arXiv subjects

Yuji Aoki

Publications and source records attributed to Yuji Aoki.

At least 19 recordsLinked to original sources

arXiv2026

Incommensurate Magnetic Ordered Phase with Enhanced Low-Temperature Magnetic Specific Heat in SmAu$_3$Al$_7$

Neutron scattering and muon spin rotation ($\mu$SR) measurements on single-crystal SmAu$_3$Al$_7$ reveal magnetically ordered states associated with successive transitions at $T_{\rm N}$ = 2.8 K and $T^*$ = 0.9 K. Magnetic Bragg peaks appear below $T_{\rm N}$ with an incommensurate (IC) propagation vector ${\bf q}$ = (0.30, 0, 1.33). $\mu$SR detects spontaneous internal fields below $T_{\rm N}$, and the spectral shape is consistent with the IC magnetic ordering. No anomalies are observed at $T^*$, indicating that the magnetic structure remains essentially unchanged below and above $T^*$. The magnetic order is revealed to be a spatially homogeneous long-range ordered state, rather than a partially disordered state proposed in earlier studies. The possible connection between the IC magnetic order and the enhanced low-temperature magnetic specific heat is discussed.

cond-mat.str-el↗

arXiv2024

Linear Magnetoresistance and Type-I Superconductivity in $\beta$-IrSn$_4$

Layered material $\beta$-IrSn$_4$ ($I4_1/acd$, $D^{20}_{4h}$, #142), whose electron bands have symmetry-enforced Dirac points, was investigated using high-quality single crystals. It exhibits a pronounced linear field-dependence of magnetoresistance (LMR), which cannot be explained by currently existing models. Structures in the field-angle dependence of magnetoresistance and Hall resistivity are attributable to the Fermi surface topology; the presence of open orbits is inferred. At the superconducting (SC) transition, the specific-heat jump exhibits a significant increase in applied fields, revealing the type-I SC nature. This feature is attributable to the high Fermi velocity of linearly dispersive multibands. To clarify the mechanism of the puzzling LMR, investigations into the topological nature of those multibands in applied fields are highly desired.

cond-mat.supr-con↗

arXiv2024

Fermi Surface Nesting Driving the RKKY Interaction in the Centrosymmetric Skyrmion Magnet Gd2PdSi3

The magnetic skyrmions generated in a centrosymmetric crystal were recently first discovered in Gd2PdSi3. In light of this, we observe the electronic structure by angle-resolved photoemission spectroscopy (ARPES) and unveil its direct relationship with the magnetism in this compound. The Fermi surface and band dispersions are demonstrated to have a good agreement with the density functional theory (DFT) calculations carried out with careful consideration of the crystal superstructure. Most importantly, we find that the three-dimensional Fermi surface has extended nesting which matches well the q-vector of the magnetic order detected by recent scattering measurements. The consistency we find among ARPES, DFT, and the scattering measurements suggests the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction involving itinerant electrons to be the formation mechanism of skyrmions in Gd2PdSi3.

cond-mat.mtrl-sci↗

arXiv2023

Pseudogap and Fermi arc induced by Fermi surface nesting in a centrosymmetric skyrmion magnet

Skyrmions in noncentrosymmetric materials are believed to occur due to the Dzyaloshinskii-Moriya interaction. By contrast, the skyrmion formation mechanism in centrosymmetric materials remains elusive. Here, we reveal the intrinsic electronic structure of the centrosymmetric GdRu2Si2 by selectively measuring magnetic domains using angle-resolved photoemission spectroscopy (ARPES). We found robust Fermi surface (FS) nesting, consistent with the magnetic modulation q-vector detected by the previous resonant x-ray scattering measurements. The pseudogap opens at the nested FS portions, which vary for different magnetic domains. The anomalous pseudogap disconnects the FS to generate Fermi arcs with twofold symmetry. These results indicate that the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction plays a decisive role in generating the screw spin modulation responsible for the skyrmion formation in GdRu2Si2. Furthermore, we demonstrate the flexible nature of magnetism in GdRu2Si2 by manipulating magnetic domains with magnetic field and temperature cyclings, providing potential future applications for data storage and processing devices.

cond-mat.mtrl-sci↗

arXiv2021

Superconductivity in In-doped AgSnBiTe3 with possible band inversion

We investigated the chemical pressure effects on structural and electronic properties of SnTe-based material using partial substitution of Sn by Ag0.5Bi0.5, which results in lattice shrinkage. For Sn1-2x(AgBi)xTe, single-phase polycrystalline samples were obtained with a wide range of x. On the basis of band calculations, we confirmed that the Sn1-2x(AgBi)xTe system is basically possessing band inversion and topologically preserved electronic states. To explore new superconducting phases related to the topological electronic states, we investigated the In-doping effects on structural and superconducting properties for x = 0.33 (AgSnBiTe3). For (AgSnBi)(1-y)/3InyTe, single-phase polycrystalline samples were obtained for y = 0-0.5 by high-pressure synthesis. Superconductivity was observed for y = 0.2-0.5. For y = 0.4, the transition temperature estimated from zero-resistivity state was 2.4 K, and the specific heat investigation confirmed the emergence of bulk superconductivity. Because the presence of band inversion was theoretically predicted, and the parameters obtained from specific heat analyses were comparable to In-doped SnTe, we expect that the (AgSnBi)(1-y)/3InyTe and other (Ag,In,Sn,Bi)Te phases are candidate systems for studying topological superconductivity.

cond-mat.supr-con↗

arXiv2020

Superconductivity enhanced by $d$-band filling in La$Tr_2$Al$_{20}$ with $Tr$ = Mo and W

Electrical resistivity, magnetic susceptibility, and specific heat measurements on single crystals of La$Tr_2$Al$_{20}$ with $Tr$ = Mo and W revealed that these compounds exhibit superconductivity with transition temperatures $T_c$ = 3.22 and 1.81 K, respectively, achieving the highest values in the reported La$Tr_2$Al$_{20}$ compounds. There appears a positive correlation between $T_c$ and the electronic specific heat coefficient, which increases with increasing the number of $4d$- and $5d$-electrons. This finding indicates that filling of the upper $e_g$ orbitals in the $4d$ and $5d$ bands plays an essential role for the significant enhancement of the superconducting condensation energy. Possible roles played by the $d$ electrons in the strongly correlated electron phenomena appearing in $RTr_{2}$Al$_{20}$ are discussed.

cond-mat.supr-con↗

arXiv2020

Bulk superconductivity induced by Se substitution in self-doped BiCh2-based compound CeOBiS2-xSex

We report the Se substitution effects on the crystal structure, superconducting properties, and valence states of self-doped BiCh2-based compound CeOBiS2-xSex. Polycrystalline CeOBiS2-xSex samples with x = 0-1.0 were synthesized. For x = 0.4 and 0.6, bulk superconducting transitions with a large shielding volume fraction were observed in magnetic susceptibility measurements; the highest transition temperature (Tc) was 3.0 K for x = 0.6. A superconductivity phase diagram of CeOBiS2-xSex was established based on Tc estimated from the electrical resistivity and magnetization measurements. The emergence of superconductivity in CeOBiS2-xSex was explained with two essential parameters of in-plane chemical pressure and carrier concentration, which systematically changed with increasing Se concentration.

cond-mat.supr-con↗

arXiv2020

Superconducting properties of high-entropy-alloy tellurides M-Te (M: Ag, In, Cd, Sn, Sb, Pb, Bi) with a NaCl-type structure

High-entropy-alloy-type tellurides M-Te, which contain five different metals of M = Ag, In, Cd, Sn, Sb, Pb, and Bi, were synthesized using high pressure synthesis. Structural characterization revealed that all the obtained samples have a cubic NaCl-type structure. Six samples, namely AgCdSnSbPbTe5, AgInSnSbPbTe5, AgCdInSnSbTe5, AgCdSnPbBiTe5, AgCdInPbBiTe5, and AgCdInSnBiTe5 showed superconductivity. The highest transition temperature (Tc) among those samples was 1.4 K for AgInSnSbPbTe5. A sample of AgCdInSbPbTe5 showed a semiconductor-like transport behavior. From the relationship between Tc and lattice constant, it was found that a higher Tc is observed for a telluride with a larger lattice constant.

cond-mat.supr-con↗

arXiv2019

Superconductivity in Se-doped La2O2Bi2Pb2S6-xSex with a Bi2Pb2Ch4-type thick conducting layer

La2O2Bi2Pb2S6 is a layered Bi-based oxychalcogenide with a thick four-layer-type conducting layer. Although La2O2Bi2Pb2S6 is a structural analogue of La2O2Bi3AgS6, which is a superconductor, insulating behavior has been observed in La2O2Bi2Pb2S6 at low temperatures, and no superconductivity has been reported. Herein, we demonstrate superconductivity in La2O2Bi2Pb2S6-xSex via partial substitution of Se in the S sites. Owing to the Se doping, the normal state electrical resistivity of La2O2Bi2Pb2S6-xSex at low temperatures was dramatically suppressed, and superconductivity was observed at a transition temperature (Tc) of 1.15 K for x = 0.5. Tc increased with increasing Se concentration: Tc = 1.9 K for x = 1.0. The emergence of metallicity and superconductivity was explained via in-plane chemical pressure effects that can suppress local disorder and carrier localization, which are commonly observed in two-layer-type BiS2-based systems.

cond-mat.supr-con↗

arXiv2019

Detection of hole pockets in the candidate type-II Weyl semimetal MoTe$_2$ from Shubnikov-de Haas quantum oscillations

The bulk electronic structure of $T_d$-MoTe$_2$ features large hole Fermi pockets at the Brillouin zone center ($\Gamma$) and two electron Fermi surfaces along the $\Gamma-X$ direction. However, the large hole pockets, whose existence has important implications for the Weyl physics of $T_d$-MoTe$_2$, has never been conclusively detected in quantum oscillations. This raises doubt about the realizability of Majorana states in $T_d$-MoTe$_2$, because these exotic states rely on the existence of Weyl points, which originated from the same band structure predicted by density functional theory (DFT). Here, we report an unambiguous detection of these elusive hole pockets via Shubnikov-de Haas (SdH) quantum oscillations. At ambient pressure, the quantum oscillation frequencies for these pockets are 988 T and 1513 T, when the magnetic field is applied along the $c$-axis. The quasiparticle effective masses $m^*$ associated with these frequencies are 1.50 $m_e$ and 2.77 $m_e$, respectively, indicating the importance of Coulomb interactions in this system. We further measure the SdH oscillations under pressure. At 13 kbar, we detected a peak at 1798 T with $m^*$ = 2.86 $m_e$. Relative to the oscillation data at a lower pressure, the amplitude of this peak experienced an enhancement, which can be attributed to the reduced curvature of the hole pockets under pressure. Combining our experimental data with DFT + $U$ calculations, where $U$ is the Hubbard parameter, our results shed light on why these important hole pockets have not been detected until now.

cond-mat.supr-con↗

arXiv2019

Enhanced superconductivity by Na doping in SnAs-based layered compound Na$_{1+x}$Sn$_{2-x}$As$_2$

Superconducting transition temperature (Tc) reported in SnAs-based layered compound NaSn$_2$As$_2$ varies from 1.2 to 1.6 K, implying that its superconductivity is critically sensitive to non-stoichiometry. Here, we demonstrate that Na-doping on the Sn site (Na$_{1+x}$Sn$_{2-x}$As$_2$) is effective in enhancing superconductivity, leading to Tc = 2.1 K for x = 0.4. First-principles calculation indicates that such a doping, or Na$_{\rm Sn}$ antisite defects, is energetically favored over other cation vacancies. Our results pave the way for increasing Tc of layered tin pnictide superconductors.

cond-mat.supr-con↗

arXiv2019

Effect of Indium doping on the superconductivity of layered oxychalcogenide La2O2Bi3Ag1-xInxS6

We report on the substitution effect of Indium (In) at the Ag site of layered oxychalcogenide La2O2Bi3Ag1-xInxS6. The Tc decreases with increasing In concentration. A hump in the normal state resistivity at an anomaly temperature (T*) near 180 K was observed for all the samples. The anomaly in the resistivity at T* is indicating the possible occurrence of a charge-density-wave (CDW) transition. The T* does not markedly change by In doping. The x dependence of Seebeck coefficient suggests that carrier concentration does not change by In doping. The EDX analysis indicates small amount of Bi deficiency, which suggests that the Bi site is slightly substituted by In. The CDW transition is robust against the In substitution at Ag site, while Tc is decreasing due to the Bi site substitution by In. On the basis of those analyses, we propose that the suppression of superconductivity in the In-doped La2O2Bi3Ag1-xInxS6 system is caused by negative in-plane chemical pressure effect and partial substitution of In for the in-plane Bi site.

cond-mat.supr-con↗

arXiv2018

Bulk superconductivity in La2O2M4S6-type layered oxychalcogenide La2O2Bi3Ag0.6Sn0.4S5.7Se0.3

Recently, we reported the observation of superconductivity at ~0.5 K in a La2O2M4S6-type (M: metal) layered oxychalcogenide La2O2Bi3AgS6, which is a layered compound related to the BiS2-based superconductor system but possesses a thicker Bi3AgS6-type conducting layer. In this study, we have developed the La2O2Bi3AgS6-type materials by element substitutions to increase the transition temperature (Tc) and to induce bulk nature of superconductivity. A resistivity anomaly observed at 180 K in La2O2Bi3AgS6 was systematically suppressed by Sn substitution for the Ag site. By the Sn substitution, Tc increased, and the shielding volume fraction estimated from magnetization measurements also increased. The highest Tc (= 2.3 K) and the highest shielding volume fraction (~20%) was observed for La2O2Bi3Ag0.6Sn0.4S6. The superconducting properties were further improved by Se substitutions for the S site. By the combinational substitutions of Sn and Se, bulk-superconducting phase of La2O2Bi3Ag0.6Sn0.4S5.7Se0.3 with a Tc of 3.0 K (Tconset = 3.6 K) was obtained.

cond-mat.supr-con↗

arXiv2018

Superconductivity in layered Oxychalcogenide La2O2Bi3AgS6

We report the superconductivity in layered oxychalcogenide La2O2Bi3AgS6 compound. The La2O2Bi3AgS6 compound has been reported recently by our group, which has a tetragonal structure with the space group P4/nmm. The crystal structure of La2O2Bi3AgS6 can be regarded as alternate stacks of LaOBiS2-type layer and rock-salt-type (Bi,Ag)S layer. We measured low-temperature electrical resistivity and observed superconductivity at 0.5 K. The observation of superconductivity in the La2O2Bi3AgS6 should provide us with the successful strategy for developing new superconducting phases by the insertion of a rock-salt-type chalcogenide layer into the van der Waals gap of BiS2-based layered compound like LaOBiS2.

cond-mat.supr-con↗

arXiv2018

Na1-xSn2P2 as a new member of van der Waals-type layered tin pnictide superconductors

Superconductors with a van der Waals (vdW) structure have attracted a considerable interest because of the possibility for truly two-dimensional (2D) superconducting systems. We recently reported NaSn2As2 as a novel vdW-type superconductor with transition temperature (Tc) of 1.3 K. Herein, we present the crystal structure and superconductivity of new material Na1-xSn2P2 with Tc = 2.0 K. Its crystal structure consists of two layers of a buckled honeycomb network of SnP, bound by the vdW forces and separated by Na ions, as similar to that of NaSn2As2. Amount of Na deficiency (x) was estimated to be 0.074(18) using synchrotron X-ray diffraction. Bulk nature of superconductivity was confirmed by the measurements of electrical resistivity, magnetic susceptibility, and specific heat. First-principles calculation using density functional theory shows that Na1-xSn2P2 and NaSn2As2 have comparable electronic structure, suggesting higher Tc of Na1-xSn2P2 is resulted from increased density of states at the Fermi level due to Na deficiency. Because there are various structural analogues with tin-pnictide (SnPn) conducting layers, our results indicate that SnPn-based layered compounds can be categorized into a novel family of vdW-type superconductors, providing a new platform for studies on physics and chemistry of low-dimensional superconductors.

cond-mat.supr-con↗

arXiv2018

Superconductivity in Cage Compounds La$Tr_{2}$Al$_{20}$ with $Tr$ = Ti, V, Nb, and Ta

Electrical resistivity, magnetic susceptibility, and specific heat measurements on single crystals of La$Tr_{2}$Al$_{20}$ ($Tr$ = Ti, V, Nb, and Ta) revealed that these four compounds exhibit weak-coupling superconductivity with transition temperatures $T_{\rm c}$ = 0.46, 0.15, 1.05, and 1.03 K, respectively. LaTi$_{2}$Al$_{20}$ is most probably a type-I superconductor, which is quite rare among intermetallic compounds. Single-crystal X-ray diffraction suggests "rattling" anharmonic large-amplitude oscillations of Al ions (16$c$ site) on the Al$_{16}$ cage, while no such feature is suggested for the cage-center La ion. Using a parameter $d_{\rm GFS}$ quantifying the "guest free space" of the cage-center ion, we demonstrate that nonmagnetic $RTr_{2}$Al$_{20}$ superconductors are classified into two groups, i.e., (A) $d_{\rm GFS} \ne 0$ and $T_{\rm c}$ correlates with $d_{\rm GFS}$, and (B) $d_{\rm GFS} \simeq 0$ and $T_{\rm c}$ seems to be governed by other factors.

cond-mat.supr-con↗

arXiv2017

Anomalous magnetotransport properties of high-quality single crystals of Weyl semimetal WTe2: Sign change of Hall resistivity

We report on a systematic study of Hall effect using high quality single crystals of type-II Weyl semimetal WTe2 with the applied magnetic field B//c. The residual resistivity ratio of 1330 and the large magnetoresistance of 1.5\times10^6 % in 9 T at 2 K, being in the highest class in the literature, attest to their high quality. Based on a simple two-band model, the densities (n_e and n_h) and mobilities (\mu_e and \mu_h) for electron and hole carriers have been uniquely determined combining both Hall- and electrical-resistivity data. The difference between ne and nh is ~1% at 2 K, indicating that the system is in an almost compensated condition. The negative Hall resistivity growing rapidly below ~20 K is due to a rapidly increasing \mu_h/\mu_e approaching one. Below 3 K in a low field region, we found the Hall resistivity becomes positive, reflecting that \mu_h/\mu_e finally exceeds one in this region. These anomalous behaviors of the carrier densities and mobilities might be associated with the existence of a Lifshitz transition and/or the spin texture on the Fermi surface.

cond-mat.mtrl-sci↗

arXiv2017

SnAs-based layered superconductor NaSn2As2

Superconductivity with exotic properties has often been discovered in materials with a layered (two-dimensional) crystal structure. The low dimensionality affects the electronic structure of materials, which could realize a high transition temperature (Tc) and/or unconventional pairing mechanisms. Here, we report the superconductivity in a layered tin arsenide NaSn2As2. The crystal structure consists of (Sn2As2)2- bilayers, which is bound by van-der-Waals forces, separated by Na+ ions. Measurements of electrical resistivity and specific heat confirm the bulk nature of superconductivity of NaSn2As2 with Tc of 1.3 K. Our results propose that the SnAs layers will be a basic structure providing another universality class of a layered superconducting family, and it provides a new platform for the physics and chemistry of low-dimensional superconductors with lone pair electrons.

cond-mat.supr-con↗