arXiv subjects

M. Shi

Publications and source records attributed to M. Shi.

At least 37 records · Page 2Linked to original sources

arXiv2020

Oxide Fermi liquid universality revealed by electron spectroscopy

We present a combined soft x-ray and high-resolution vacuum-ultraviolet angle-resolved photoemission spectroscopy study of the electron-overdoped cuprate Pr$_{1.3-x}$La$_{0.7}$Ce$_{x}$CuO$_4$ (PLCCO). Demonstration of its highly two-dimensional band structure enabled precise determination of the in-plane self-energy dominated by electron-electron scattering. Through analysis of this self-energy and the Fermi-liquid cut-off energy scale, we find -- in contrast to hole-doped cuprates -- a momentum isotropic and comparatively weak electron correlation in PLCCO. Yet, the self-energies extracted from multiple oxide systems combine to demonstrate a logarithmic divergent relation between the quasiparticle scattering rate and mass. This constitutes a spectroscopic version of the Kadowaki-Woods relation with an important merit -- the demonstration of Fermi liquid quasiparticle lifetime and mass being set by a single energy scale.

cond-mat.str-el↗

arXiv2020

Experimental Evidence of Stable 2$H$ Phase on the Surface of Layered 1$T'$-TaTe$_2$

We report on the low-energy electronic structure of Tantalum ditelluride (1$T'$-TaTe$_2$), one of the charge density wave (CDW) materials from the group V transition metal dichalcogenides using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT). We find that the Fermi surface topology of TaTe$_2$ is quite complicated compared to its isovalent compounds such as TaS$_2$, TaSe$_2$, and isostructural compound NbTe$_2$. More importantly, we discover that the surface electronic structure of 1$T'$-TaTe$_2$ has more resemblance to the 2$H$-TaTe$_2$, while the bulk electronic structure has more resemblance to the hypothetical 1$T$-TaTe$_2$. These experimental observations are thoroughly compared with our DFT calculations performed on 1$T$-, 2$H$- and 2$H$ (monolayer)/1$T$- TaTe$_2$. We further notice that the Fermi surface topology is temperature independent up to 180 K, confirming that the 2$H$ phase on the surface is stable up to 180 K and the CDW order is not due to the Fermi surface nesting.

cond-mat.mtrl-sci↗

arXiv2020

Link between superconductivity and a Lifshitz transition in intercalated Bi$_2$Se$_3$

Topological superconductivity is an exotic phase of matter in which the fully gapped superconducting bulk hosts gapless Majorana surface states protected by topology. Intercalation of copper, strontium or niobium between the quintuple layers of the topological insulator Bi$_2$Se$_3$ increases the carrier density and leads to superconductivity that is suggested to be topological. Here we study the electronic structure of strontium-intercalated Bi$_2$Se$_3$ using angle resolved photoemission spectroscopy (ARPES) and Shubnikov-de Haas (SdH) oscillations. Despite the apparent low Hall number of $\sim2 \times 10 ^{19}$cm$^{-3}$, we show that the Fermi surface is shaped as an open cylinder with a larger carrier density of $\sim 10 ^{20}$cm$^{-3}$. We suggest that superconductivity in intercalated Bi$_2$Se$_3$ emerges with the appearance of a quasi-2D open Fermi surface.

cond-mat.supr-con↗

arXiv2020

Anomalous Hall resistivity and possible topological Hall effect in the EuAl$_4$ antiferromagnet

We report the observation of anomalous Hall resistivity in single crystals of EuAl$_4$, a centrosymmetric tetragonal compound, which exhibits coexisting antiferromagnetic (AFM) and charge-density-wave (CDW) orders with onset at $T_\mathrm{N} \sim 15.6$ K and $T_\mathrm{CDW} \sim 140$ K, respectively. In the AFM state, when the magnetic field is applied along the $c$-axis direction, EuAl$_4$ undergoes a series of metamagnetic transitions. Within this field range, we observe a clear hump-like anomaly in the Hall resistivity, representing part of the anomalous Hall resistivity. By considering different scenarios, we conclude that such a hump-like feature is most likely a manifestation of the topological Hall effect, normally occurring in noncentrosymmetric materials known to host nontrivial topological spin textures. In view of this, EuAl$_4$ would represent a rare case where the topological Hall effect not only arises in a centrosymmetric structure, but it also coexists with CDW order.

cond-mat.str-el↗

arXiv2020

Multigap superconductivity in the Mo$_5$PB$_2$ boron-phosphorus compound

The tetragonal Mo$_5$PB$_2$ compound was recently reported to show superconductivity with a critical temperature up to 9.2 K. In search of evidence for multiple superconducting gaps in Mo$_5$PB$_2$, comprehensive measurements, including magnetic susceptibility, electrical resistivity, heat capacity, and muon-spin rotation and relaxation ($μ$SR) measurements were carried out. Data from both low-temperature superfluid density and electronic specific heat suggest a nodeless superconducting ground state in Mo$_5$PB$_2$. Two superconducting energy gaps $Δ_0$ = 1.02 meV (25%) and 1.49 meV (75%) are required to describe the low-$T$ electronic specific-heat data. The multigap features are clearly evidenced by the field dependence of the electronic specific-heat coefficient and the Gaussian relaxation rate in the superconducting state (i.e., superfluid density), as well as by the temperature dependence of the upper critical field. By combining our extensive experimental results with numerical band-structure calculations, we provide compelling evidence of multigap superconductivity in Mo$_5$PB$_2$.

cond-mat.supr-con↗

arXiv2020

Decoupling of Lattice and Orbital Degrees of Freedom in an Iron-Pnictide Superconductor

The interplay of structural and electronic phases in iron-based superconductors is a central theme in the search for the superconducting pairing mechanism. While electronic nematicity, defined as the breaking of four-fold symmetry triggered by electronic degrees of freedom, is competing with superconductivity, the effect of purely structural orthorhombic order is unexplored. Here, using x-ray diffraction (XRD), we reveal a new structural orthorhombic phase with an exceptionally high onset temperature ($T_\mathrm{ort} \sim 250$ K), which coexists with superconductivity ($T_\mathrm{c} = 25$ K), in an electron-doped iron-pnictide superconductor far from the underdoped region. Furthermore, our angle-resolved photoemission spectroscopy (ARPES) measurements demonstrate the absence of electronic nematic order as the driving mechanism, in contrast to other underdoped iron pnictides where nematicity is commonly found. Our results establish a new, high temperature phase in the phase diagram of iron-pnictide superconductors and impose strong constraints for the modeling of their superconducting pairing mechanism.

cond-mat.supr-con↗

arXiv2020

Time-reversal symmetry breaking in the noncentrosymmetric Zr$_3$Ir superconductor

We report the discovery of Zr$_3$Ir as a new type of unconventional noncentrosymmetric superconductor (with $T_c = 2.3$ K), here investigated mostly via muon-spin rotation/relaxation ($μ$SR) techniques. Its superconductivity was characterized using magnetic susceptibility, electrical resistivity, and heat capacity measurements. The low-temperature superfluid density, determined via transverse-field $μ$SR and electronic specific heat, suggests a fully-gapped superconducting state. The spontaneous magnetic fields, revealed by zero-field $μ$SR below $T_c$, indicate the breaking of time-reversal symmetry in Zr$_3$Ir and, hence, the unconventional nature of its superconductivity. By using symmetry arguments and electronic-structure calculations we obtain a superconducting order parameter that is fully compatible with the experimental observations. Hence, our results clearly suggest that Zr$_3$Ir represents a new member of noncentrosymmetric superconductors with broken time-reversal symmetry.

cond-mat.supr-con↗

arXiv2020

Anomalous band renormalization due to high energy $kink$ in the colossal thermoelectric material K$_{0.65}$RhO$_2$

We report on low-energy electronic structure and electronic correlations of K$_{0.65}$RhO$_2$, studied using high-resolution angle-resolved photoemission spectroscopy (ARPES) technique and density functional theory (DFT) calculations. We observe a highly correlated hole pocket on the Fermi surface. We further notice that the correlations are momentum dependent. Most importantly, two $kinks$ at binding energies of 75 meV and 195 meV have been observed from the band dispersion in the vicinity of the Fermi level. While the low energy $kink$ at 75 meV can be understood as a result of the electron-phonon interaction, the presence of high energy $kink$ at 195 meV is totally a new discovery of this system leading to an anomalous band renormalization. Based on systematic analysis of our experimental data, we propose high frequency bosonic excitations as a plausible origin of the high energy anomaly. Further, we notice that the high energy anomaly has important implications in obtaining the colossal thermoelectric power of K$_{0.65}$RhO$_2$.

cond-mat.str-el↗

arXiv2020

Superconductivity and topological aspects of the rock-salt carbides NbC and TaC

Superconducting materials with a nontrivial band structure are potential candidates for topological superconductivity. Here, by combining muon-spin rotation and relaxation ($μ$SR) methods with theoretical calculations, we investigate the superconducting and topological properties of the rock-salt-type compounds NbC and TaC (with$T_c$ = 11.5 and 10.3 K, respectively). At a macroscopic level, the magnetization and heat-capacity measurements under applied magnetic field provide an upper critical field of 1.93 and 0.65 T for NbC and TaC, respectively. The low-temperature superfluid density, determined by transverse-field $μ$SR and electronic specific-heat data, suggest a fully-gapped superconducting state in both NbC and TaC, with a zero-temperature gap $Δ_0 = 1.90$ and 1.45 meV, and a magnetic penetration depth $λ_0$ = 141 and 77 nm, respectively. Band-structure calculations suggest that the density of states at the Fermi level is dominated by the Nb $4d$- (or Ta $5d$-) orbitals, which are strongly hybridized with the C $p$-orbitals to produce large cylinder-like Fermi surfaces, similar to those of high-$T_c$ iron-based superconductors. Without considering the spin-orbit coupling (SOC) effect, the first Brillouin zone contains three closed node lines in the bulk band structure, protected by time-reversal and space-inversion symmetry. When considering SOC, its effects in the NbC case appear rather modest. Therefore, the node lines may be preserved in NbC, hence proposing it as a potential topological superconductor.

cond-mat.supr-con↗

arXiv2020

Simultaneous Nodal Superconductivity and Time-Reversal Symmetry Breaking in the Noncentrosymmetric Superconductor CaPtAs

By employing a series of experimental techniques, we provide clear evidence that CaPtAs represents a rare example of a noncentrosymmetric superconductor which simultaneously exhibits nodes in the superconducting gap and broken time-reversal symmetry (TRS) in its superconducting state (below $T_c$ $\approx$ 1.5 K). Unlike in fully-gapped superconductors, the magnetic penetration depth $λ(T)$ does not saturate at low temperatures, but instead it shows a $T^2$-dependence, characteristic of gap nodes. Both the superfluid density and the electronic specific heat are best described by a two-gap model comprising of a nodeless gap and a gap with nodes, rather than by single-band models. At the same time, zero-field muon-spin spectra exhibit increased relaxation rates below the onset of superconductivity, implying that TRS is broken in the superconducting state of CaPtAs, hence indicating its unconventional nature. Our observations suggest CaPtAs to be a new remarkable material which links two apparently disparate classes, that of TRS-breaking correlated magnetic superconductors with nodal gaps and the weakly-correlated noncentrosymmetric superconductors with broken TRS, normally exhibiting only a fully-gapped behavior.

cond-mat.supr-con↗

arXiv2020

Observation of flat bands due to band hybridization in 3d-electron heavy-fermion compound CaCu3Ru4O12

We report angle-resolved photoemission spectroscopy and first-principles numerical calculations for the band structure evolution of the 3d heavy-fermion compound CaCu3Ru4O12. Below 200 K, we observed an emergent hybridization gap between the Cu 3d electron-like band and the Ru 4d hole-like band and the resulting flat band features near the Fermi energy centered around the Brillouin zone corner. Our results confirm the non-Kondo nature of CaCu3Ru4O12, in which the Cu 3dxy electrons are less correlated and not in the Kondo limit. Comparison between theory and experiment also suggests that other mechanism such as nonlocal interactions or spin fluctuations beyond the local dynamical mean-field theory may be needed in order to give a quantitative explanation of the peculiar properties in this material.

cond-mat.str-el↗

arXiv2020

Strong- to weak-coupling superconductivity in high-$T_c$ bismuthates: revisiting the phase diagram via $μ$SR

Several decades after the discovery of superconductivity in bismuthates, the strength of their electron-phonon coupling and its evolution with doping remain puzzling. To clarify these issues, polycrystalline hole-doped Ba$_{1-x}$K$_{x}$BiO$_3$ ($0.1 \le x \le 0.6$) samples were systematically synthesized and their bulk- and microscopic superconducting properties were investigated by means of magnetic susceptibility and muon-spin rotation/relaxation ($μ$SR), respectively. The phase diagram of Ba$_{1-x}$K$_{x}$BiO$_3$ was reliably extended up to $x = 0.6$, which is still found to be a bulk superconductor. The lattice parameter $a$ increases linearly with K-content, implying a homogeneous chemical doping. The low-temperature superfluid density, measured via transverse-field (TF)-$μ$SR, indicates an isotropic fully-gapped superconducting state with zero-temperature gaps $Δ_0/k_\mathrm{B}T_c$ = 2.15, 2.10, and 1.75, and magnetic penetration depths $λ_0$ = 219, 184, and 279 nm for $x$ = 0.3, 0.4, and 0.6, respectively. A change in the superconducting gap, from a nearly ideal BCS value (1.76 $k_\mathrm{B}$$T_c$ in the weak coupling case) in the overdoped $x$ = 0.6 region, to much higher values in the optimally-doped case, implies a gradual decrease in electron-phonon coupling with doping.

cond-mat.supr-con↗

arXiv2019

Electronic Structure Studies of FeSi: A Chiral Topological System

Most recent observation of topological Fermi arcs on the surface of manyfold degenerate B20 systems, CoSi and RhSi, have attracted enormous research interests. Although an another isostructural system, FeSi, has been predicted to show bulk chiral fermions, it is yet to be clear theoretically and as well experimentally that whether FeSi possesses the topological surface Fermi arcs associated with the exotic chiral fermions in vicinity of the Fermi level. In this contribution, using angle-resolved photoemission spectroscopy (ARPES) and density functional theory (DFT), we present the low-energy electronic structure of FeSi. We further report the surface state calculations to provide insights into the surface band structure of FeSi near the Fermi level. Unlike in CoSi or RhSi, FeSi has no topological Fermi arcs near the Fermi level as confirmed both from ARPES and surface state calculations. Further, the ARPES data show spin-orbit coupling (SOC) band splitting of 40 meV, which is in good agreement with bulk band structure calculations. We noticed an anomalous temperature dependent resistivity in FeSi which can be understood through the electron-phonon interactions as we find a Debye energy of 80 meV from the ARPES data.

cond-mat.mtrl-sci↗

arXiv2019

Spin fluctuation induced Weyl semimetal state in the paramagnetic phase of EuCd$_2$As$_2$

Weyl fermions as emergent quasiparticles can arise in Weyl semimetals (WSMs) in which the energy bands are nondegenerate, resulting from inversion or time-reversal symmetry breaking. Nevertheless, experimental evidence for magnetically induced WSMs is scarce. Here, using photoemission spectroscopy, we observe that the degeneracy of Bloch bands is already lifted in the paramagnetic phase of EuCd$_2$As$_2$. We attribute this effect to the itinerant electrons experiencing quasistatic and quasi-long-range ferromagnetic fluctuations. Moreover, the spin nondegenerate band structure harbors a pair of ideal Weyl nodes near the Fermi level. Hence, we show that long-range magnetic order and the spontaneous breaking of time-reversal symmetry are not an essential requirement for WSM states in centrosymmetric systems, and that WSM states can emerge in a wider range of condensed-matter systems than previously thought.

cond-mat.str-el↗

arXiv2019

Enhanced $T_c$ and multiband superconductivity in the fully-gapped ReBe$_{22}$ superconductor

In search of the origin of superconductivity in diluted rhenium superconductors and their significantly enhanced $T_c$ compared to pure Be (0.026 K), we investigated the intermetallic ReBe$_{22}$ compound, mostly by means of muon-spin rotation/relaxation ($μ$SR). At a macroscopic level, its bulk superconductivity (with $T_c=9.4$ K) was studied via electrical resistivity, magnetization, and heat-capacity measurements. The superfluid density, as determined from transverse-field $μ$SR and electronic specific-heat measurements, suggest that ReBe$_{22}$ is a fully-gapped superconductor with some multigap features. The larger gap value, $Δ_0^l=1.78$ k$_\mathrm{B}T_c$, with a weight of almost 90\%, is slightly higher than that expected from the BCS theory in the weak-coupling case. The multigap feature, rather unusal for an almost elemental superconductor, is further supported by the field-dependent specific-heat coefficient, the temperature dependence of the upper critical field, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below $T_c$, as determined from zero-field $μ$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ReBe$_{22}$. In general, we find that a dramatic increase in the density of states at the Fermi level and an increase in the electron-phonon coupling strength, both contribute to the highly enhanced $T_c$ value of ReBe$_{22}$.

cond-mat.supr-con↗

arXiv2019

Moderate hybridization effects revealed by ARPES in heavy-fermion Ce2IrIn8

We utilized high-resolution resonant angle-resolved photoemission spectroscopy (ARPES) to study the band structure and hybridization effect of the heavy-fermion compound Ce2IrIn8. We observe a nearly flat band at the binding energy of 7 meV below the coherent temperature Tcoh ~ 40 K, which characterizes the electrical resistance maximum indicating the onset temperature of hybridization. However, the Fermi vector kF and the Fermi surface (FS) volume have little change around Tcoh, challenging the widely believed evolution from a high-temperature small FS to a low-temperature large FS. Our experimental results of the band structure fit well with the density functional theory plus dynamic mean-field theory (DFT+DMFT) calculations.

cond-mat.str-el↗

arXiv2019

Nodeless superconductivity and preserved time-reversal symmetry in the noncentrosymmetric Mo3P superconductor

We report a comprehensive study of the noncentrosymmetric superconductor Mo$_3$P. Its bulk superconductivity, with $T_c = 5.5$ K, was characterized via electrical resistivity, magnetization, and heat-capacity measurements, while its microscopic electronic properties were investigated by means of muon-spin rotation/relaxation ($μ$SR) and nuclear magnetic resonance (NMR) techniques. In the normal state, NMR relaxation data indicate an almost ideal metallic behavior, confirmed by band-structure calculations, which suggest a relatively high electron density of states, dominated by the Mo $4d$-orbitals. The low-temperature superfluid density, determined via transverse-field $μ$SR and electronic specific heat, suggest a fully-gapped superconducting state in Mo$_3$P, with $Δ_0= 0.83$ meV, the same as the BCS gap value in the weak-coupling case, and a zero-temperature magnetic penetration depth $λ_0 = 126$ nm. The absence of spontaneous magnetic fields below the onset of superconductivity, as determined from zero-field $μ$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of Mo$_3$P and, hence, spin-singlet pairing.

cond-mat.supr-con↗

arXiv2019

High-resolution photoemission on Sr2RuO4 reveals correlation-enhanced effective spin-orbit coupling and dominantly local self-energies

We explore the interplay of electron-electron correlations and spin-orbit coupling in the model Fermi liquid Sr2RuO4 using laser-based angle-resolved photoemission spectroscopy. Our precise measurement of the Fermi surface confirms the importance of spin-orbit coupling in this material and reveals that its effective value is enhanced by a factor of about two, due to electronic correlations. The self-energies for the $β$ and $γ$ sheets are found to display significant angular dependence. By taking into account the multi-orbital composition of quasiparticle states, we determine self-energies associated with each orbital component directly from the experimental data. This analysis demonstrates that the perceived angular dependence does not imply momentum-dependent many-body effects, but arises from a substantial orbital mixing induced by spin-orbit coupling. A comparison to single-site dynamical mean-field theory further supports the notion of dominantly local orbital self-energies, and provides strong evidence for an electronic origin of the observed non-linear frequency dependence of the self-energies, leading to `kinks' in the quasiparticle dispersion of Sr2RuO4.

cond-mat.str-el↗