arXiv subjects

Zenji Hiroi

Publications and source records attributed to Zenji Hiroi.

At least 19 recordsLinked to original sources

arXiv2026

Intrinsic Magnetic Excitations and Heavy-Fermion Formation in the Frustrated Mn Pyrochlore System YMn$_{2+\delta}$Zn$_{20-x}X_x$ ($X$ = In and Al) Revealed by Nuclear Magnetic Resonance and Nuclear Quadrupole Resonance Measurements

We performed nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the microscopic electronic states of the $d$-electron heavy-fermion candidates $\mathrm{YMn_{2+\delta}Zn_{20-x}In_x}$ and $\mathrm{YMn_{2+\delta}Zn_{20-x}Al_x}$. In these compounds, magnetic fluctuations of the Mn pyrochlore lattice are expected to play an important role in heavy-fermion formation; however, excess Mn atoms complicate the interpretation of the physical properties. Our spectral analysis reveals that In substitution exhibits much higher site selectivity and introduces significantly less disorder in local structure than Al substitution. The temperature dependence of the nuclear spin-lattice relaxation rate divided by temperature $1/T_1T$ measured by $^{55}$Mn-NQR shows a clear enhancement at low temperatures, indicating the development of low-energy excitations associated with heavy-fermion formation. However, its absolute magnitude is approximately 20 times smaller than that in the related compound YMn$_2$, which hosts stronger antiferromagnetic correlations, indicating that the magnetic interactions are substantially weakened by the enlarged Mn-Mn distance. These results demonstrate that the heavy-fermion state in this system arises from the Mn pyrochlore network and is more closely associated with frustration-induced magnetic excitations with low energy than with conventional antiferromagnetic quantum-critical fluctuations.

cond-mat.str-el↗

arXiv2026

Introduction to High-Temperature Superconductivity for Solid State Chemists

Superconductivity is one of the most amazing properties that metallic conductors exhibit. Electrical resistance is completely eliminated below the critical temperature (Tc), which is the most important parameter in superconductivity. Since the discovery of copper oxide superconductors 39 years ago, many solid state chemists have made significant contributions to the field by discovering new compounds and producing high-quality samples for physical measurements. However, superconductivity research remains challenging for most solid state chemists because it requires knowledge of complicated solid state physics. This manuscript aims to provide a simple, intuitive introduction to superconductivity using only fundamental physics concepts that solid state chemists are familiar with. The author investigates a wide range of materials and classifies them according to the superconductivity mechanisms that may drive them. Specifically focusing on a series of copper oxide superconductors with the highest Tc at ambient conditions, the remarkable material dependence of Tc and the underlying, unconventional superconductivity mechanism that leads to the high Tc are thoroughly examined. Although our understanding of cuprate superconductivity is still fragmented, the author believes that once the branches and leaves are removed, the story will be fairly simple, similar to the phonon-based superconductivity mechanism revealed by the BCS theory. Furthermore, potential strategies for raising the Tc of cuprates and other superconductors are discussed. The author hopes that this article will pique interest in superconductors in young solid state chemists and encourage them to pursue the discovery of still unknown and unexplored room-temperature superconductors in the future.

cond-mat.supr-con↗

arXiv2026

Coexisting electronic smectic liquid crystal and superconductivity in a Si square-net semimetal

Electronic nematic and smectic liquid crystals are spontaneous symmetry-breaking phases that are seen to precede or coexist with enigmatic unconventional superconducting states in multiple classes of materials. In this Letter we describe scanning tunneling microscopy observations of a short ranged charge stripe (smectic) order in NaAlSi, whose superconductivity is speculated to have an unconventional origin. As well as this we resolve a clear spatial modulation of the superconducting gap amplitude, which arises due to the intertwined superconducting and smectic orders. Numerical calculations help to understand the possible driving mechanism as a suppression of kinetic energy on the Fermi surface formed in part by two large, flat-topped hole pockets of p-orbital character.

cond-mat.supr-con↗

arXiv2025

Ferroaxial order of the monolayer ice in martyite

Ice Ih, the most stable phase of water at ambient pressure, is a stacking of the honeycomb network of water molecules H2O. What if one layer of ice is exfoliated and confined to a two-dimensional (2D) sheet? Martyite Zn3(V2O7)(OH)2 2H2O, a mineral with the honeycomb lattice of H2O in the porous framework, is an ideal system for studying such monolayer ice. Due to the geometrical frustration and 2D nature, H2O molecules are dynamically disordered at room temperature. In this study, we reveal disorder-order transitions of H2O in martyite using single-crystal x-ray diffraction (XRD). The XRD results visualize the formation of hydrogen-bonded toroidal H2O hexamers, leading to the ferroaxial order below 200 K. Combined with the molecular dynamics simulations, we discuss the formation process of the H2O hexamers and how they compromise the molecular arrangement towards lower temperatures. Our results unveil the ground state of monolayer ice, a fundamental knowledge to understand the polymorphism of H2O.

cond-mat.mtrl-sci↗

arXiv2025

Site-selective observation of spin dynamics of a Tomonaga-Luttinger liquid in frustrated Heisenberg chains

Low-energy spin dynamics is investigated by $^{35}$ Cl NMR measurements in a frustrated antiferromagnet Ca$_3$ReO$_5$Cl$_2$. The local spin susceptibility measured with the Knight shift behaves as a one-dimensional Heisenberg antiferromagnet and remains constant down to low temperatures, as expected in a gapless Tomonaga-Luttinger liquid. The nuclear spin-lattice relaxation rate $T_1^{-1}$ demonstrates a slowing down of atomic motions and a power-law evolution of spin correlation. The Luttinger parameter is enhanced in a site-selective manner depending on the form factor of dynamical spin susceptibility. The strong anisotropy of $T_1^{-1}$ reflects the strong spin-orbit coupling through Dzyaloshinskii-Moriya interaction. The ground state exhibits an incommensurate antiferromagnetic ordering with low-lying magnon excitations.

cond-mat.str-el↗

arXiv2024

Topological Semimetal KAlGe with Novel Electronic Instability

Compounds with the anti-PbFCl structure exhibit a variety of electronic instabilities and intriguing physical properties. NaAlSi and NaAlGe are similar topological nodal-line semimetals, but they have distinct properties. NaAlSi is a superconductor at 6.8 K, whereas NaAlGe is an insulator with a pseudogap of approximately 100 K. Using the potassium-indium flux method, we succeeded in synthesizing a single crystal of KAlGe, a new anti-PbFCl compound. First principles electronic structure calculations reveal that KAlGe is isoelectronic with NaAlSi and NaAlGe. KAlGe undergoes a metal-to-metal transition at 89 K and exhibits no superconductivity above 1.8 K. The low temperature phase has significantly lower carrier density and extremely high mobility, similar to Dirac electron systems. Furthermore, X-ray diffraction experiments show a structural change that breaks the fourfold symmetry during the phase transition. Electron-phonon interactions may be responsible for superconductivity in NaAlSi, whereas excitonic electron-hole interactions are thought to play an important role in KAlGe and possibly NaAlGe. Our findings demonstrate that fascinating physics lies within the compound family.

cond-mat.str-el↗

arXiv2023

Superconductivity induced by doping holes in the nodal-line semimetal NaAlGe

The nodal-line semimetals NaAlSi and NaAlGe have significantly different ground states despite having similar electronic structures: NaAlSi exhibits superconductivity below 7 K, while NaAlGe exhibits semiconductive electrical conductivity at low temperatures, indicating the formation of a pseudogap at approximately 100 K. The origin of the pseudogap in NaAlGe is unknown but may be associated with excitonic instability. We investigated hole-doping effects on the ground state in the solid solution Na(Al1-xZnx)Ge and discovered that the pseudogap is suppressed continuously with increasing Zn content, followed by the appearance of a superconducting dome with the highest transition temperature of 2.8 K. This superconductivity most likely results from excitonic fluctuations.

cond-mat.supr-con↗

arXiv2023

Successive phase transitions of the spin-orbit-coupled metal Cd2Re2O7 probed by high-resolution synchrotron x-ray diffraction

The 5d pyrochlore oxide superconductor Cd2Re2O7 (CRO) has attracted significant interest as a spin-orbit-coupled metal (SOCM) that spontaneously undergoes a phase transition to an odd-parity multipole phase by breaking the spatial inversion symmetry due to the Fermi liquid instability caused by strong spin-orbit coupling. Despite the significance of structural information during the transition, previous experimental results regarding lattice deformation have been elusive. We have conducted ultra-high resolution synchrotron radiation x-ray diffraction experiments on a high-quality CRO single crystal. The temperature-dependent splitting of the 0 0 16 and 0 0 14 reflections, which are allowed and forbidden, respectively, in the high-temperature cubic phase I (space group Fd-3m), has been clearly observed and reveals the following significant facts: inversion symmetry breaking and tetragonal distortion occur simultaneously at Ts1 = 201.5(1) K; the previously believed first-order transition between phase II (I-4m2) and phase III (I4122) at Ts2 ~ 120 K consists of two close second-order transitions at Ts2 = 115.4(1) K and Ts3 ~ 100 K; there is a new orthorhombic phase XI (F222) in between. The order parameters (OPs) of these continuous transitions are uniquely represented by a two-dimensional irreducible representation Eu of the Oh point group, and the OPs of phase XI are a linear combination of those of phases II and III. Each phase is believed to correspond to a distinct odd-parity multipole order, and the complex successive transitions observed may be the result of an electronic phase transition that resolves the Fermi liquid instability in the SOCM.

cond-mat.str-el↗

arXiv2023

Fermi surface and light quasi particles in hourglass nodal chain metal \b{eta}-ReO2

Quantum oscillations in magnetic torque and electrical resistivity were measured to investigate the electronic structure of \b{eta}-ReO2, a candidate hourglass nodal chain metal (Dirac loop chain metal). All the de Haas-van Alphen oscillation branches measured at 30 mK in magnetic fields of up to 17.5 T were consistent with first-principles calculations predicting four Fermi surfaces (FSs). The small-electron FS of the four FSs exhibited a very small cyclotron mass, 0.059 times that of the free electrons, which is likely to be related to the linear dispersion of the energy band. The consistency between the quantum oscillation results and band calculations indicates the presence of the hourglass nodal chain predicted for \b{eta}-ReO2 in the vicinity of the Fermi energy.

cond-mat.str-el↗

arXiv2023

Linear Trimer Formation by Three-Center-Four-Electron Bonding in RuP

In molecules like hydrogen, most chemical bonds are formed by sharing two electrons from each atom in the bonding molecular orbital (two-center-two-electron (2c2e) bonding). There are, however, different kinds of chemical bonding. The I3- molecule, for example, is noteworthy because three iodine atoms are linearly united by sharing four electrons (three-center-four-electron (3c4e) bonding). Some inorganic solids undergo phase transitions that result in the formation of "molecules" in their crystalline frameworks, which are often accompanied by dramatic changes in physical properties; the metal-to-insulator transition (MIT) in vanadium dioxide, for example, occurs with the formation of dimer molecules with 2c2e bonding. We repot the creation of a linear ruthenium trimer with 3c4e bonding in ruthenium monopnictide at its MIT. Charge transfer from polymerized phosphorous to ruthenium produces this unusual molecule, with all conduction electrons trapped by the bonding molecular orbital. Our results demonstrate that molecules are crucial even in solid crystals as they impact their electronic properties.

cond-mat.str-el↗

arXiv2023

Inelastic Neutron Scattering Study of the Spin Dynamics in the Breathing Pyrochlore System LiGa0.95In0.05Cr4O8

The A-site ordered chromate spinels LiGa1-xInxCr4O8 host a network of size-alternating spin-3/2 Cr3+ tetrahedra known as a 'breathing' pyrochlore lattice. For the x=0.05 composition, the complex magneto-structural ordering observed in the parent x=0 material is replaced by a single transition at Tf=11 K, ascribed to the collinear nematic order caused by strong spin-lattice coupling. We present here an inelastic neutron scattering study of the spin dynamics in this composition. Above Tf , the dynamical scattering function S(Q,E) is ungapped and quasi-elastic, similar to undoped LiGaCr4O8. Below Tf , the spectral weight splits between a broad inelastic feature at 5.8 meV and toward the elastic line. The former feature can be ascribed to spin precessions within antiferromagnetic loops, lifted to finite energy by the effective biquadratic spin-lattice term in the spin Hamiltonian.

cond-mat.str-el↗

arXiv2022

Pseudogap Formation in the Nodal-Line Semimetal NaAlGe

NaAlSi and NaAlGe are isostructural and isoelectronic semimetals with topological nodal lines close to the Fermi level. Despite having virtually identical electronic structures, NaAlSi exhibits superconductivity below Tc = 6.8 K, whereas NaAlGe does not. We investigate NaAlGe by measuring its electrical resistivity, Hall effect, magnetic susceptibility, and heat capacity using single crystals. It is revealed that NaAlGe is not a simple semimetal but rather has an unusual ground state with a small pseudogap of approximately 100 K close to the Fermi level. We argue that the formation of the pseudogap in NaAlGe is due to an unexpected Fermi surface instability, such as an excitonic instability, as opposed to the electron-phonon instability that leads to the formation of the superconducting gap in NaAlSi.

cond-mat.mtrl-sci↗

arXiv2022

Negative-chirality order in $S=1/2$ kagome antiferromagnet CdCu$_{3}$(OH)$_{6}$(NO$_{3}$)$_{2}\cdot $H$_{2}$O

The neutron diffraction and nuclear magnetic resonance (NMR) measurements have been used to microscopically analyze the magnetic structure in the $S = 1/2$ kagome antiferromagnet CdCu$_{3}$(OH)$_{6}$(NO$_{3}$)$_{2}\cdot $H$_{2}$O. Below the magnetic ordering temperature $T_N\simeq 4$ K, magnetic Bragg reflections at (110) and (100) were found in the neutron diffraction pattern, which suggests a $q=0$ magnetic structure. Furthermore, the vector spin chirality for the $q=0$ structure was successfully identified from the internal field direction obtained by the $^{14}$N-NMR measurement. Our findings point to a chirality-ordered magnetic structure with negative vector chirality and $\langle 100 \rangle$ anisotropy.

cond-mat.str-el↗

arXiv2021

Pyrochlore Oxide Hg2Os2O7 on Verge of Metal-Insulator Boundary

Semimetallic osmium pyrochlore oxide Cd2Os2O7 undergoes a magnetic transition to an all-in-all-out (AIAO)-type order at 227 K, followed by a crossover to an AIAO insulator at around 210 K. Here, we studied the isostructural and isoelectronic compound Hg2Os2O7 through thermodynamic measurements, muSR spectroscopy and neutron diffraction experiments. A similar magnetic transition, probably to an AIAO-type order, was observed at 88 K, while the resistivity showed a decrease at the transition and remained metallic down to 2 K. Thus, the ground state of Hg2Os2O7 is most likely an AIAO semimetal, which is analogous to the intermediate-temperature state of Cd2Os2O7. Hg2Os2O7 exists on the verge of the metal-insulator boundary on the metal side and provides an excellent platform for studying the electronic instability of 5d electrons with moderate electron correlations and strong spin-orbit interactions.

cond-mat.str-el↗

arXiv2021

Unusual Resistive Transitions in the Nodal-Line Semimetallic Superconductor NaAlSi

NaAlSi is a quasi-two-dimensional semimetal with superconductivity below Tc = 6.8 K and a band structure characterized by nodal lines near the Fermi level and potential topological surface states. Electrical resistivity measurements on its superconducting transitions in magnetic fields were made using plate-like single crystals. In the magnetic field-temperature phase diagram, we observed a substantial reduction in resistivity in a pre-transitional zone above the bulk superconducting regime only when the magnetic fields were perpendicular to the plane, rather than parallel to it. Significant sample (thickness) dependence, reentrant behavior, and sensitivity to electrode configurations all indicate that a portion of the crystal has an upper critical field greater than the bulk superconductivity in the pre-transitional region. This fractional superconductivity may occur on the side surface of the crystal.

cond-mat.supr-con↗

arXiv2021

Quantum antiferromagnet bluebellite comprising a maple-leaf lattice made of spin-1/2 Cu$^{2+}$ ions

Spin-1/2 maple leaf lattice antiferromagnets are expected to show interesting phenomena originating from frustration effects and quantum fluctuations. We report the hydrothermal synthesis of a powder sample of bluebellite Cu$_6$IO$_3$(OH)$_{10}$Cl as a first potential candidate. Magnetization and heat capacity measurements reveal moderate frustration with a Curie-Weiss temperature of $-35$ K, and a magnetic transition at $T_N$ = 17 K. Surprisingly, the magnetic susceptibility and heat capacity above $T_N$ are well reproduced by the Bonner-Fisher model, which suggests that a one-dimensional spin correlation with a magnetic interaction of 25 K occurs in the apparently two-dimensional lattice. This emergent one-dimensionality cannot be explained by orbital ordering or dimensional reduction due to geometrical frustration. We believe that there is an unknown mechanism to cause one-dimensionality in the spin-1/2 maple leaf lattice antiferromagnet.

cond-mat.str-el↗

arXiv2021

Interplay between quadrupolar and magnetic interactions in 5$d^{1}$ double perovskite Ba$_2$MgReO$_6$ under pressure

We measured the electrical resistivity, AC magnetic susceptibility, and specific heat of the cubic double perovskite Ba$_2$MgReO$_6$ under pressure. The application of pressure leads to a gradual increase in the transition temperature to the canted antiferromagnetic order, while the quadrupolar order is suppressed above 5 GPa, at which a collinear antiferromagnetic state appears. The obtained unit cell volume-temperature phase diagram for the compound as well as the related compounds suggests that Ba$_2$MgReO$_6$ exists near the phase boundary between the noncollinear and the collinear antiferromagnetic order with multipole decrees of freedom.

cond-mat.str-el↗

arXiv2021

Dimer Crystallization Induced by Elemental Substitution in the Honeycomb Lattice of Ru1-xOsxCl3

Substitution effects of Os for Ru in {\alpha}-RuCl3 are investigated in a wide composition range of 0 =< x =< 0.67 in Ru1-xOsxCl3 by X-ray and electron diffraction, magnetic susceptibility, heat capacity, and Raman spectroscopy measurements. Apart from the Kitaev physics with antiferromagnetic interactions increasing with x, a rich phase diagram is obtained, which includes an antiferromagnetic long-range order below 12 K for x =< 0.15, a dome-shaped spin-singlet dimer phase below 130 K for 0.15 =< x =< 0.40, and a magnetic short-range order for x > 0.40. A dimerization as similarly observed in {\alpha}-RuCl3 under high pressure occurs in the spin-singlet phase. It is suggested that Ru-Os pairs in the solid solutions tend to form dimers with short bonds and trigger the first-order transition in the presence of pseudo-threefold rotational symmetry for dimerization around a substituted Os atom only at low substitutions. This is a rare example of molecular orbital crystallization induced by elemental substitution in a highly disordered system. The short-range order at high substitutions may be related to a random-singlet state stabilized by bond disorder in the honeycomb net.

cond-mat.mtrl-sci↗