arXiv subjects

H. Sakai

Publications and source records attributed to H. Sakai.

At least 37 records · Page 2Linked to original sources

arXiv2022

Structural 130-K Phase Transition and Emergence of a Two-Ion Kondo State in HT-Ce$_2$Rh$_2$Ga Explored by $^{69,71}$Ga Nuclear Quadrupole Resonance

We have studied the microscopic magnetic properties, the nature of the 130-K phase transition, and the ground state in the recently synthesized compound Ce$_2$Rh$_2$Ga by use of $^{69,71}$Ga nuclear quadrupole resonance (NQR). The NQR spectra clearly show an unusual phase transition at $T_t$ $\sim$ 130 K yielding a splitting of the high-temperature single NQR line into two clearly resolved NQR lines, providing evidence for two crystallographically inequivalent Ga sites. The NQR frequencies are in good agreement with fully-relativistic calculations of the band structure. Our NQR results indicate the absence of magnetic or charge order down to 0.3 K. The temperature dependence of the spin-lattice relaxation rate, 1/$T_1$, shows three distinct regimes, with onset temperatures at $T_t$ and 2 K. The temperature-independent 1/$T_1$, observed between $T_t$ and 2 K, crosses over to a Korringa process, 1/$T_1$ $\propto$ $T$, below $\sim$ 2 K, which evidences a rare two-ion Kondo scenario: the system goes into a dense Kondo coherent state at 2.0 and 0.8 K for the two different Ga sites.

cond-mat.str-el↗

arXiv2022

Field-tunable Weyl points and large anomalous Hall effects in degenerate magnetic semiconductor EuMg$_2$Bi$_2$

Magnets, with topologically-nontrivial Dirac/Weyl points, have recently attracted significant attention owing to the unconventional physical properties, such as large anomalous Hall effects. However, they typically have a high carrier density and complicated band structure near the Fermi energy. In this study, we report degenerate magnetic semiconductor EuMg$_2$Bi$_2$, which exhibits a single valley at the $Γ$ point, where the field-tunable Weyl points form via the magnetic exchange interaction with the local Eu spins. By the high-field measurements on high-quality single crystals, we observed the quantum oscillations in resistivity, elastic constant, and surface impedance, which enabled us to determine the position of the Fermi energy. In combination with the first-principles calculation, we revealed that the Weyl points are located in the vicinity of the Fermi energy when the Eu spins are fully polarized. Furthermore, we observed large anomalous Hall effect (Hall angle $Θ_{\mathrm{AH}}\sim0.07$) in the forced ferromagnetic phase, which is consistent with this field variation of band structure.

cond-mat.str-el↗

arXiv2022

Effect of uranium deficiency on normal and superconducting properties in unconventional superconductor UTe$_2$

Single crystals of the unconventional superconductor UTe$_2$ have been grown in various conditions which result in different superconducting transition temperature as well as normal state properties. Stoichiometry of the samples has been characterized by the single-crystal X-ray crystallography and electron microprobe analyses. Superconducting samples are nearly stoichiometric within an experimental error of about 1 \%, while non-superconducting sample significantly deviates from the ideal composition. The superconducting UTe$_2$ showed that the large density of states was partially gapped in the normal state, while the non-superconducting sample is characterized by the relatively large electronic specific heat as reported previously.

cond-mat.supr-con↗

arXiv2022

Chiral superconductivity in UTe2 probed by anisotropic low-energy excitations

Chiral spin-triplet superconductivity is a topologically nontrivial pairing state with broken time-reversal symmetry, which can host Majorana quasiparticles. The recently discovered heavy-fermion superconductor UTe$_2$ exhibits peculiar properties of spin-triplet pairing, and the possible chiral state has been actively discussed. However, the symmetry and nodal structure of its order parameter in the bulk, which determine the Majorana surface states, remains controversial. Here we focus on the number and positions of superconducting gap nodes in the ground state of UTe$_2$. Our magnetic penetration depth measurements for three field orientations in the Meissner state reveal the power-law temperature dependence with exponents nearly equal to 2 or less than 2, which excludes single-component spin-triplet states. The anisotropy of low-energy quasiparticle excitations indicates multiple point nodes near the $k_y$- and $k_z$-axes, evidencing that the order parameter has multiple components in a chiral complex form. We find that most consistent is a chiral $B_{3u}+iA_u$ non-unitary state, which provides fundamentals of the topological properties in UTe$_2$.

cond-mat.supr-con↗

arXiv2021

Anomalous Nernst Effect in Nonmagnetic Nodal Line Semimetal PbTaSe$_2$

PbTaSe$_2$ is a unique topological material, in which the number of nodal lines is expected to change at the structural transition between the lower temperature/pressure "L" phase and the higher temperature/pressure "H" phase. We report the anomalous Nernst effect attributed to the Berry curvature of nodal lines and its change with the structural transition. In the L phase, the Nernst coefficient ($S_{yx}$) shows the step-like magnetic field dependence reminiscent of the anomalous Nernst effect of nonmagnetic Dirac/Weyl semimetals. By applying hydrostatic pressure, we discovered that the amplitude of the anomalous component significantly decreases at the transition to the H phase, which might correspond to the partial annihilation of nodal line structures.

cond-mat.str-el↗

arXiv2021

Proton-$\rm ^3He$ elastic scattering at intermediate energies

We present a precise measurement of the cross section, proton and $\rm ^3He$ analyzing powers, and spin correlation coefficient $C_{y,y}$ for $p$-$\rm ^3He$ elastic scattering near 65 MeV, and a comparison with rigorous four-nucleon scattering calculations based on realistic nuclear potentials and a model with $Δ$-isobar excitation. Clear discrepancies are seen in some of the measured observables in the regime around the cross section minimum. Theoretical predictions using scaling relations between the calculated cross section and the $\rm ^3 He$ binding energy are not successful in reproducing the data. Large sensitivity to the $NN$ potentials and rather small $Δ$-isobar effects in the calculated cross section are noticed as different features from those in the deuteron-proton elastic scattering. The results obtained above indicate that $p$-$\rm ^3He$ scattering at intermediate energies is an excellent tool to explore nuclear interactions not accessible by three-nucleon scattering.

nucl-ex↗

arXiv2020

$^{181}$Ta Nuclear quadrupole resonance study of non-centrosymmetric superconductor PbTaSe$_2$

We report on a pure $^{181}$Ta-nuclear quadrupole resonance (NQR) measurement of PbTaSe$_2$ at zero magnetic field, which has the advantage of directly probing the intrinsic superconducting phase and electronic states of the TaSe$_2$ layer. We observed the $^{181}$Ta-NQR spectrum of the intrinsic structure with space group $P6$-$m2$, which agrees well with density functional theory (DFT) calculations. The nuclear spin relaxation rate ($1/T_1$) shows an exponential decrease well below $T_{\rm_c}$, indicating that the superconducting state is fully gapped in the framework of Bardeen-Cooper-Schrieffer (BCS) theory. The gap size obtained by $^{181}$Ta-NQR was smaller than the value in previous reports, which may imply that the Fermi surfaces composed of Ta-5$d$ orbitals, where the average pairing interactions are expected to be weaker than in BCS model, are primarily probed. The temperature dependence of $1/T_1$ below $T_{\rm_c}$ can be reproduced well by the superposition of quadrupole and magnetic relaxation mechanisms, together with the distribution of superconducting gap size inherent to multiple Fermi surfaces theoretically proposed in PbTaSe$_2$.

cond-mat.supr-con↗

arXiv2020

Switching of band inversion and topological surface states by charge density wave

Topologically nontrivial materials host protected edge states associated with the bulk band inversion through the bulk-edge correspondence. Manipulating such edge states is highly desired for developing new functions and devices practically using their dissipation-less nature and spin-momentum locking. Here we introduce a transition-metal dichalcogenide VTe$_2$, that hosts a charge density wave (CDW) coupled with the band inversion involving V3$d$ and Te5$p$ orbitals. Spin- and angle-resolved photoemission spectroscopy with first-principles calculations reveal the huge anisotropic modification of the bulk electronic structure by the CDW formation, accompanying the selective disappearance of Dirac-type spin-polarized topological surface states that exist in the normal state. Thorough three dimensional investigation of bulk states indicates that the corresponding band inversion at the Brillouin zone boundary dissolves upon CDW formation, by transforming into anomalous flat bands. Our finding provides a new insight to the topological manipulation of matters by utilizing CDWs' flexible characters to external stimuli.

cond-mat.mtrl-sci↗

arXiv2020

Nanoscale imaging of unusual photo-acoustic waves in thin flake VTe$_2$

Controlling acoustic phonons, the carriers of sound and heat, has been attracting great attention toward the manipulation of sonic and thermal properties in nanometric devices. In particular, the photo-acoustic effect using ultrafast optical pulses has a promising potential to optically manipulate phonons in picoseconds time regime. However, its mechanism has been so far mostly based on the commonplace thermoelastic expansion in isotropic media, limiting the spectrum of potential applications. We investigate a conceptually new mechanism of photo-acoustic effect involving the structural instability, by utilizing a transition-metal dichalcogenide VTe$_2$ with the ribbon-type charge-density-wave (CDW). Ultrafast electron microscope imaging and diffraction measurements reveal the generation and propagation of unusual acoustic waves in the nanometric thin plate associated with the optically induced instantaneous charge-density-wave dissolution. Our results highlight the capability of photo-induced structural instability as a source of coherent acoustic waves.

cond-mat.mtrl-sci↗

arXiv2020

Field-induced spin reorientation in the antiferromagnetic Dirac material EuMnBi$_2$ revealed by neutron and resonant x-ray diffraction

Field-dependent magnetic structure of a layered Dirac material EuMnBi$_2$ was investigated in detail by the single crystal neutron diffraction and the resonant x-ray magnetic diffraction techniques. On the basis of the reflection conditions in the antiferromagnetic phase at zero field, the Eu moments were found to be ordered ferromagnetically within the $ab$ plane and stacked antiferromagnetically along the $c$ axis in the sequence of up-up-down-down. Upon the spin-flop transition under the magnetic field parallel to the $c$ axis, the Eu moments are reoriented from the $c$ to the $a$ or $b$ directions forming two kinds of spin-flop domains, whereas the antiferromagnetic structure of the Mn sublattice remains intact as revealed by the quantitative analysis of the change in the neutron diffraction intensities. The present study provides a concrete basis to discuss the dominant role of the Eu sublattice on the enhanced two-dimensionality of the Dirac fermion transport in EuMnBi$_2$.

cond-mat.mtrl-sci↗

arXiv2020

Ferroelectric atomic displacement in multiferroic tetragonal perovskite Sr$_{1/2}$Ba$_{1/2}$MnO$_3$

We investigate the crystal structure in multiferroic tetragonal perovskite Sr$_{1/2}$Ba$_{1/2}$MnO$_3$ with high accuracy of the order of 10$^{-3}$ Angstrom for an atomic displacement. The large atomic displacement for Mn ion from the centerosymmetric position, comparable with the off-centering distortion in the tetragonal ferroelectric BaTiO$_3$, is observed in the ferroelectric phase ($T_\mathrm{N}$ $\leq$ $T$ $\leq$ $T_\mathrm{C}$). In stark contrast, in the multiferroic phase ($T$ $\leq$ $T_\mathrm{N}$), the atomic displacement for Mn ion is suppressed, but those for O ions are enlarged. The atomic displacements in the polar crystal structures are also analyzed in terms of the ferroelectric modes. In the ferroelectric phase, the atomic displacements are decomposed into dominant positive Slater, negative Last, and small positive Axe modes. The suppression of Slater and Last modes, the sign change of Last mode, and the enlargement of Axe mode are found in the multiferroic phase. The ferroelectric distortion is well reproduced by a first-principles calculation based on Berry phase method, providing an additional information on competing mechanisms to induce the polarization, electronic $p$-$d$ hybridization vs. magnetic exchange-striction.

cond-mat.mtrl-sci↗

arXiv2020

Bulk quantum Hall effect of spin-valley-coupled Dirac fermions in a polar antiferromagnet BaMnSb$_2$

Unconventional features of relativistic Dirac/Weyl quasi-particles in topological materials are most evidently manifested in the 2D quantum Hall effect (QHE), whose variety is further enriched by their spin and/or valley polarization. Although its extension to three dimensions has been long-sought and inspired theoretical proposals, material candidates have been lacking. Here we have discovered valley-contrasting spin-polarized Dirac fermions in a multilayer form in bulk antiferromagnet BaMnSb$_2$, where the out-of-plane Zeeman-type spin splitting is induced by the in-plane inversion symmetry breaking and spin-orbit coupling (SOC) in the distorted Sb square net. Furthermore, we have observed well-defined quantized Hall plateaus together with vanishing interlayer conductivity at low temperatures as a hallmark of the half-integer QHE in a bulk form. The Hall conductance of each layer is found to be nearly quantized to $2(N+1/2)e^2/h$ with $N$ being the Landau index, which is consistent with two spin-polarized Dirac valleys protected by the strong spin-valley coupling.

cond-mat.str-el↗

arXiv2020

Large Enhancement of Thermoelectric Efficiency Due to a Pressure-Induced Lifshitz Transition in SnSe

Lifshitz transition, a change in Fermi surface topology, is likely to greatly influence exotic correlated phenomena in solids, such as high-temperature superconductivity and complex magnetism. However, since the observation of Fermi surfaces is generally difficult in the strongly correlated systems, a direct link between the Lifshitz transition and quantum phenomena has been elusive so far. Here, we report a marked impact of the pressure-induced Lifshitz transition on thermoelectric performance for SnSe, a promising thermoelectric material without strong electron correlation. By applying pressure up to 1.6 GPa, we have observed a large enhancement of thermoelectric power factor by more than 100% over a wide temperature range (10-300 K). Furthermore, the high carrier mobility enables the detection of quantum oscillations of resistivity, revealing the emergence of new Fermi pockets at ~0.86 GPa. The observed thermoelectric properties linked to the multi-valley band structure are quantitatively reproduced by first-principles calculations, providing novel insight into designing the SnSe-related materials for potential valleytronic as well as thermoelectric applications.

cond-mat.mtrl-sci↗

arXiv2019

Measurement of 3He analyzing power for p-3He scattering using the polarized 3He target

Proton-3He scattering is one of the good probes to study the T=3/2 channel of three--nucleon forces. We have measured 3He analyzing powers for p-3He elastic scattering with the polarized 3He target at 70 and 100 MeV. The data are compared with the theoretical predictions based on the modern nucleon--nucleon potentials. Large discrepancies are found between the data and the calculations at the angles where the 3He analyzing power takes the minimum and maximum values, which are not explained by taking into account Delta-isobar degrees of freedom.

nucl-ex↗

arXiv2019

Orbitally defined field-induced electronic state in a Kondo lattice

CeRhIn$_{5}$ is a Kondo-lattice prototype in which a magnetic field B$\bf{^{\ast}\simeq}$ 30 T induces an abrupt Fermi-surface (FS) reconstruction and pronounced in-plane electrical transport anisotropy all within its antiferromagnetic state. Though the antiferromagnetic order at zero field is well-understood, the origin of an emergent state at B$^{\ast}$ remains unknown due to challenges inherent to probing states microscopically at high fields. Here, we report low-temperature Nuclear Magnetic Resonance (NMR) measurements revealing a discontinuous decrease in the $^{115}$In formal Knight shift, without changes in crystal or magnetic structures, of CeRhIn$_{5}$ at fields spanning B$^{\ast}$. We show that the emergent state above B$^{\ast}$ results from a change in Ce's 4f orbitals that arises from field-induced evolution of crystal-electric field (CEF) energy levels. This change in orbital character enhances hybridisation between the 4f and the conduction electrons (c.e.) that leads ultimately to an itinerant quantum-critical point at B$\bf{_{c0} \simeq}$ 50 T.

cond-mat.str-el↗

arXiv2018

${}^{239}$Pu nuclear magnetic resonance in the candidate topological insulator PuB$_4$

We present a detailed nuclear magnetic resonance (NMR) study of ${}^{239}$Pu in bulk and powdered single-crystal plutonium tetraboride (PuB$_4$), which has recently been investigated as a potential correlated topological insulator. This study constitutes the second-ever observation of the ${}^{239}$Pu NMR signal, and provides unique on-site sensitivity to the rich $f$-electron physics and insight into the bulk gap-like behavior in PuB$_4$. The ${}^{239}$Pu NMR spectra are consistent with axial symmetry of the shift tensor showing for the first time that ${}^{239}$Pu NMR can be observed in an anisotropic environment and up to room temperature. The temperature dependence of the ${}^{239}$Pu shift, combined with a relatively long spin-lattice relaxation time ($T_1$), indicate that PuB$_4$ adopts a non-magnetic state with gap-like behavior consistent with our density functional theory (DFT) calculations. The temperature dependencies of the NMR Knight shift and $T_1^{-1}$--microscopic quantities sensitive only to bulk states--imply bulk gap-like behavior confirming that PuB$_4$ is a good candidate topological insulator. The large contrast between the ${}^{239}$Pu orbital shifts in the ionic insulator PuO$_2$ ($\sim$~+24.7~\%) and PuB$_4$ ($\sim$~-0.5~\%) provides a new tool to investigate the nature of chemical bonding in plutonium materials.

cond-mat.str-el↗

arXiv2018

How different is the core of $^{25}$F from $^{24}$O$_{g.s.}$?

The neutron-shell structure of $^{25}$F was studied using quasi-free (p,2p) knockout reaction at 270A MeV in inverse kinematics. The sum of spectroscopic factors of $π$0d$_{5/2}$ orbital is found to be $1.0 \pm 0.3$. However, the spectroscopic factor for the ground-state to ground-state transition ($^{25}$F, $^{24}$O$_{g.s.}$) is only $0.36\pm 0.13$, and $^{24}$O excited states are produced from the 0d$_{5/2}$ proton knockout. The result shows that the $^{24}$O core of $^{25}$F nucleus significantly differs from a free $^{24}$O nucleus, and the core consists of 35% $^{24}$O$_{g.s}$. and 65% excited $^{24}$O.

nucl-ex↗

arXiv2018

Negative-pressure-induced helimagnetism in ferromagnetic cubic perovskites Sr$_{1-x}$Ba$_{x}$CoO$_{3}$

Helimagnetic materials are identified as promising for novel spintronic applications. Since helical spin order is manifested as a compromise of competing magnetic exchange interactions, its emergence is limited by unique constraints imposed by the crystalline lattice and the interaction geometries, as exemplified by the multiferroic perovskite manganites with large orthorhombic distortion. Here we show that a simple cubic perovskite SrCoO$_3$ with room-temperature ferromagnetism has the potential to host helimagnetic order upon isotropic lattice expansion. Increasing the Ba content $x$ in Sr$_{1-x}$Ba$_x$CoO$_3$ continuously expands the cubic lattice, eventually suppressing the ferromagnetic order near $x$=0.4, where helimagnetic correlations are observed as incommensurate diffuse magnetic scattering by neutron diffraction measurements. The emergence of helimagnetism is semi-quantitatively reproduced by first-principles calculations, leading to the conjecture that a simple cubic lattice with strong $d$-$p$ hybridisation can exhibit a variety of novel magnetic phases originating from competing exchange interactions.

cond-mat.str-el↗