arXiv subjects

KeYuan Ma

Publications and source records attributed to KeYuan Ma.

14 recordsLinked to original sources

arXiv2026

Structure, Composition, and High-Field Superconductivity in Metal-Rich $\mathrm{\eta}$-Carbide-Type Compounds

$\mathrm{\eta}$-Carbide-type compounds have recently emerged as a diverse class of materials in the study of superconductivity. These phases contribute to a growing family of metal-rich quantum materials that exhibit unusual superconducting properties emerging from complex metallic bonding. Several members of the $\mathrm{\eta}$-carbide-type phases have been found to be bulk superconductors -- such as Nb$_4$Rh$_2$C$_{1-\delta}$, Ta$_4$Rh$_2$C$_{1-\delta}$, Ti$_4$Ir$_2$O$_{1-\delta}$, and Ti$_4$Co$_2$O$_{1-\delta}$ -- with transition temperatures up to $T_{\rm c} \approx$ 10 K and upper critical fields as high as $\mu_0 H_{\rm c2}(0) \approx$ 30 T. Whereas the transition temperatures may fall within the range typical for intermetallic superconductors, the pronounced violation of the weak-coupling Pauli limit in many of these crystallographically high-symmetry materials is noteworthy. Here, we review recent progress on superconducting $\mathrm{\eta}$-carbide-type phases, emphasizing how crystal symmetry, synthetic challenges, transition-metal composition, and electronic structure govern their superconducting properties. Furthermore, we outline open questions and future directions, including the possible discovery of new $\mathrm{\eta}$-carbide-type materials.

cond-mat.supr-con↗

arXiv2025

Suppression of stripe-ordered structural phases in monolayer IrTe$_2$ by a gold substrate

Metal-assisted exfoliation of two-dimensional (2D) materials has emerged as an efficient route to isolating large-area monolayer crystals, yet the influence of the supporting metal substrate on their intrinsic properties remains poorly understood. Here, we demonstrate successful gold-assisted exfoliation of monolayer IrTe$_2$ up to the millimeter scale. Angle-resolved photoemission spectroscopy (ARPES), combined with first-principles calculations, reveals that the low-energy electronic structure closely resembles that of a freestanding monolayer 1T-IrTe$_2$. We find that quasi-covalent hybridization together with substrate-induced strain leads to only modest modifications of the electronic bands. Although strain contributes to phase stability, it is essentially hybridization that drives the stabilization of the 1T-phase of the monolayer IrTe$_2$ by suppressing stripe-ordered phase transitions. These results establish gold-assisted exfoliation as a robust route to prepare a large-area monolayer IrTe$_2$ and highlight the role of metal-substrate interaction in engineering 2D materials with tailored structural phases.

cond-mat.mtrl-sci↗

arXiv2025

Discovery of the Type-II Superconductor Ta$_4$Rh$_2$C$_{1-δ}$ with a High Upper Critical Field

We report on the discovery of superconductivity in the previously unknown compound Ta$_4$Rh$_2$C$_{1-δ}$. Ta$_4$Rh$_2$C$_{1-δ}$ crystallizes in the $η$-carbide structure type, in the cubic space group $Fd\bar{3}m$ (No.227) with a unit cell parameter of $a = $ 11.7947 Å. Temperature-dependent magnetic susceptibility, resistivity, and specific heat capacity measurements reveal that Ta$_4$Rh$_2$C$_{1-δ}$ is a type-II bulk superconductor with a critical temperature of $T_{\rm c}$ = 6.4 K, and a normalized specific heat jump $ΔC/γT_{\rm c}$ = 1.56. Notably, we find Ta$_4$Rh$_2$C$_{1-δ}$ has a high upper critical field of $μ_0 H_{\rm c2}{\rm (0)}$ = 17.4 T, which is exceeding the BCS weak coupling Pauli limit of $μ_0 H_{\rm Pauli}$ = 11.9 T.

cond-mat.supr-con↗

arXiv2025

Oxygen-isotope effect on density wave transitions in La$_3$Ni$_2$O$_{7}$

TThe isotope effect is a powerful probe of electron-phonon interactions in solid-state systems, offering key insights into how atomic mass influences emergent quantum states. Here, the impact of oxygen isotope substitution ($^{16}{\rm O}\rightarrow \; ^{18}{\rm O}$) on charge- and spin-density wave (CDW and SDW) transitions in the double-layer Ruddlesden-Popper nickelate La$_3$Ni$_2$O$_7$ is investigated. A clear isotope effect is observed in the CDW transition: the transition temperature ($T_{\rm CDW}$) increases upon $^{18}$O substitution. In contrast, the SDW transition temperature remains unaffected within experimental uncertainty. These findings point to a strong involvement of lattice vibrations in the formation of charge order, while spin order appears to be predominantly of electronic origin. The results suggest that electron-phonon coupling, manifested through the CDW response to isotope substitution, may be relevant to the superconducting pairing mechanism in Ruddlesden-Popper nickelates.

cond-mat.supr-con↗

arXiv2024

Dome-Shaped Superconducting Phase Diagram Linked to Charge Order in LaRu$_{3}$Si$_{2}$

The interplay between superconductivity and charge order is a central focus in condensed matter research, with kagome lattice systems offering unique insights. The kagome superconductor LaRu$_{3}$Si$_{2}$ ($T_{\rm c}$ ${\simeq}$ 6.5 K) exhibits a hierarchy of charge order transitions: primary ($T_{\rm co,I}$ ${\simeq}$ 400 K), secondary ($T_{\rm co,II}$ ${\simeq}$ 80 K), and an additional transition at ($T^{*}$ $\simeq$ 35 K). The transitions at $T_{\rm co,II}$ and $T^{*}$ are linked to electronic and magnetic responses as revealed by muon-spin rotation and magnetotransport experiments. However, the connection between superconductivity, charge order, and electronic responses has remained elusive. By employing magnetotransport and X-ray diffraction techniques under pressures of up to 40 GPa, we observe that $T_{\rm c}$ rises to 9 K at 2 GPa, remains nearly constant up to 12 GPa, and then decreases to 2 K at 40 GPa, resulting in a dome-shaped phase diagram. The resistivity anomaly at $T^{*}$ and magnetoresistance also exhibit a similar dome-shaped pressure dependence. Furthermore, we find that charge order transitions from long-range to short-range above 12 GPa, correlating with the suppression of $T_{\rm c}$, suggesting superconductivity is closely tied to the charge-ordered state. Specifically, $T_{\rm c}$ peaks when charge order and the normal-state electronic responses are optimized. In contrast to systems like the cuprates, transition metal dichalcogenides, and other kagome materials, where superconductivity typically competes with charge order, LaRu$_{3}$Si$_{2}$ displays a pronounced interdependence between these two phenomena. This distinctive behavior sheds new light on the connection between superconductivity and charge order, offering avenues for theoretical advancements in understanding superconductivity.

cond-mat.supr-con↗

arXiv2024

Pressure-induced superconductivity in monoclinic RhBi$_2$

RhBi$_2$ is a polymorphic system that exhibits two distinct phases. RhBi$_2$ in the triclinic phase has been identified as a weak topological insulator with a van Hove singularity point close to the Fermi energy. Thus, triclinic RhBi$_2$ is expected to exhibit exotic quantum properties under strain or pressure. In this study, we report on the emergence of superconductivity in the monoclinic RhBi$_2$ under external pressures. The electrical resistivity behavior of the monoclinic RhBi$_2$ single crystal is studied at a wide range of applied external pressures up to 40 GPa. We observe a pressure-induced superconductivity with a dome-shaped dependence of the critical temperature on pressure at pressures above 10 GPa. A maximum critical temperature ($T_\mathrm{c}$) value of $T_\mathrm{c}$ = 5.1 K is reached at the pressure of 16.1 GPa. Furthermore, we performed detailed ab initio calculations to understand the electronic band structures of monoclinic RhBi$_2$ under varying pressures. The combination of topology and pressure-induced superconductivity in the RhBi$_2$ polymorphic system may provide us with a new promising material platform to investigate topological superconductivity.

cond-mat.supr-con↗

arXiv2024

Ti4Ir2O a time-reversal-invariant fully gapped unconventional superconductor

Here we report muon spin rotation (muSR) experiments on the temperature and field dependence of the effective magnetic penetration depth (lambda) in the eta-carbide-type suboxide Ti4Ir2O, a superconductor with an considerably high upper critical field. Temperature dependence of penetration depth, obtained from transverse-field (TF)-muSR measurements, is in perfect agreement with an isotropic fully gaped superconducting state. Furthermore, our ZF muSR results confirm that the time-reversal symmetry is preserved in the superconducting state. We find, however, a notably low ratio of 1.22 between the superconducting critical temperature and the superfluid density. This value is close to most unconventional superconductors, showing that a very small superfluid density is present in the superconducting state of Ti4Ir2O. The presented results will pave the way for further theoretical and experimental investigations to obtain a microscopic understanding of the origin of such a high upper critical field in an isotropic single gap superconducting system.

cond-mat.supr-con↗

arXiv2023

Electronic structure of few-layer black phosphorus from $μ$-ARPES

Black phosphorus (BP) stands out among two-dimensional (2D) semiconductors because of its high mobility and thickness dependent direct band gap. However, the quasiparticle band structure of ultrathin BP has remained inaccessible to experiment thus far. Here we use a recently developed laser-based micro-focus angle resolved photoemission ($μ$-ARPES) system to establish the electronic structure of 2-9 layer BP from experiment. Our measurements unveil ladders of anisotropic, quantized subbands at energies that deviate from the scaling observed in conventional semiconductor quantum wells. We quantify the anisotropy of the effective masses and determine universal tight-binding parameters which provide an accurate description of the electronic structure for all thicknesses.

cond-mat.mes-hall↗

arXiv2021

Group-9 Transition Metal Suboxides Adopting the Filled-Ti$_2$Ni Structure: A Class of Superconductors Exhibiting Exceptionally High Upper Critical Fields

The Ti$_2$Ni and the related $η$-carbide structure are known to exhibit various intriguing physical properties. The Ti$_2$Ni structure with the cubic space group $Fd\bar{3}m$ is surprisingly complex, consisting of a unit cell with 96 metal atoms. The related $η$-carbide compounds correspond to a filled version of the Ti$_2$Ni structure. Here, we report on the structure and superconductivity in the $η$-carbide type suboxides Ti$_4$M$_2$O with M = Co, Rh, Ir. We have successfully synthesized all three compounds in single phase form. We find all three compounds to be type-II bulk superconductors with transition temperatures of $T_{\rm c}$ = 2.7, 2.8, and 5.4 K, and with normalized specific heat jumps of $ΔC/γT_{\rm c}$ = 1.65, 1.28, and 1.80 for Ti$_4$Co$_2$O, Ti$_4$Rh$_2$O, and Ti$_4$Ir$_2$O, respectively. We find that all three superconductors, exhibit high upper-critical fields. Particularly noteworthy is Ti$_4$Ir$_2$O with an upper critical field of $μ_0 H_{\rm c2}{\rm (0)}$ =~16.06~T, which exceeds by far the weak-coupling Pauli limit of 9.86~T. The role of the void filling light atom X has so far been uncertain for the overall physical properties of these materials. Herein, we have successfully grown single crystals of Ti$_2$Co. In contrast to the metallic $η$-carbide type suboxides Ti$_4$M$_2$O, we find that Ti$_2$Co displays a semimetallic behavior. Hence, the octahedral void-filling oxygen plays a crucial role for the overall physical properties, even though its effect on the crystal structure is small. Our results indicate that the design of new superconductors by incorporation of electron-acceptor atoms may in the Ti$_2$Ni-type structures and other materials with crystallographic void position be a promising future approach. The remarkably high upper critical fields, in this family of compounds, may furthermore spark significant future interest.

cond-mat.supr-con↗

arXiv2021

Synthetic control over polymorph formation in the d-band semiconductor system FeS$_2$

Pyrite, also known as fool's gold is the thermodynamic stable polymorph of FeS$_2$. It is widely considered as a promising d-band semiconductor for various applications due to its intriguing physical properties. Marcasite is the other naturally occurring polymorph of FeS$_2$. Measurements on natural crystals have shown that it has similarly promising electronic, mechanical, and optical properties as pyrite. However, it has been only scarcely investigated so far, because the laboratory-based synthesis of phase-pure samples or high-quality marcasite single crystal has been a challenge until now. Here, we report the targeted phase formation via hydrothermal synthesis of marcasite and pyrite. The formation condition and phase purity of the FeS$_2$ polymorphs are systematically studied in the form of a comprehensive synthesis map. We, furthermore, report on a detailed analysis of marcasite single crystal growth by a space-separated hydrothermal synthesis. We observe that single phase product of marcasite forms only on the surface under the involvement of H$_2$S and sulphur vapor. The availability of high-quality crystals of marcasite allows us to measure the fundamental physical properties, including an allowed direct optical bandgap of 0.76 eV, temperature independent diamagnetism, an electronic transport gap of 0.11 eV, and a room-temperature carrier concentration of 4.14 $\times$ 10$^{18}$ cm$^{-3}$. X-ray absorption/emission spectroscopy are employed to measure the band gap of the two FeS$_2$ phases. We find marcasite has a band gap of 0.73 eV, while pyrite has a band gap of 0.87 eV. Our results indicate that marcasite -- that is now synthetically available in a straightforward fashion -- is as equally promising as pyrite as candidate for various semiconductor applications based on earth abundant elements.

cond-mat.mtrl-sci↗

arXiv2021

Break of symmetry at the surface of IrTe$_2$ upon phase transition measured by X-ray photoelectron diffraction

IrTe$_2$ undergoes a series of charge-ordered phase transitions below room temperature that are characterized by the formation of stripes of Ir dimers of different periodicities. Full hemispherical X-ray photoelectron diffraction (XPD) experiments have been performed to investigate the atomic position changes undergone near the surface of $1T-$IrTe$_2$ in the first-order phase transition, from the $(1\times1)$ phase to the $(5\times1)$ phase. Comparison between experiment and simulation allows us to identify the consequence of the dimerization on the Ir atoms local environment. We report that XPD permits to unveil the break of symmetry of IrTe$_2$ trigonal to a monoclonic unit cell and confirm the occurence of the $(5\times1)$ reconstruction within the first few layers below the surface with a staircase-like stacking of dimers.

cond-mat.mtrl-sci↗

arXiv2021

Superconductivity with High Upper Critical Field in the Cubic Centrosymmetric $η$-Carbide Nb$_4$Rh$_2$C$_{1-δ}$

The upper critical field is a fundamental measure of the strength of superconductivity in a material. It is also a cornerstone for the realization of superconducting magnet applications. The critical field arises because of the Copper pair breaking at a limiting field, which is due to the Pauli paramagnetism of the electrons. The maximal possible magnetic field strength for this effect is commonly known as the Pauli paramagnetic limit given as $μ_0 H_{\rm Pauli} \approx 1.86{\rm [T/K]} \cdot T_{\rm c}$ for a weak-coupling BCS superconductor. The violation of this limit is only rarely observed. Exceptions include some low-temperature heavy fermion and some strongly anisotropic superconductors. Here, we report on the superconductivity at 9.75 K in the centrosymmetric, cubic $η$-carbide-type compound Nb$_4$Rh$_2$C$_{1-δ}$, with a normalized specific heat jump of $ΔC/γT_{\rm c} =$ 1.64. We find that this material has a remarkably high upper critical field of $μ_0 H_{\rm c2}{\rm (0)}$ =~28.5~T, which is exceeding by far its weak-coupling BCS Pauli paramagnetic limit of $μ_0 H_{\rm Pauli}$~=~18.1 T. Determination of the origin and consequences of this effect will represent a significant new direction in the study of critical fields in superconductors.

cond-mat.supr-con↗

arXiv2020

Two-gap to Single-gap Superconducting Transition on a Honeycomb Lattice in Ca$_{1-x}$Sr$_{x}$AlSi

It is a well-established fact that the physical properties of compounds follow their crystal symmetries. This has especially pronounced implications on emergent collective quantum states in materials. Specifically, the effect of crystal symmetries on the properties of superconductors is widely appreciated, although the clarification of this relationship is a core effort of on-going research. Emergent phenomena on honeycomb lattices are of special interest, as they can give rise to spectacular phenomenology, as manifested by the recent discovery of correlated states in magic-angle graphene, or by the high-temperature superconductivity in MgB$_2$. Here, we report on the structural and microscopic superconducting properties of a class of ternary superconductors with Al/Si honeycomb layers, i.e. Ca$_{1-x}$Sr$_{x}$AlSi. We show that this solid solution is a remarkable model system with a highly tunable two-gap to single-gap superconducting system on a honeycomb lattice, where the superconductivity is enhanced by a subtle structural instability, i.e. the buckling of the Al/Si layers.

cond-mat.supr-con↗

arXiv2019

Superconductivity in the $η$-carbide-type oxides Zr4Rh2Ox

We report on the synthesis and the superconductivity of Zr$_4$Rh$_2$O$_{x}$ ($x$ = 0.4, 0.5, 0.6, 0.7, 1.0). These compounds crystallize in the $η$-carbide structure, which is a filled version of the complex intermetallic Ti$_2$Ni structure. We find that in the system Zr$_4$Rh$_2$O$_{x}$, already a small amount ($x$ $\geq$ 0.4) of oxygen addition stabilizes the $η$-carbide structure over the more common intermetallic CuAl$_2$ structure-type, in which Zr$_2$Rh crystallizes. We show that Zr$_4$Rh$_2$O$_{0.7}$ and Zr$_4$Rh$_2$O are bulk superconductors with critical temperatures of $T_c \approx$ 2.8 K and 4.7 K in the resistivity, respectively. Our analysis of the superconducting properties reveal both compounds to be strongly type-II superconductors with critical fields up to $μ_0 H_{c1}$(0) $\approx$ 8.8 mT and $μ_0 H_{c2}$(0) $\approx$ 6.08 T. Our results support that the $η$-carbides are a versatile family of compounds for the investigation of the interplay of interstitial doping on physical properties, especially for superconductivity.

cond-mat.supr-con↗