arXiv subjects

Toshiro Sakakibara

Publications and source records attributed to Toshiro Sakakibara.

At least 37 records · Page 2Linked to original sources

arXiv2019

Superconductivity of Electron-Doped NdOBiS2 by Substitution of Mixed-Valence Ce ions

Superconductivity is achieved in Nd1-xCexOBiS2 via electron doping using mixed-valence Ce ions. Single crystals with x = 0.2, 0.3, 0.4, and 0.5 are generated using a CsCl flux method. Plate-like single crystals with dimensions of 0.8 0.8 0.2 mm3 were obtained. The magnetic susceptibility chi(T) indicates large diamagnetism, and the electrical resistivity rho(T) indicates zero resistivity. The maximum value of Tc is observed at 4.7 K in Nd0.8Ce0.2OBiS2 from chi(T). From the rho(T) measurements taken in several magnetic fields, the upper critical field mu0Hc2(0) is estimated to be about 12 and 0.34 T for the ab- and c-planes, respectively. We redetermined mu0Hc2(0) of NdO0.7F0.3BiS2, as about 35 and 0.78 T for the ab- and c-planes, respectively. The anisotropic parameter is estimated to be about 35 for Nd0.7Ce0.3OBiS2 and 45 for NdO0.7F0.3BiS2. The mu0Hc2(0) of Nd0.7Ce0.3OBiS2 is approximately two times smaller than that of NdO0.7F0.3BiS2, although the difference of Gamma is approximately 10.

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arXiv2019

Superconductivity in PtSbS with a Noncentrosymmetric Cubic Crystal Structure

We report the synthesis, electronic properties, and electronic structure of ullmannite-type PtSbS, which has a cubic crystal structure without space inversion symmetry. Electrical resistivity and magnetization measured at low temperatures suggested that this compound is a bulk superconductor with a superconducting transition temperature of Tc = 0.15 K. First principles calculations indicated that Fermi surfaces of PtSbS include strongly nested hole pockets, which can make this compound interesting if they contribute to the emergence of superconductivity.

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arXiv2019

Quasi-particle evidence for the nematic state above $T_{\rm{c}}$ in Sr$_x$Bi$_2$Se$_3$

In the electronic nematic state, an electronic system has a lower symmetry than the crystal structure of the same system. Electronic nematic states have been observed in various unconventional superconductors such as cuprate- and iron-based, heavy-fermion, and topological superconductors. The relation between nematicity and superconductivity is a major unsolved problem in condensed matter physics. By angle-resolved specific heat measurements, we report bulk quasi-particle evidence of nematicity in the topological superconductor Sr$_x$Bi$_2$Se$_3$. The specific heat exhibited a clear 2-fold symmetry despite the 6-fold symmetric lattice. Most importantly, the 2-fold symmetry appeared in the normal state above the superconducting transition temperature. This is explained by the angle-dependent Zeeman effect due to the anisotropic density of states in the nematic phase. Such results highlight the interrelation between nematicity and unconventional superconductivity.

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arXiv2019

Field-Induced Switching of Ferro-Quadrupole Order Parameter in PrTi2Al20

We report magnetic-field-induced first-order phase transitions in the ferro-quadrupole (FQ) ordered state of PrTi2Al20, in which non-Kramers Pr3+ ions with two 4f electrons have a non-magnetic Gamma3 doublet ground state in the cubic Td crystalline electric field. For magnetic fields along [111], 27Al-NMR and magnetization experiments reveal Qz \propto 3z2-r2 type FQ order below 2 K independent of field strength. Magnetic fields along [001] or [110], however, induce discontinuous switching of order parameters within the two dimensional space spanned by Qz and Qx \propto x2-y2 at small field values less than a few tesla. A symmetry-based theoretical analysis shows that the transitions can be caused by competition between the magnetic Zeeman interaction and anisotropy in the quadrupole-quadrupole interactions, if the latter dominates over the former in low fields and vice versa in high fields. Furthermore, striking violation of proportionality between NMR Knight shift and magnetic susceptibility is observed in the symmetry-broken FQ phases, indicating significant influence of FQ order on the hybridization between conduction and f electrons, which in turn mediates the RKKY-type quadrupole interaction causing the FQ order. This feedback effect may be a specific feature of quadrupole orders not commonly observed in magnetic phase transitions and play a key role for inducing the discontinuous transitions.

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arXiv2019

Quantum critical behaviour and Lifshitz transition in intermediate valence $α$-YbAlB$_4$

Intermetallic compounds containing $f$-electron elements have been prototypical materials for investigating strong electron correlations and quantum criticality (QC). Their heavy fermion ground state evoked by the magnetic $f$-electrons is susceptible to the onset of quantum phases, such as magnetism or superconductivity, due to the enhanced effective mass ($m^{*}$) and a corresponding decrease of the Fermi temperature. However, the presence of $f$-electron valence fluctuations to a non-magnetic state is regarded an anathema to QC, as it usually generates a paramagnetic Fermi-liquid state with quasiparticles of moderate $m^{*}$. Such systems are typically isotropic, with a characteristic energy scale $T_0$ of the order of hundreds of kelvins that require large magnetic fields or pressures to promote a valence or magnetic instability. Here we show that the intermediate valence compound $α$-YbAlB$_4$ surprisingly exhibits both quantum critical behaviour and a Lifshitz transition under low magnetic field, which is attributed to the anisotropy of the hybridization between the conduction and localized $f$-electrons. These findings suggest a new route to bypass the large valence energy scale in developing the QC.

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arXiv2018

Disorder-sensitive node-like small gap in FeSe

We investigate the band structure, nematic state and superconducting gap structure of two selected FeSe single crystals containing different amount of disorder. Transport and angle-resolved photoemission spectroscopy measurements show that the small amount of disorder has little effect to the band structure and the nematic state of FeSe. However, temperature and magnetic field dependencies of specific heat for the two samples are quite different. Wave-vector-dependent gap structure are obtained from the three dimensional field-angle-resolved specific heat measurements. A small gap with two vertical-line nodes or gap minima along the $k_z$ direction is found only in the sample with higher quality. Such symmetry-unprotected nodes or gap minima are found to be smeared out by small amount of disorder, and the gap becomes isotropic in the sample of lower quality. Our study reveals that the reported controversy on the gap structure of FeSe is due to the disorder-sensitive node-like small gap.

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arXiv2018

Searching for gap zeros in Sr2RuO4 via field-angle-dependent specific-heat measurement

The gap structure of Sr$_2$RuO$_4$, which is a longstanding candidate for a chiral p-wave superconductor, has been investigated from the perspective of the dependence of its specific heat on magnetic field angles at temperatures as low as 0.06 K ($\sim 0.04T_{\rm c}$). Except near $H_{\rm c2}$, its fourfold specific-heat oscillation under an in-plane rotating magnetic field is unlikely to change its sign down to the lowest temperature of 0.06 K. This feature is qualitatively different from nodal quasiparticle excitations of a quasi-two-dimensional superconductor possessing vertical lines of gap minima. The overall specific-heat behavior of Sr$_2$RuO$_4$ can be explained by Doppler-shifted quasiparticles around horizontal line nodes on the Fermi surface, whose in-plane Fermi velocity is highly anisotropic, along with the occurrence of the Pauli-paramagnetic effect. These findings, in particular, the presence of horizontal line nodes in the gap, call for a reconsideration of the order parameter of Sr$_2$RuO$_4$.

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arXiv2018

Field-rotational magnetocaloric effect: A new experimental technique for accurate measurement of the anisotropic magnetic entropy

We developed a new technique for measuring the thermodynamic entropy as a function of the magnetic field angle. This technique enables high-resolution angle-resolved measurements of the entropy in an unprecedentedly short measuring time. When the magnetic field is rotated under adiabatic conditions, the sample temperature changes owing to the field-angle variation of its entropy, which is referred to as the rotational magnetocaloric effect. By investigating this effect along with the specific heat, the field-angle dependence of the entropy can be determined. To demonstrate this technique, we chose the spin-ice compound Dy$_2$Ti$_2$O$_7$ as a benchmark and showed good agreement between the measured and theoretical entropies as a function of the field angle. This development provides a new approach to studying condensed-matter physics, in which multiple degrees of freedom play an important role.

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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.

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arXiv2017

Gap structure of FeSe determined by field-angle-resolved specific heat measurements

Quasiparticle excitations in FeSe were studied by means of specific heat ($C$) measurements on a high-quality single crystal under rotating magnetic fields. The field dependence of $C$ shows three-stage behavior with different slopes, indicating the existence of three gaps ($Δ_1$, $Δ_2$, and $Δ_3$). In the low-temperature and low-field region, the azimuthal-angle ($ϕ$) dependence of $C$ shows a four-fold symmetric oscillation with sign change. On the other hand, the polar-angle ($θ$) dependence manifests as an anisotropy-inverted two-fold symmetry with unusual shoulder behavior. Combining the angle-resolved results and the theoretical calculation, the smaller gap $Δ_1$ is proved to have two vertical-line nodes or gap minima along the $k_z$ direction, and is determined to reside on the electron-type $\varepsilon$ band. $Δ_2$ is found to be related to the electron-type $δ$ band, and is isotropic in the $ab$-plane but largely anisotropic out of the plane. $Δ_3$ residing on the hole-type $α$ band shows a small out-of-plane anisotropy with a strong Pauli-paramagnetic effect.

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arXiv2017

Wing structure in the phase diagram of the Ising Ferromagnet URhGe close to its tricritical point investigated by angle-resolved magnetization measurements

High-precision angle-resolved dc magnetization and magnetic torque studies were performed on a single-crystalline sample of URhGe, an orthorhombic Ising ferromagnet with the $c$ axis being the magnetization easy axis, in order to investigate the phase diagram around the ferromagnetic (FM) reorientation transition in a magnetic field near the $b$ axis. We have clearly detected first-order transition in both the magnetization and the magnetic torque at low temperatures, and determined detailed profiles of the wing structure of the three-dimensional $T$-$H_{b}$-$H_{c}$ phase diagram, where $H_{c}$ and $H_{b}$ denotes the field components along the $c$ and the $b$ axes, respectively. The quantum wing critical points are located at $μ_0H_c\sim\pm$1.1 T and $μ_0H_b\sim$13.5 T. Two second-order transition lines at the boundaries of the wing planes rapidly tend to approach with each other with increasing temperature up to $\sim 3$ K. Just at the zero conjugate field ($H_c=0$), however, a signature of the first-order transition can still be seen in the field derivative of the magnetization at $\sim 4$ K, indicating that the tricritical point exists in a rather high temperature region above 4 K. This feature of the wing plane structure is consistent with the theoretical expectation that three second-order transition lines merge tangentially at the triciritical point.

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arXiv2017

Quasiparticle Excitations and Evidence for Superconducting Double Transitions in Monocrystalline U0.97Th0.03Be13

Superconducting (SC) gap symmetry and magnetic response of cubic U0.97Th0.03Be13 are studied by means of high-precision heat-capacity and dc magnetization measurements using a single crystal, in order to address the long-standing question of its second phase transition at Tc2 in the SC state below Tc1. The absence (presence) of an anomaly at Tc2 in the field-cooling (zero-field-cooling) magnetization indicates that this transition is between two different SC states. There is a qualitative difference in the field variation of the transition temperatures; Tc2(H) is isotropic whereas Tc1(H) exhibits a weak anisotropy between [001] and [111] directions. In the low temperature phase below Tc2(H), the angle-resolved heat-capacity $C(T,H, ϕ)$ reveals that the gap is fully opened over the Fermi surface, narrowing down the possible gap symmetry.

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arXiv2016

Thermodynamic Investigation of Metamagnetic Transitions and Partial Disorder in the Quasi-Kagome Kondo Lattice CePdAl

Low-temperature ($T$) thermodynamic properties in magnetic fields ($B$) of the quasi-Kagome antiferromagnet CePdAl ($T_{\rm N}\sim$ 2.7 K) were studied by means of magnetization and specific-heat $C(T,B)$ measurements. The unusual magnetic phase diagram, including three thermodynamic phases I-III, and crossover anomalies, has been obtained. In low-field phase I, the existence of the partial Kondo screening state on the one-third Ce sites is supported from the thermodynamic point of view: (i) an appearance of the paramagnetic moment with one-third of the full moment around 3 T, and (ii) an occurrence of the entropy release $\sim$0.3$R$ln2 around the observed crossover. On the other hand, a strong enhancement of $C/T$ is found around the point, where $T_{\rm N}(B)$ meets the boundary of phase III, indicating the possible presence of a tri-critical point. Furthermore, it is suggested that the three metamagnetic transitions at $B_{{\rm m}i}$ ($i=$1,2,3) come from successive increments of the Ising-type magnetic moment along the $c$-axis. In high-field state above 20 T, the behavior of $f$ electron can be rather described by the crystal-electric-field model.

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arXiv2016

Thermodynamic study of gap structure and pair-breaking effect by magnetic field in the heavy-fermion superconductor CeCu2Si2

This paper presents the results of specific-heat and magnetization measurements, in particular their field-orientation dependence, on the first discovered heavy-fermion superconductor CeCu$_2$Si$_2$ ($T_{\rm c} \sim 0.6$ K). We discuss the superconducting gap structure and the origin of the anomalous pair-breaking phenomena, leading e.g., to the suppression of the upper critical field $H_{\rm c2}$, found in the high-field region. The data show that the anomalous pair breaking becomes prominent below about 0.15 K in any field direction, but occurs closer to $H_{\rm c2}$ for $H \parallel c$. The presence of this anomaly is confirmed by the fact that the specific-heat and magnetization data satisfy standard thermodynamic relations. Concerning the gap structure, field-angle dependences of the low-temperature specific heat within the $ab$ and $ac$ planes do not show any evidence for gap nodes. From microscopic calculations in the framework of a two-band full-gap model, the power-law-like temperature dependences of $C$ and $1/T_1$, reminiscent of nodal superconductivity, have been reproduced reasonably. These facts further support multiband full-gap superconductivity in CeCu$_2$Si$_2$.

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arXiv2016

Superconductivity and non-Fermi-liquid behavior in the heavy-fermion compound CeCo$_{1-x}$Ni$_x$In$_5$

The effect of off-plane impurity on superconductivity and non-Fermi-liquid (NFL) behavior in the layered heavy-fermion compound CeCo$_{1-x}$Ni$_x$In$_5$ is investigated by specific heat, magnetization, and electrical resistivity measurements. These measurements reveal that the superconducting (SC) transition temperature T$_c$ monotonically decreases from 2.3 K (x=0) to 0.8 K (x=0.20) with increasing x, and then the SC order disappears above x=0.25. At the same time, the Ni substitution yields the NFL behavior at zero field for x=0.25, characterized by the -ln T divergence in specific heat divided by temperature, C$_p$/T, and magnetic susceptibility, M/B. The NFL behavior in magnetic fields for x=0.25 is quite similar to that seen at around the SC upper critical field in pure CeCoIn$_5$, suggesting that both compounds are governed by the same antiferromagnetic quantum criticality. The resemblance of the doping effect on the SC order among Ni- , Sn-, and Pt-substituted CeCoIn5 supports the argument that the doped carriers are primarily responsible for the breakdown of the SC order. The present investigation further reveals the quantitative differences in the trends of the suppression of superconductivity between Ce(Co,Ni)In$_5$ and the other alloys, such as the rates of decrease in T$_c$, dT$_c$/dx, and specific heat jump at T$_c$, d($Δ$C$_p$/T$_c$)/dx. We suggest that the occupied positions of the doped ions play an important role in the origin of these differences.

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arXiv2016

Evidence for chiral d-wave superconductivity in URu2Si2 from the field-angle variation of its specific heat

Low-energy quasiparticle (QP) excitations in the heavy-fermion superconductor URu$_2$Si$_2$ were investigated by specific-heat $C(T, H, ϕ, θ)$ measurements of a high-quality single crystal. The occurrence of QP excitations due to the Doppler-shift effect was detected regardless of the field direction in $C(H)$ of the present clean sample, which is in sharp contrast to a previous report. Furthermore, the polar-angle-dependent $C(θ)$ measured under a rotating magnetic field within the ac plane exhibits a shoulder-like anomaly at $θ\sim 45$ deg and a sharp dip at $θ= 90$ deg ($H \parallel a$) in the moderate-field region. These features are supported by theoretical analyses based on microscopic calculations assuming the gap symmetry of $k_z(k_x+ik_y)$, whose gap structure is characterized by a combination of a horizontal line node at the equator and point nodes at the poles. The present results have settled the previous controversy over the gap structure of URu$_2$Si$_2$ and have authenticated its chiral $d$-wave superconductivity.

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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.

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arXiv2015

$S$ = 1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain in a zinc-verdazyl complex

We successfully synthesized the zinc-verdazyl complex [Zn(hfac)$_2$]$\cdot$($o$-Py-V) [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; $o$-Py-V = 3-(2-pyridyl)-1,5-diphenylverdazyl], which is an ideal model compound with an $S$ = 1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain (F-AF AHC). $Ab$ $initio$ molecular orbital (MO) calculations indicate that two dominant interactions $J_{\rm{F}}$ and $J_{\rm{AF}}$ form the $S=1/2$ F-AF AHC in this compound. The magnetic susceptibility and magnetic specific heat of the compound exhibit thermally activated behavior below approximately 1 K. Furthermore, its magnetization curve is observed up to the saturation field and directly indicates a zero-field excitation gap of 0.5 T. These experimental results provide evidence for the existence of a Haldane gap. We successfully explain the results in terms of the $S=1/2$ F-AF AHC through quantum Monte Carlo calculations with $|J_{\rm{AF}}/J_{\rm{F}}|$ = 0.22. The $ab$ $initio$ MO calculations also indicate a weak AF interchain interaction $J'$ and that the coupled F-AF AHCs form a honeycomb lattice. The $J'$ dependence of the Haldane gap is calculated, and the actual value of $J'$ is determined to be less than 0.01$|J_{\rm{F}}|$.

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