arXiv subjects

Yusei Shimizu

Publications and source records attributed to Yusei Shimizu.

At least 55 records · Page 3Linked to original sources

arXiv2020

Multiple Superconducting Phases and Unusual Enhancement of the Upper Critical Field in UTe2

We performed AC calorimetry and magnetoresistance measurements under pressure for H || a-axis (easy-magnetization axis) in the novel heavy-fermion superconductor UTe2. Thanks to the thermodynamic information, multiple superconducting phases have been revealed under pressure and magnetic field. The (H,T) phase diagram of superconductivity under pressure displays an abrupt increase of the upper critical field (Hc2) at low temperature and in the high field region, and a strong convex curvature of Hc2 at high temperature. This behavior of Hc2 and the multiple superconducting phases require a state for the superconducting order parameter more complex than a spin-triplet equal spin pairing. Above the superconducting critical pressure, Pc, we find strong indications that the possible magnetic order is closer to antiferromagnetism than to ferromagnetism.

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arXiv2020

Anisotropy of the Upper Critical Field in the Heavy-Fermion Superconductor UTe2 under Pressure

We studied the anisotropy of the superconducting upper critical field $H_{\rm c2}$ in the heavy-fermion superconductor UTe$_2$ under hydrostatic pressure by magnetoresistivity measurements. In agreement with previous experiments we confirm that superconductivity disappears near a critical pressure $p_{\rm c} \approx 1.5$~GPa, and a magnetically ordered state appears. The unusual $H_{\rm c2}(T)$ at low temperatures for $H \parallel a$ suggests that the multiple superconducting phases which appear under pressure have quite different $H_{\rm c2}$. For a field applied along the hard magnetization $b$ axis $H_{\rm c2} (0)$ is glued to the metamagnetic transition $H_{\rm m}$ which is suppressed near $p_{\rm c}$. The suppression of $H_{\rm m}$ with pressure follows the decrease of temperature $T_χ^{\rm max}$, at the maximum in the susceptibility along $b$. The strong reinforcement of $H_{\rm c2}$ at ambient pressure for $H \parallel b$ above 16~T is rapidly suppressed under pressure due to the increase of $T_{\rm sc}$ and the decrease of $H_{\rm m}$. The change in the hierarchy of the anisotropy of $H_{\rm c2}(0)$ on approaching $p_{\rm c}$ points out that the $c$ axis becomes the hard magnetization axis.

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arXiv2020

Spin-Triplet Superconductivity in UTe2 and Ferromagnetic Superconductors

The spin-triplet state is most likely realized in uranium ferromagnetic superconductors, UGe2, URhGe, UCoGe. The microscopic coexistence of ferromagnetism and superconductivity means that the Cooper pair should be realized under the strong internal field due the ferromagnetism, leading to the spin-triplet state with equal spin pairing. The field-reinforced superconductivity, which is observed in all three materials when the ferromagnetic fluctuations are enhanced, is one of the strong evidences for the spin-triplet superconductivity. We present here the results of a newly discovered spin-triplet superconductor, UTe2, and compare those with the results of ferromagnetic superconductors. Although no magnetic order is found in UTe2, there are similarities between UTe2 and ferromagnetic superconductors. For example, the huge upper critical field exceeding the Pauli limit and the field-reentrant superconductivity for H || b-axis are observed in UTe2, URhGe and UCoGe. We also show the specific heat results on UTe2 in different quality samples, focusing on the residual density of states in the superconducting phase.

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arXiv2019

Superconducting Properties of Heavy Fermion UTe$_2$ Revealed by $^{125}$Te-Nuclear Magnetic Resonance

We have performed the $^{125}$Te-nuclear magnetic resonance (NMR) measurement in the field along the $b$ axis on the newly discovered superconductor UTe$_2$, which is a candidate of a spin-triplet superconductor. The nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ abruptly decreases below a superconducting (SC) transition temperature $T_c$ without showing a coherence peak, indicative of UTe$_2$ being an unconventional superconductor. It was found that the temperature dependence of $1/T_1T$ in the SC state cannot be understood by a single SC gap behavior but can be explained by a two SC gap model. The Knight shift, proportional to the spin susceptibility, decreases below $T_c$, but the magnitude of the decrease is much smaller than the decrease expected in the spin-singlet pairing. Rather, the small Knight-shift decrease as well as the absence of the Pauli-depairing effect can be interpreted by the spin triplet scenario.

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arXiv2019

125Te-NMR Study on a Single Crystal of Heavy Fermion Superconductor UTe2

We report 125Te-NMR studies on a newly discovered heavy fermion superconductor UTe2. Using a single crystal, we have measured the 125Te-NMR Knight shift K and spin-lattice relaxation rate 1/T1 for fields along the three orthorhombic crystal axes. The data confirm a moderate Ising anisotropy for both the static (K) and dynamical susceptibilities (1/T1) in the paramagnetic state above about 20 K. Around 20 K, however, we have observed a sudden loss of NMR spin-echo signal due to sudden enhancement of the NMR spin-spin relaxation rate 1/T2, when the field is applied along the easy axis of magnetization (=a axis). This behavior suggests the development of longitudinal magnetic fluctuations along the a axis at very low frequencies below 20 K.

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arXiv2019

Metamagnetic Transition in Heavy Fermion Superconductor UTe2

We have studied the magnetization of the recently discovered heavy fermion superconductor UTe$_2$ up to 56 T in pulsed-magnetic fields. A first-order metamagnetic transition has been clearly observed at $H_{\rm m}$ =34.9 T when the magnetic field $H$ is applied along the orthorhombic hard-magnetization $b$-axis. The transition has a critical end point at $\sim$11 K and 34.8 T, where the first order transition terminates and changes into a crossover regime. Using the thermodynamic Maxwell relation, we have evaluated the field dependence of the Sommerfeld coefficient of the specific heat directly related to the superconducting pairing. From the analysis, we found a significant enhancement of the effective mass centered at $H_{\rm m}$, which is reminiscent of the field-reentrant superconductivity of the ferromagnet URhGe in transverse fields. We discuss the origin of their field-robust superconductivity.

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arXiv2019

Unconventional Superconductivity in Heavy Fermion UTe2

We grew single crystals of the recently discovered heavy fermion superconductor UTe2, and measured the resistivity, specific heat and magnetoresistance. Superconductivity (SC) was clearly detected at Tsc=1.65K as sharp drop of the resistivity in a high quality sample of RRR=35. The specific heat shows a large jump at Tsc indicating strong coupling. The large Sommerfeld coefficient, 117mJ K-2mol-1 extrapolated in the normal state and the temperature dependence of C/T below Tsc are the signature of unconventional SC. The discrepancy in the entropy balance at Tsc between SC and normal states points out that hidden features must occur. Surprisingly, a large residual value of the Sommerfeld coefficient seems quite robust (gamma_0/gamma ~ 0.5). The large upper critical field Hc2 along the three principal axes favors spin-triplet SC. For H // b-axis, our experiments do not reproduce the huge upturn of Hc2 reported previously. This discrepancy may reflect that Hc2 is very sensitive to the sample quality. A new perspective in UTe2 is the proximity of a Kondo semiconducting phase predicted by the LDA band structure calculations.

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arXiv2018

Magnetic field effect on the chiral magnetism of noncentrosymmetric UPtGe: experiment and theory

The effect of differently oriented magnetic field on chiral incommensurate helimagnet UPtGe is studied both experimentally and theoretically. The magnetization measurements up to the field above the saturation have revealed an isotropic magnetic response below 20 T and a remarkable nonmonotonic anisotropy in high fields. Moreover, the two principally different phase transitions from the noncollinear incommensurate to the field-induced ferromagnetic state have been observed. These properties are successfully explained by density-functional theory calculations taking into account the noncollinearity of the magnetic structures, arbitrary directed magnetic field, and relativistic effects. We also estimate the strength of different competing magnetic interactions and discuss possible scenarios of the field-induced phase transformations.

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arXiv2018

Low-Temperature X-ray Crystal Structure Analysis of the Cage-Structured Compounds MBe13 (M = La, Sm, and U)

The beryllides MBe13 (M = rare earths and actinides) crystallize in a NaZn13-type cubic structure, which can be categorized as a cage-structured compound. In this study, powder X-ray diffraction measurements have been performed on LaBe13, SmBe13, and UBe13 in the temperature range between 7 and 300 K in order to investigate their crystallographic characteristics systematically. They keep the NaZn13-type cubic structure down to the lowest temperature. We estimated their Debye temperature to be 600 - 750 K from analyses of the temperature dependence of a lattice parameter, being in good agreement with the values reported previously. Rietveld refinements on the obtained powder patterns revealed that the M atom in the 8a site is located in an almost ideal snub cube formed by 24 Be atoms in the 96i site, whose caged structure is unchanged even at the low temperatures. In addition, it is argued from the temperature variation of an isotropic mean-square displacement parameter that the MBe13 compounds commonly have a low-energy phonon mode, which can be described by a model assuming an Einstein oscillation of the M atom with a characteristic temperature of ~ 160 K.

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arXiv2018

Splitting Fermi Surfaces and Heavy Electronic States in Non-Centrosymmetric U3Ni3Sn4

We report the single-crystal growth of the non-centrosymmetric paramagnet U3Ni3Sn4 by the Bridgman method and the Fermi surface properties detected by de Haas-van Alphen (dHvA) experiments. We have also investigated single-crystal U3Ni3Sn4 by single-crystal X-ray diffraction, magnetization, electrical resistivity, and heat capacity measurements. The angular dependence of the dHvA frequencies reveals many closed Fermi surfaces, which are nearly spherical in topology. The experimental results are in good agreement with local density approximation (LDA) band structure calculations based on the 5f-itinerant model. The band structure calculation predicts many Fermi surfaces, mostly with spherical shape, derived from 12 bands crossing the Fermi energy. To our knowledge, the splitting of Fermi surfaces due to the non-centrosymmetric crystal in 5f-electron systems is experimentally detected for the first time. The temperature dependence of the dHvA amplitude reveals a large cyclotron effective mass of up to 35m0, indicating the heavy electronic state of U3Ni3Sn4 due to the proximity of the quantum critical point. From the field dependence of the dHvA amplitude, a mean free path of conduction electrons of up to 1950A is detected, reflecting the good quality of the grown crystal. The small splitting energy related to the antisymmetric spin-orbit interaction is most likely due to the large cyclotron effective mass.

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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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arXiv2017

Magnetically Ordered State and Crystalline-Electric-Field Effects in SmBe$_{13}$

The physical properties of single-crystalline SmBe$_{13}$ with a NaZn$_{13}$-type cubic structure have been studied by electrical resistivity ($ρ$), specific heat ($C$), and magnetization ($M$) measurements in magnetic fields of up to 9 T. The temperature ($T$) dependence of $ρ$ shows normal metallic behavior without showing the Kondo -ln$T$ behavior, suggesting the weak hybridization effect in this system. Analyses of the temperature dependence of $C$ suggest that the Sm ions of this compound are trivalent and that the crystalline-electric-field (CEF) ground state is a $Γ_8$ quartet with a first-excited state of a $Γ_7$ doublet located at the energy scale of $\sim$ 90 K. Mean-field calculations based on the suggested CEF level scheme can reasonably well reproduce the $T$ dependence of magnetic susceptibility ($χ$) below $\sim$ 70 K. These results in the paramagnetic state strongly indicate that the 4$f$ electrons are well localized with the Sm$^{3+}$ configuration. At low temperatures, the 4$f$ electrons undergo a magnetic order at $T_{\rm M}$ $\sim$ 8.3 K, where $χ$($T$) shows an antiferromagnetic-like cusp anomaly. From the positive Curie--Weiss temperature obtained from the mean-field calculations and from a constructed magnetic phase diagram with multiple regions, we discussed the magnetic structure of SmBe$_{13}$ below $T_{\rm M}$, by comparing with other isostructural MBe$_{13}$ compounds showing helical-magnetic ordering.

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arXiv2017

Observation of Low-Energy Einstein Phonon and Superconductivity in Single-Crystalline LaBe$_{13}$

The thermal and electrical transport properties of single-crystalline LaBe$_{13}$ have been investigated by specific-heat ($C$) and electrical-resistivity ($ρ$) measurements. The specific-heat measurements in a wide temperature range revealed the presence of a hump anomaly near 40 K in the $C$($T$)/$T$ curve, indicating that LaBe$_{13}$ has a low-energy Einstein-like-phonon mode with a characteristic temperature of $\sim$ 177 K. In addition, a superconducting transition was observed in the $ρ$ measurements at the transition temperature of 0.53 K, which is higher than the value of 0.27 K reported previously by Bonville et al. Furthermore, an unusual $T^3$ dependence was found in $ρ$($T$) below $\sim$ 50 K, in contrast to the behavior expected from the electron--electron scattering or the electron--Debye phonon scattering.

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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

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

Unconventional Strong Spin-Fluctuation Effects around the Critical Pressure of the Itinerant Ising-Type Ferromagnet URhAl

Resistivity measurements were performed for the itinerant Ising-type ferromagnet URhAl at temperatures down to 40 mK under high pressure up to 7.5 GPa, using single crystals. We found that the critical pressure of the Curie temperature exists at around $P_c$ ~ 5.2 GPa. Near $P_c$, the $A$-coefficient of the $AT^{2}$ Fermi-liquid resistivity term below $T^*$ is largely enhanced with a maximum around 5.2-5.5 GPa. Above $P_c$, the exponent of the resistivity $ρ(T)$ deviates from 2. At $P_c$, it is close to $n = 5/3$, which is expected by the theory of three-dimensional ferromagnetic spin fluctuations for a 2nd-order quantum-critical point (QCP). However, $T_C(P)$ disappears as a 1st-order phase transition, and the critical behavior of resistivity in URhAl cannot be explained by the theory of a 2nd-order QCP. The 1st-order nature of the phase transition is weak, and the critical behavior is still dominated by the spin fluctuation at low temperature. With increasing pressure, the non-Fermi-liquid behavior is observed in higher fields. Magnetic field studies point out a ferromagnetic wing structure with a tri-critical point (TCP) at ~ 4.8-4.9 GPa in URhAl. One open possibility is that the switch from the ferromagnetic to the paramagnetic states does not occur simply but an intermediate state arises below the TCP as suggested theoretically recently. Quite generally, if a drastic Fermi-surface change occurs through $P_c$, the nature of the interaction itself may change and lead to the observed unconventional behavior.

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