anomalously enhanced. We found that the restoration of the Fermi liquid state with applied pressure leads to a gradual suppression of this dramatic enhancement. Moreover, the enhancement in $R_H$ was found to be confined to an intermediate temperature window, where inelastic electron-electron scattering is dominant. Our results strongly support the presence of cold and hot spots on the Fermi surface probably due to anisotropic scattering by antiferromagnetic fluctuations, which may also prove relevant for the debate on the anomalous normal-state properties of high-$T_c$ cuprates.

cond-mat.str-el↗
arXiv2005

Universality in heavy-fermion systems with general degeneracy

We discuss the relation between the T^{2}-coefficient of electrical resistivity $A$ and the T-linear specific-heat coefficient $γ$ for heavy-fermion systems with general $N$, where $N$ is the degeneracy of quasi-particles. A set of experimental data reveals that the Kadowaki-Woods relation; $A/γ^{2} = 1*10^{-5} {μΩ}(K mol/mJ)^{2}$, collapses remarkably for large-N systems, although this relation has been regarded to be commonly applicable to the Fermi-liquids. Instead, based on the Fermi-liquid theory we propose a new relation; $\tilde{A}/\tildeγ^2=1\times10^{-5}$ with $\tilde{A} = A/(1/2)N(N-1)$ and $\tildeγ = γ/(1/2)N(N-1)$. This new relation exhibits an excellent agreement with the data for whole the range of degenerate heavy-fermions.

cond-mat.str-el↗
arXiv2003

Normal-state Hall Angle and Magnetoresistance in quasi-2D Heavy Fermion CeCoIn_5 near a Quantum Critical Point

The normal-state Hall effect and magnetoresisitance (MR) have been measured in the quasi-2D heavy fermion superconductor CeCoIn_5. In the non-Fermi liquid region where the reistivity rho_xx exhibits an almost perfect T-linear dependence, the Hall angle varies as cot theta_H propto T^2 and the MR displays a strong violation of Kohler's rule. We demonstrate a novel relation between the MR and the Hall conductivity, Delta rho_xx/rho_xx propto (sigma_xy rho_xx)^2. These results bear a striking resemblance to the normal-state properties of high-T_c cuprates, indicating universal transport properties in the presence of quasi-2D antiferromagnetic fluctuations near a quantum critical point.

cond-mat.supr-con↗