arXiv subjects

Satoru Nakatsuji

Publications and source records attributed to Satoru Nakatsuji.

At least 73 records · Page 4Linked to original sources

arXiv2018

Effect of Anisotropic Hybridization in YbAlB$_4$ Probed by Linear Dichroism in Core-Level Hard X-ray Photoemission Spectroscopy

We have probed the crystalline electric-field ground states of pure $|J = 7/2, J_z = \pm 5/2\rangle$ as well as the anisotropic $c$-$f$ hybridization in both valence fluctuating systems $α$- and $β$-YbAlB$_4$ by linear polarization dependence of angle-resolved core level photoemission spectroscopy. Interestingly, the small but distinct difference between \abyb was found in the polar angle dependence of linear dichroism, indicating the difference in the anisotropy of $c$-$f$ hybridization which may be essential to a heavy Fermi liquid state in $α$-YbAlB$_4$ and a quantum critical state in $β$-YbAlB$_4$.

cond-mat.str-el↗

arXiv2018

Anomalous Hall Effect in Thin Film of the Weyl Antiferromagnet Mn$_3$Sn

The Weyl antiferromagnet Mn$_3$Sn has recently attracted significant attention as it exhibits various useful functions such as large anomalous Hall effect that are normally absent in antiferromagnets. Here we report the thin film fabrication of the single phase of Mn$_3$Sn and the observation of the large anomalous Hall effect at room temperature despite its vanishingly small magnetization. Our work on the high-quality thin film growth of the Weyl antiferromagnet paves the path for developing the antiferromagnetic spintronics.

cond-mat.mtrl-sci↗

arXiv2018

Spin-Orbital Entangled Liquid State in the Copper Oxide Ba$_3$CuSb$_2$O$_9$

Structure with orbital degeneracy is unstable toward spontaneous distortion. Such orbital correlation usually has a much higher energy scale than spins, and therefore, magnetic transition takes place at a much lower temperature, almost independently from orbital ordering. However, when the energy scales of orbitals and spins meet, there is a possibility of spin-orbital entanglement that would stabilize novel ground state such as spin-orbital liquid and random singlet state. Here we review on such a novel spin-orbital magnetism found in the hexagonal perovskite oxide Ba$_3$CuSb$_2$O$_9$, which hosts a self-organized honeycomblike short-range order of a strong Jahn-Teller ion Cu$^{2+}$. Comprehensive structural and magnetic measurements have revealed that the system has neither magnetic nor Jahn-Teller transition down to the lowest temperatures, and Cu spins and orbitals retain the hexagonal symmetry and paramagnetic state. Various macroscopic and microscopic measurements all indicate that spins and orbitals remain fluctuating down to low temperatures without freezing, forming a spin-orbital entangled liquid state.

cond-mat.str-el↗

arXiv2018

Giant anomalous Nernst effect and quantum-critical scaling in a ferromagnetic semimetal

In metallic ferromagnets, the Berry curvature of underlying quasiparticles can cause an electric voltage perpendicular to both magnetization and an applied temperature gradient, a phenomenon called the anomalous Nernst effect (ANE). Here, we report the observation of a giant ANE in the full-Heusler ferromagnet Co$_2$MnGa, reaching $S_{yx}\sim -6$ $μ$V/K at room $T$, one order of magnitude larger than the maximum value reported for a magnetic conductor. With increasing temperature, the transverse thermoelectric conductivity or Peltier coefficient $α_{yx}$ shows a crossover between $T$-linear and $-T \log(T)$ behaviors, indicating the violation of Mott formula at high temperatures. Our numerical and analytical calculations indicate that the proximity to a quantum Lifshitz transition between type-I and type-II magnetic Weyl fermions is responsible for the observed crossover properties and an enhanced $α_{yx}$. The $T$ dependence of $α_{yx}$ in experiments and numerical calculations can be understood in terms of a quantum critical scaling function predicted by the low energy effective theory over more than a decade of temperatures. Moreover, the observation of chiral anomaly or an unsaturated positive longitudinal magnetoconductance also provide evidence for the existence of Weyl fermions in Co$_2$MnGa.

cond-mat.mtrl-sci↗

arXiv2018

Large magneto-optical Kerr effect and imaging of magnetic octupole domains in an antiferromagnetic metal

When a polarized light beam is incident upon the surface of a magnetic material, the reflected light undergoes a polarization rotation. This magneto-optical Kerr effect (MOKE) has been intensively studied in a variety of ferro- and ferrimagnetic materials because it provides a powerful probe for electronic and magnetic properties as well as for various applications including magneto-optical recording. Recently, there has been a surge of interest in antiferromagnets (AFMs) as prospective spintronic materials for high-density and ultrafast memory devices, owing to their vanishingly small stray field and orders of magnitude faster spin dynamics compared to their ferromagnetic counterparts. In fact, the MOKE has proven useful for the study and application of the antiferromagnetic (AF) state. Although limited to insulators, certain types of AFMs are known to exhibit a large MOKE, as they are weak ferromagnets due to canting of the otherwise collinear spin structure. Here we report the first observation of a large MOKE signal in an AF metal at room temperature. In particular, we find that despite a vanishingly small magnetization of $M \sim$0.002 $μ_{\rm B}$/Mn, the non-collinear AF metal Mn$_3$Sn exhibits a large zero-field MOKE with a polar Kerr rotation angle of 20 milli-degrees, comparable to ferromagnetic metals. Our first-principles calculations have clarified that ferroic ordering of magnetic octupoles in the non-collinear Neel state may cause a large MOKE even in its fully compensated AF state without spin magnetization. This large MOKE further allows imaging of the magnetic octupole domains and their reversal induced by magnetic field. The observation of a large MOKE in an AF metal should open new avenues for the study of domain dynamics as well as spintronics using AFMs.

cond-mat.mtrl-sci↗

arXiv2018

Giant Anisotropic Magnetoresistance due to Purely Orbital Rearrangement in the Quadrupolar Heavy Fermion Superconductor PrV$_2$Al$_{20}$

We report the discovery of giant and anisotropic magnetoresistance due to the orbital rearrangement in a non-magnetic correlated metal. In particular, we measured the magnetoresistance under fields up to 31.4 T in the cubic Pr-based heavy fermion superconductor PrV$_2$Al$_{20}$ with a non-magnetic $Γ_3$ doublet ground state, exhibiting antiferro-quadrupole ordering below 0.7 K. For the [100] direction, we find that the high-field phase appears between 12 T and 25 T, accompanied by a large jump at 12 T in the magnetoresistance ($ΔMR \sim $ 100 $\% $) and in the anisotropic magnetoresistivity (AMR) ratio by $\sim $ 20 $\% $. These observations indicate that the strong hybridization between the conduction electrons and anisotropic quadrupole moments leads to the Fermi surface reconstruction upon crossing the field-induced antiferro-quadrupole (orbital) rearrangement.

cond-mat.str-el↗

arXiv2018

Evolution of Magnetic Excitations Across the Metal-Insulator Transition in a Pyrochlore Iridate Eu$_{2}$Ir$_{2}$O$_{7}$

We report Resonant Inelastic X-ray Scattering (RIXS) study of the magnetic excitation spectrum in a highly insulating Eu$_{2}$Ir$_{2}$O$_{7}$ single crystal that exhibits a metal-insulator transition at $T_{MI}$ = 111(7) K. A propagating magnon mode with 20 meV bandwidth and 28 meV magnon gap is found in the excitation spectrum at 7 K, which is expected in the all-in-all-out (AIAO) magnetically ordered state. This magnetic excitation exhibits substantial softening as temperature is raised towards $T_{MI}$, and turns into highly damped excitation in the paramagnetic phase. Remarkably, the softening occurs throughout the whole Brillouin zone including the zone boundary. This observation is inconsistent with magnon renormalization expected in a local moment system, and indicates that the strength of electron correlation in Eu$_{2}$Ir$_{2}$O$_{7}$ is only moderate, so that electron itinerancy should be taken into account in describing its magnetism.

cond-mat.str-el↗

arXiv2017

Spontaneous Hall effect induced by strain in Pr$_2$Ir$_2$O$_7$ epitaxial thin films

Strongly correlated iridate pyrochlores with geometrically frustrated spins have been recognized as a potentially interesting group of oxide materials where novel topological phases may appear. A particularly attractive system is the metallic Pr$_2$Ir$_2$O$_7$, as it is known as a Fermi node semimetal characterized by quadratic band touching at the Brillouin zone center, suggesting that the topology of its electronic states can be tuned by moderate lattice strain. In this work we report the growth of epitaxial Pr$_2$Ir$_2$O$_7$ thin films grown by solid-state epitaxy. We show that the strained parts of the films give rise to a spontaneous Hall effect that persists up to 50 K without having spontaneous magnetization within our experimental accuracy. This indicates that a macroscopic time reversal symmetry (TRS) breaking appears at a temperature scale that is too high for the magnetism to be due to Pr 4$f$ moments, and must thus be related to magnetic order of the iridium 5$d$ electrons. The magnetotransport and Hall analysis results are consistent with the formation of a Weyl semimetal state that is induced by a combination of TRS breaking and cubic symmetry breaking due to lattice strain.

cond-mat.str-el↗

arXiv2017

Orthogonal Magnetization and Symmetry Breaking in Pyrochlore Iridate Eu2Ir2O7

Electrons in the pyrochore iridates experience a large interaction energy in addition to a strong spin-orbit interaction. Both features make the iridates promising for realizing novel states such as the Topological Mott Insulator. The pyrochlore iridate Eu$_2$Ir$_2$O$_7$ shows a metal-insulator transition at $T_N \sim$ 120 K below which a magnetically ordered state develops. Using torque magnetometry, we uncover a highly unusual magnetic response. A magnetic field $\bf H$ applied in its $a$-$b$ plane produces a nonlinear magnetization $M_\perp$ orthogonal to the plane. $M_\perp$ displays a $d$-wave field-angle pattern consistent with octupolar order, with a handedness dictated by field cooling, leading to symmetry breaking of the chirality $ω$. A surprise is that the lobe orientation of the $d$-wave pattern is sensitive to the direction of the field when the sample is field-cooled below $T_N$, suggestive of an additional order parameter $η$ already present at 300 K.

cond-mat.str-el↗

arXiv2017

Large anomalous Nernst effect at room temperature in a chiral antiferromagnet

Temperature gradient in a ferromagnetic conductor may generate a spontaneous transverse voltage drop in the direction perpendicular to both magnetization and heat current. This anomalous Nernst effect (ANE) has been considered to be proportional to the magnetization, and thus observed only in ferromagnets, while recent theories indicate that ANE provides a measure of the Berry curvature at the Fermi energy $E_{\rm F}$. Here we report the observation of a large ANE at zero field in the chiral antiferromagnet Mn$_3$Sn. Despite a very small magnetization $\sim 0.002$ $μ_{\rm B}/$Mn, the transverse Seebeck coefficient at zero field is $\sim 0.35~μ$V/K at room temperature and reaches $\sim 0.6~μ$V/K at 200 K, comparable with the maximum value known for a ferromagnetic metal. Our first-principles calculation reveals that the large ANE comes from a significantly enhanced Berry curvature associated with the Weyl points nearby $E_{\rm F}$. The ANE is geometrically convenient for the thermoelectric power generation, as it enables a lateral configuration of the modules to efficiently cover the heat source. Our observation of the large ANE in an antiferromagnet paves a way to develop a new class of thermoelectric material using topological magnets to fabricate an efficient, densely integrated thermopile.

cond-mat.mes-hall↗

arXiv2017

Frustrated magnetism in the Heisenberg pyrochlore antiferromagnets $A$Yb$_2$$X$$_4$ ($A$ = Cd, Mg, $X$ = S, Se)

Our polycrystalline sample study on the Yb-based chalcogenide spinels $A$Yb$_2$$X_4$ ($A =$ Cd, Mg, $X =$ S, Se) has revealed frustrated magnetism due to the antiferromagnetically coupled Heisenberg spin on the pyrochlore lattice. Our crystal electric field analysis indicates the Yb ground state has nearly Heisenberg spins with a strong quantum character of the ground doublet. All the materials exhibit an antiferromagnetic order at 1.4-1.8 K, much lower temperature than the antiferromagnetic exchange coupling scale of $\sim 10$ K. The magnetic specific heat $C_{\rm M}$ shows a $T^{3}$ dependence, indicating the gapless feature in the Yb-based chalcogenide spinels. The magnetic entropy change much smaller than $R$ ln 2 below the Néel temperature and the small local magnetic field estimated from the $μ$SR measurements strongly suggest the significantly reduced size of the ordered moment in comparison with the bare moment size 1.33 $μ_{\rm B}$/Yb supporting strong fluctuations in the commensurate and incommensurate ordered states in CdYb$_2$S$_4$ and MgYb$_2$S$_4$, respectively.

cond-mat.str-el↗

arXiv2017

Thermal Hall effect in a phonon-glass Ba$_3$CuSb$_2$O$_9$

A distinct thermal Hall signal is observed in a quantum spin liquid candidate Ba$_3$CuSb$_2$O$_9$. The transverse thermal conduction shows a power-law temperature dependence below 40 K where a spin gap opens. We suggest that, through the very low longitudinal thermal conductivity and the thermal Hall signals, a phonon Hall effect is induced by strong phonon scatterings by orphan Cu$^{2+}$ spins formed in random domains of Cu$^{2+}$-Sb$^{5+}$ dumbbells.

cond-mat.str-el↗

arXiv2016

Pressure-induced magnetic transition exceeding 30 K in the Yb-based heavy fermion superconductor $β$-YbAlB$_4$

Measurements of the electric resistivity $ρ(T)$ under pressure up to 8 GPa were performed on high-quality single-crystals of the Yb-based heavy fermion system $β$-YbAlB$_4$ in the temperature range $2<T<$ 300 K. In the resistivity data, we observed pressure-induced magnetic ordering above the critical pressure $P_{\rm c} \sim$ 2 GPa. Clear difference in the phase diagram under pressure using two types of pressure mediums indicates that the transition temperature may be further enhanced under application of uniaxial pressure. With pressure, this phase transition temperature $T_{\rm M}$ is enhanced reaching 32 K at 8 GPa, which is the highest transition temperature so far recorded for the Yb-based heavy fermion compounds. The power-law exponent $α$ in $ρ=ρ_0+ AT^α$ below $T_{\rm M}$ gradually changes from 3/2 to 5/2 with increasing pressure from 2 to 8 GPa. In contrast, the resistivity exhibits a $T$-linear behavior in the temperature range 2 $\le T \le$ 20 K and is insensitive to pressure below $P_{\rm c}$. In this pressure regime, the magnetization is also nearly independent of pressure and shows no anomaly above 2 K. Our results indicate that a quantum critical point for $β$-YbAlB$_4$ is also located near $P_{\rm c}$ in addition to the strange metal region near the ambient pressure.

cond-mat.str-el↗

arXiv2016

NMR Observation of Ferro-quadrupole Order in PrTi2Al20

We report the results of 27Al-NMR measurements on a single crystal of PrTi2Al20. This compound shows a phase transition near 2 K associated with the quadrupole degree of freedom of Pr ions, for which the ground state of the crystalline electric field is a nonmagnetic doublet. When a magnetic field is applied along the <111> direction, the NMR lines split upon entering the low-temperature phase, indicating the breaking of the three fold rotation symmetry due to the field-induced magnetic dipole perpendicular to the field. This provides microscopic evidence for a ferro order of the O20 quadrupole. Although a first-order transition is expected theoretically, the line splitting evolves continuously within the experimental resolution.

cond-mat.str-el↗

arXiv2016

Giant Anomalous Hall Effect in the Chiral Antiferromagnet Mn3Ge

The external field control of antiferromagnetism is a significant subject both for basic science and technological applications. As a useful macroscopic response to detect magnetic states, the anomalous Hall effect (AHE) is known for ferromagnets, but it has never been observed in antiferromagnets until the recent discovery in Mn3Sn. Here we report another example of the AHE in a related antiferromagnet, namely, in the hexagonal chiral antiferromagnet Mn3Ge. Our single-crystal study reveals that Mn3Ge exhibits a giant anomalous Hall conductivity $|σ_{xz}|$ ~ 60 per ohm per cm at room temperature and approximately 380 per ohm per cm at 5 K in zero field, reaching nearly half of the value expected for the quantum Hall effect per atomic layer with Chern number of unity. Our detailed analyses on the anisotropic Hall conductivity indicate that in comparison with the in-plane-field components $|σ_{xz}|$ and $|σ_{zy}|$, which are very large and nearly comparable in size, we find $|σ_{yx}|$ obtained in the field along the c axis is found to be much smaller. The anomalous Hall effect shows a sign reversal with the rotation of a small magnetic field less than 0.1 T. The soft response of the AHE to magnetic field should be useful for applications, for example, to develop switching and memory devices based on antiferromagnets.

cond-mat.str-el↗

arXiv2015

Optical evidence for a Weyl semimetal state in pyrochlore Eu2Ir2O7

A Weyl semimetallic state with pairs of nondegenerate Dirac cones in three dimensions was recently predicted to occur in the antiferromagnetic state of the pyrochlore iridates. Here, we show that the THz optical conductivity and temperature dependence of free carriers in the pyrochlore Eu2Ir2O7 match the predictions for a Weyl semimetal and suggest novel Dirac liquid behavior. The interband optical conductivity vanishes continuously at low frequencies signifying a semimetal. The metal-insulator transition at T_N = 110 K is manifested in the Drude spectral weight, which is independent of temperature in the metallic phase, and which decreases smoothly in the ordered phase. The temperature dependence of the free carrier weight below T_N is in good agreement with theoretical predictions for a Dirac material. The data yield a Fermi velocity v_F=4x10^7 cm/s, a logarithmic renormalization scale Lambda_L=600 K, and require a Fermi temperature of T_F=100 K associated with residual unintentional doping to account for the low temperature optical response and dc resistivity.

cond-mat.str-el↗

arXiv2015

Spin Fluctuations from Hertz to Terahertz on a Triangular Lattice

The temporal magnetic correlations of the triangular lattice antiferromagnet NiGa$_2$S$_4$ are examined through thirteen decades ($10^{-13}-1$~sec) using ultra-high-resolution inelastic neutron scattering, muon spin relaxation, AC and nonlinear susceptibility measurements. Unlike the short-ranged {\it spatial} correlations, the temperature dependence of the {\it temporal} correlations show distinct anomalies. The spin fluctuation rate decreases precipitously upon cooling towards $T^{\ast}=8.5$~K, but fluctuations on the microsecond time scale then persist in an anomalous dynamical regime for 4 K $<T\le T^{\ast}$. As this time scale exceeds that of single site dynamics by six orders of magnitude, these fluctuations bear evidence of emergent degrees of freedom within the short-range correlated incommensurate state of NiGa$_2$S$_4$.

cond-mat.str-el↗

arXiv2015

Unstable Spin-Ice Order in the Stuffed Metallic Pyrochlore Pr$_{2+x}$Ir$_{2-x}$O$_{7-δ}$

Specific heat, elastic neutron scattering, and muon spin rotation ($μ$SR) experiments have been carried out on a well-characterized sample of "stuffed" (Pr-rich) Pr$_{2+x}$Ir$_{2-x}$O$_{7-δ}$. Elastic neutron scattering shows the onset of long-range spin-ice "2-in/2-out" magnetic order at $T_M = 0.93$ K, with an ordered moment of 1.7(1)$μ_\mathrm{B}$/Pr ion at low temperatures. Approximate lower bounds on the correlation length and correlation time in the ordered state are 170 Å and 0.7 ns, respectively. $μ$SR experiments yield an upper bound 2.6(7) mT on the local field $B_\mathrm{loc}^{4f}$ at the muon site, which is nearly two orders of magnitude smaller than the expected dipolar field for long-range spin-ice ordering of 1.7$μ_B$ moments (120--270 mT, depending on muon site). This shortfall is due in part to splitting of the non-Kramers crystal-field ground-state doublets of near-neighbor Pr$^{3+}$ ions by the $μ^+$-induced lattice distortion. For this to be the only effect, however, $\sim$160 Pr moments out to a distance of $\sim$14 Å must be suppressed. An alternative scenario, which is consistent with the observed reduced nuclear hyperfine Schottky anomaly in the specific heat, invokes slow correlated Pr-moment fluctuations in the ordered state that average $B_\mathrm{loc}^{4f}$ on the $μ$SR time scale (${\sim}10^{-7}$ s), but are static on the time scale of the elastic neutron scattering experiments (${\sim}10^{-9}$ s). In this picture the dynamic muon relaxation suggests a Pr$^{3+}$ $4f$ correlation time of a few nanoseconds, which should be observable in a neutron spin echo experiment.

cond-mat.str-el↗