arXiv subjects

Kazuyoshi Yoshimura

Publications and source records attributed to Kazuyoshi Yoshimura.

At least 19 recordsLinked to original sources

arXiv2024

Molecular Orbital Electronic Instability in the van der Waals Kagome Semiconductor Nb$_3$Cl$_8$: Exploring Future Directions

Nb$_3$Cl$_8$, a cluster Mott insulator with a distinctive magnetic molecular orbital structure organized into a breathing kagome lattice, showcases critical phase transitions under specialized conditions. By transitioning from paramagnetic to nonmagnetic states below 90 K, we clarified this behavior through combined nuclear magnetic resonance and low-temperature X-ray diffraction studies, pointing to charge disproportionation as the driving force. Subsequent investigations via angle-resolved photoemission spectroscopy and first-principles calculations have disclosed topologically flat bands, confirming advanced electronic characteristics in Nb$_3$Cl$_8$. These discoveries not only deepen our comprehension of Mott insulators but also broaden our grasp of the dynamic interrelations among topology, electron interactions, and quantum phenomena in two-dimensional systems. The research on Nb$_3$Cl$_8$ thus lays foundational knowledge for advancing the exploration of quantum states in complex material systems, marking it as a critical model in the ongoing evolution of condensed matter physics.

cond-mat.mtrl-sci↗

arXiv2022

Spin Seebeck effect in quantum magnet Pb2V3O9

Spin Seebeck effect (SSE), the generation of spin current from heat, has been extensively studied in a large variety of magnetic materials, including ferromagnets, antiferromagnets, paramagnets, and quantum spin liquids. In this paper, we report the study of the SSE in the single crystalline Pb2V3O9, a spin-gapped quantum magnet candidate with quasi-one-dimensional spin-1/2 chain. Detailed temperature and magnetic field dependences of the SSE are investigated, and the temperature-dependent critical magnetic fields show a strong correlation to the Bose-Einstein condensation phase of the quantum magnet Pb2V3O9. This work shows the potential of using spin current as a probe to study the spin correlation and phase transition properties in quantum magnets.

cond-mat.mtrl-sci↗

arXiv2021

Pressure driven magnetic order in Sr$_{1-x}$Ca$_x$Co$_2$P$_2$

The magnetic phase diagram of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ as a function of hydrostatic pressure and temperature is investigated by means of high pressure muon spin rotation, relaxation and resonance ($\mu^+$SR). The weak pressure dependence for the $x\neq1$ compounds suggests that the rich phase diagram of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ as a function of $x$ at ambient pressure may not only be attributed to solely chemical pressure effects. The $x=1$ compound on the other hand reveals a high pressure dependence, where the long range magnetic order is fully suppressed at $p_{\rm c2}\approx9.8$~kbar, which seem to be a first order transition. In addition, an intermediate phase consisting of dilute ferromagnetic islands (FMI) is formed above $p_{\rm c1}\approx8$~kbar where they co-exist with a magnetically disordered state. Moreover, such FMI phase seems to consist of an high- (FMI-\textcircled{\small{1}}) and low-temperature (FMI-\textcircled{\small{2}}) region, respectively, separated by a phase boundary at $T_{\rm i}\approx20$~K.

cond-mat.str-el↗

arXiv2021

Co-existence of short- and long-range magnetic order in LaCo$_2$P$_2$

The ferromagnetic (FM) nature of the metallic LaCo$_2$P$_2$ was investigated with the positive muon spin rotation, relaxation and resonance ($μ^+$SR) technique. Transverse and zero field $μ^+$SR measurements revealed that the compound enters a long range FM ground state at $T_{\rm C}=130.91(65)$~K, consistent with previous studies. Based on the reported FM structure, the internal magnetic field was computed at the muon sites, which were predicted with first principles calculations. The computed result agree well with the experimental data. Moreover, although LaCo$_2$P$_2$ is a paramagnet at higher temperatures $T>160$~K, it enters a short range ordered (SRO) magnetic phase for $T_{\rm C}<T\leq160$~K. Measurements below the vicinity of $T_{\rm C}$ revealed that the SRO phase co-exists with the long range FM order at temperatures $124\leq T\leq T_{\rm C}$. Such co-existence is an intrinsic property and stems from competition between the 2D and 3D interactions/fluctuations.

cond-mat.str-el↗

arXiv2020

Investigation of Ionic and Anomalous Magnetic Behavior in CrSe$_2$ Using $^8$Li $β$-NMR

We have studied a mosaic of 1T-CrSe$_2$ single crystals using $β$-detected nuclear magnetic resonance of $^{8}$Li from 4 to 300 K. We identify two broad resonances that show no evidence of quadrupolar splitting, indicating two magnetically distinct environments for the implanted ion. We observe stretched exponential spin lattice relaxation and a corresponding rate ($1/T_1$) that increases monotonically above 200 K, consistent with the onset of ionic diffusion. A pronounced maximum in $1/T_1$ is observed at the low temperature magnetic transition near 20 K. Between these limits, $1/T_1$ instead exhibits a broad minimum with a remarkable absence of strong features in the vicinity of structural and magnetic transitions between 150 and 200 K. Together, the results suggest $^{8}$Li$^{+}$ site occupation within the van der Waals gap between CrSe$_2$ trilayers. Possible origins of the two environments are discussed.

cond-mat.mtrl-sci↗

arXiv2018

Magnetic ordering with an XY-like anisotropy in the honeycomb lattice iridates ZnIrO$_3$ and MgIrO$_3$ synthesized via a metathesis reaction

We have successfully synthesized the novel antiferromagnets with Ir$^{4+}$ honeycomb lattice ZnIrO$_3$ and MgIrO$_3$ and investigated their magnetic and thermodynamic properties. The two iridates are isomorphic but exhibit qualitatively different magnetic properties. ZnIrO$_3$ shows antiferromagnetic ordering below 46.6 K, whereas MgIrO$_3$ displays weak ferromagnetic behavior below 31.8 K owing to formation of a canted antiferromagnetic ordering. The measurement of magnetic susceptibility with using an oriented powder sample revealed the presence of an XY-like magnetic anisotropy and a tilting magnetic structure which is possibly stabilized by the Kitaev interaction. Moreover, magnetization curves of MgIrO3 and ZnIrO3 up to 60 T show different behaviors, demonstrating that each magnetic ground state is different with each other. We discuss the difference in the ground state between MgIrO$_3$ and ZnIrO$_3$ from the viewpoint a magnetic model consisting of the Kitaev and Dzyaloshinskii-Moriya interactions with the spin frustration effect on the honeycomb lattice.

cond-mat.str-el↗

arXiv2017

Dynamics of Bound Magnon Pairs in the Quasi-One-Dimensional Frustrated Magnet LiCuVO_4

We report on the dynamics of the spin-1/2 quasi-one-dimensional frustrated magnet LiCuVO$\mathrm{_4}$ measured by nuclear spin relaxation in high magnetic fields 10--34 T, in which the ground state has spin-density-wave order. The spin fluctuations in the paramagnetic phase exhibit striking anisotropy with respect to the magnetic field. The transverse excitation spectrum probed by $^{51}$V nuclei has an excitation gap, which increases with field. On the other hand, the gapless longitudinal fluctuations sensed by $^7$Li nuclei grow with lowering temperature, but tend to be suppressed with increasing field. Such anisotropic spin dynamics and its field dependence agree with the theoretical predictions and are ascribed to the formation of bound magnon pairs, a remarkable consequence of the frustration between ferromagnetic nearest neighbor and antiferromagnetic next-nearest-neighbor interactions.

cond-mat.str-el↗

arXiv2017

Transitions from a Kondo-like diamagnetic insulator into a modulated ferromagnetic metal in $\bm{\mathrm{FeGa}_{3-y}\mathrm{Ge}_y}$

One initial and essential question of magnetism is whether the magnetic properties of a material are governed by localized moments or itinerant electrons. Here we expose the case for the weakly ferromagnetic system FeGa$_{3-y}$Ge$_y$ wherein these two opposite models are reconciled, such that the magnetic susceptibility is quantitatively explained by taking into account the effects of spin-spin correlation. With the electron doping introduced by Ge substitution, the diamagnetic insulating parent compound FeGa$_3$ becomes a paramagnetic metal as early as at $ y=0.01 $, and turns into a weakly ferromagnetic metal around the quantum critical point $ y=0.15 $. Within the ferromagnetic regime of FeGa$_{3-y}$Ge$_y$, the magnetic properties are of a weakly itinerant ferromagnetic nature, located in the intermediate regime between the localized and the itinerant dominance. Our analysis implies a potential universality for all itinerant-electron ferromagnets.

cond-mat.str-el↗

arXiv2017

Soft phonon modes in the vicinity of the structural quantum critical point

The quasi-skutterudite superconductors $A_3T_4$Sn$_{13}$ ($A$=Sr, Ca; $T$=Ir, Rh, Co) are highly tunable featuring a structural quantum critical point. We construct a temperature-lattice constant phase diagram for these isovalent compounds, establishing Ca$_{3}$Rh$_4$Sn$_{13}$ and Ca$_{3}$Co$_4$Sn$_{13}$ as members close to and far away from the structural quantum critical point, respectively. Deconvolution of the lattice specific heat and the electrical resistivity provide an approximate phonon density of states $F(ω)$ and the electron-phonon transport coupling function $α_{tr}^2F(ω)$ for Ca$_{3}$Rh$_4$Sn$_{13}$ and Ca$_{3}$Co$_4$Sn$_{13}$, enabling us to investigate the influence of the structural quantum critical point. Our results support the scenario of phonon softening close to the structural quantum critical point, and explain the enhancement of the coupling strength on approaching structural instability.

cond-mat.supr-con↗

arXiv2016

Second-order Structural Transition in Superconductor La$_3$Co$_4$Sn$_{13}$

The quasi-skutterudite superconductor La$_3$Co$_4$Sn$_{13}$ undergoes a phase transition at $T^*=152$ K. By measuring the temperature dependence of heat capacity, electrical resistivity, and the superlattice reflection intensity using X-ray, we explore the character of the phase transition at $T^*$. Our lattice dynamic calculations found imaginary phonon frequencies around the ${\bf M}$ point, when the high temperature structure is used in the calculations, indicating that the structure is unstable at the zero temperature limit. The combined experimental and computational results establish that $T^*$ is associated with a second-order structural transition with $\bf{q}$=(0.5, 0.5, 0) (or the ${\bf M}$ point). Further electronic bandstructure calculations reveal Fermi surface sheets with low curvature segments, which allow us to draw qualitative comparison with both Sr$_3$Ir$_4$Sn$_{13}$ and Sr$_3$Rh$_4$Sn$_{13}$ in which similar physics has been discussed recently.

cond-mat.supr-con↗

arXiv2016

Experimental Determination of the Fermi Surface of Sr3Ir4Sn13

The stannide family of materials A3T4Sn13 (A = La,Sr,Ca, T = Ir,Rh) is interesting due to the interplay between a tunable lattice instability and phonon-mediated superconductivity with Tc ~ 5-7 K. In Sr3Ir4Sn13 a structural transition temperature T* ~ 147 K associated with this instability has been reported, which is believed to result from a superlattice distortion of the high temperature phase on cooling. Here we report the first experimental study of the electronic structure of a member of this material family - Sr3Ir4Sn13 through measurements of quantum oscillations and comparison with density functional theory calculations. Our measurements reveal good agreement with theory using the lattice parameters consistent with a body-centred cubic lattice of symmetry I-43d of the low temperature phase. The study of the fermiology of Sr3Ir4Sn13 we present here should help inform models of multiband superconductivity in the superconducting stannides.

cond-mat.supr-con↗

arXiv2015

Strong Coupling Superconductivity in the Vicinity of the Structural Quantum Critical Point in (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$

The family of the superconducting quasi-skutterudites (Ca$_x$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ features a structural quantum critical point at $x_c=0.9$, around which a dome-shaped variation of the superconducting transition temperature $T_c$ is found. Using specific heat, we probe the normal and the superconducting states of the entire series straddling the quantum critical point. Our analysis indicates a significant lowering of the effective Debye temperature on approaching $x_c$, which we interpret as a result of phonon softening accompanying the structural instability. Furthermore, a remarkably large enhancement of $2Δ/k_BT_c$ and $ΔC/γT_c$ beyond the Bardeen-Cooper-Schrieffer (BCS) values is found in the vicinity of the structural quantum critical point. The phase diagram of (Ca$_{x}$Sr$_{1-x}$)$_3$Rh$_4$Sn$_{13}$ thus provides a model system to study the interplay between structural quantum criticality and strong electron-phonon coupling superconductivity.

cond-mat.supr-con↗

arXiv2015

Structural anomalies and short-range magnetic correlations in the orbitally degenerated system Sr$_2$VO$_4$

We report on the electronic ground state of a layered perovskite vanadium oxide Sr$_2$VO$_4$ studied by the combined use of synchrotron radiation x-ray diffraction (SR-XRD) and muon spin rotation/relaxation ($μ$SR) techniques, where $μ$SR measurements were extended down to 30 mK. We found an intermediate orthorhombic phase between $T_{\rm c2} \sim$~130 K and $T_{\rm c1} \sim$~100 K, whereas a tetragonal phase appears for $T > T_{\rm c2}$ and $T < T_{\rm c1}$. The absence of long-range magnetic order was confirmed by $μ$SR at the reentrant tetragonal phase below $T_{\rm c1}$, where the relative enhancement in the $c$-axis length versus that of the $a$-axis length was observed. However, no clear indication of the lowering of the tetragonal lattice symmetry with superlattice modulation, which is expected in the orbital order state with superstructure of $d_{yz}$ and $d_{zx}$ orbitals, was observed by SR-XRD below $T_{\rm c1}$. Instead, it was inferred from $μ$SR that a magnetic state developed below $T_{\rm c0} \sim$~10 K, which was characterized by the highly inhomogeneous and fluctuating local magnetic fields down to 30 mK. We argue that the anomalous magnetic ground state below $T_{\rm c0}$ originates from the coexistence of ferromagnetic and antiferromagnetic correlations.

cond-mat.str-el↗

arXiv2014

Anomalous Itinerant-Electron Metamagnetic Transition in the Layered Sr1-xCaxCo2P2 System

We report magnetic properties of the layered itinerant system, Sr$_{1-x}$Ca$_x$Co$_2$P$_2$ in the magnetic field up to 70 T. As for the exchange-enhanced Pauli paramagnetic metal SrCo$_2$P$_2$, the magnetization curve shows two characteristic anomalies. The low-field anomaly is small without obvious hysteresis, and the high-field one is a typical behavior of the itinerant-electron metamagnetic transition (IEMT). Such a successive transition in the magnetization curve cannot be explained by the conventional phenomenological theory for IEMT due to the Landau expansion of the free energy, but by the extended Landau expansion theory with distinguishable two energy states. In the systematical study of Sr$_{1-x}$Ca$_x$Co$_2$P$_2$, furthermore, the metamagnetic transition field decreases and goes to zero as $x$ increases up to 0.5, indicating that the ferromagnetic quantum critical point (QCP) exists at $x \sim 0.5$.

cond-mat.str-el↗

arXiv2013

Anisotropic spin fluctuations in the quasi one-dimensional frustrated magnet LiCuVO_4

We report results of NMR experiments on a single crystal of the quasi one-dimensional frustrated magnet LiCuVO_4. The NMR spectra of ^7Li and ^{51}V nuclei indicate a helical spin order in a magnetic field of 4 T with the helical spin plane perpendicular to the field and a spin-density-wave (SDW) order at 10 T with modulation in the magnitude of the moments aligned along the field, in agreement with earlier reports. The nuclear spin-lattice relaxation rate 1/T_1 at ^{51}V nuclei, which is selectively coupled to the transverse spin fluctuations perpendicular to the field, shows a pronounced peak near the helical ordering temperature in the field of 4 T applied along the a-axis. In the field of 10 T, however, such a peak is absent. Instead 1/T_1 at ^7Li nuclei probing longitudinal spin fluctuations shows divergent behavior towards the SDW ordering temperature. These results are qualitatively consistent with the theoretical description that the SDW correlation is due to bound magnon pairs, which produce an energy gap in the transverse spin excitation spectrum.

cond-mat.str-el↗

arXiv2013

Magnetic properties in layered (K, Rb and Cs)Co2Se2 with ThCr2Si2_type structure

The magnetic properties of ThCr_2Si_2-type single crystals ACo_2Se_2 (A = K, Rb and Cs) have been investigated by magnetic susceptibility and isothermal magnetization measurements at various temperatures. The ferromagnetic phase transition temperatures are estimated as $\approx$ 74 K and 76 K and 62 K for A = K, Rb and Cs (in case of high magnetic field), respectively. The susceptibility data in the paramagnetic state obey the modified Curie-Weiss law quite well and the derived effective magnetic moments of the Co atom are about 2.21, 2.04 and 2.04 μ_B/Co and the corresponding spontaneous moments derived at the ground state are 0.72, 0.59 and 0.52 μ_B/Co as well as the generalized Rhodes-Wohlfarth ratios as 3.07, 3.42 and 3.96 for A= K, Rb and Cs, respectively. The magnetic moment aligns within ab-plane and a metamagnetism-like behavior occurs at a field of 3.5 T in CsCo_2Se_2 with H//ab-plane. The magnetic properties in this system were discussed within the frameworks of the self-consistent renormalization (SCR) and Takahashi's theory of spin fluctuations.

cond-mat.supr-con↗

arXiv2012

Pressure-induced and Composition-induced Structural Quantum Phase Transition in the Cubic Superconductor (Sr/Ca)_3Ir_4Sn_{13}

We show that the quasi-skutterudite superconductor Sr_3Ir_4Sn_{13} undergoes a structural transition from a simple cubic parent structure, the I-phase, to a superlattice variant, the I'-phase, which has a lattice parameter twice that of the high temperature phase. We argue that the superlattice distortion is associated with a charge density wave transition of the conduction electron system and demonstrate that the superlattice transition temperature T* can be suppressed to zero by combining chemical and physical pressure. This enables the first comprehensive investigation of a superlattice quantum phase transition and its interplay with superconductivity in a cubic charge density wave system.

cond-mat.str-el↗

arXiv2011

Large Magnetoresistance Effects in $Ln$CoAsO with a Ferromagnetic-Antiferromagnetic Transition

A large magnetoresistance (MR) effect was observed in the layered compounds NdCoAsO and SmCoAsO, in which ferromagnetically ordered itinerant-electrons of Co are sandwiched by localized 4$f$-electrons of $Ln^{3+}$, below ferromagnetic-antiferromagnetic transition (FAFT) temperature $T_\mathrm{N}$ as observed in other FAFT compounds. In SmCoAsO, the large MR effect is also observed up to the Curie temperature $T_\mathrm{C}$, and it is found to be originating in the presence of another antiferromagnetic phase in the low-magnetic field region of the ferromagnetic phase.

cond-mat.str-el↗