arXiv subjects

T. Shiroka

Publications and source records attributed to T. Shiroka.

At least 55 records · Page 3Linked to original sources

arXiv2020

Superconductivity of underdoped PrFeAs(O,F) investigated via point-contact spectroscopy and nuclear magnetic resonance

Underdoped PrFeAs(O,F), one of the less known members of the 1111 family of iron-based superconductors, was investigated in detail by means of transport, SQUID magnetometry, nuclear magnetic resonance (NMR) measurements and point-contact Andreev-reflection spectroscopy (PCARS). PCARS measurements on single crystals evidence the multigap nature of PrFeAs(O,F) superconductivity, shown to host at least two isotropic gaps, clearly discernible in the spectra, irrespective of the direction of current injection (i.e., along the ab planes or along the c axis). Additional features at higher energy can be interpreted as signatures of a strong electron-boson coupling, as demonstrated by a model which combines Andreev reflection with the Eliashberg theory. Magnetic resonance measurements in the normal phase indicate the lack of a magnetic order in underdoped PrFeAs(O,F), while $^{75}$As NMR spin-lattice relaxation results suggest the presence of significant electronic spin fluctuations, peaking above $T_{c}$ and expected to mediate the superconducting pairing.

cond-mat.supr-con↗

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

Using uniaxial stress to probe the relationship between competing superconducting states in a cuprate with spin-stripe order

We report muon spin rotation and magnetic susceptibility experiments on in-plane stress effects on the static spin-stripe order and superconductivity in the cuprate system La2-xBaxCuO4 with x = 0.115. An extremely low uniaxial stress of 0.1 GPa induces a substantial decrease in the magnetic volume fraction and a dramatic rise in the onset of 3D superconductivity, from 10 to 32 K; however, the onset of at-least-2D superconductivity is much less sensitive to stress. These results show not only that large-volume-fraction spin-stripe order is anti-correlated with 3D superconducting (SC) coherence, but also that these states are energetically very finely balanced. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. These results strongly suggest a similar pairing mechanism for spin-stripe order, the spatially-modulated 2D and uniform 3D SC orders, imposing an important constraint on theoretical models.

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

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

Multiple- to single-gap superconductivity crossover in Nb$_5$Ir$_{3-x}$Pt$_x$O alloys

By using mostly the muon-spin rotation/relaxation ($μ$SR) technique, we investigate the superconductivity (SC) of Nb$_5$Ir$_{3-x}$Pt$_x$O ($x = 0$ and 1.6) alloys, with $T_c = 10.5$ K and 9.1 K, respectively. At a macroscopic level, their superconductivity was studied by electrical resistivity, magnetization, and specific-heat measurements. In both compounds, the electronic specific heat and the low-temperature superfluid density data suggest a nodeless SC. The superconducting gap value and the specific heat discontinuity at $T_c$ are larger than that expected from the Bardeen-Cooper-Schrieffer theory in the weak-coupling regime, indicating strong-coupling superconductivity in the Nb$_5$Ir$_{3-x}$Pt$_x$O family. In Nb$_5$Ir$_3$O, multigap SC is evidenced by the field dependence of the electronic specific heat coefficient and the superconducting Gaussian relaxation rate, as well as by the temperature dependence of the upper critical field. Pt substitution suppresses one of the gaps, and Nb$_5$Ir$_{1.4}$Pt$_{1.6}$O becomes a single-gap superconductor. By combining our extensive experimental results, we provide evidence for a multiple- to single-gap SC crossover in the Nb$_5$Ir$_{3-x}$Pt$_x$O family.

cond-mat.supr-con↗

arXiv2020

Pressure-Induced Antiferromagnetic Dome in the Heavy-Fermion $Yb_2Pd_2In_{1-x}Sn_x$ System

In the heavy-fermion system $Yb_2Pd_2In_{1-x}Sn_x$, the interplay of crystal-field splitting, Kondo effect, and Ruderman-Kittel-Kasuya-Yosida interactions leads to complex chemical-, pressure-, and magnetic-field phase diagrams, still to be explored in full detail. By using a series of techniques, we show that even modest changes of parameters other than temperature are sufficient to induce multiple quantum-critical transitions in this highly susceptible heavy-fermion family. In particular, we show that, above $\sim 10$ kbar, hydrostatic pressure not only induces an antiferromagnetic phase at low temperature, but it likely leads to a reorientation of the Yb magnetic moments and/or the competition among different antiferromagnetic configurations.

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

Two Coupled Chains are Simpler than One: Field-induced Chirality in a Frustrated Quantum Spin Ladder

Although the frustrated spin chain (zigzag chain) is a Drosophila of frustrated magnetism, the understanding of a pair of coupled zigzag chains (frustrated spin ladder) in a magnetic field is incomplete. We address this problem through nuclear magnetic resonance (NMR) experiments on $\text{BiCu}_2\text{PO}_6$ in magnetic fields up to 45 T, revealing a field-induced spiral magnetic structure. Conjointly, we present advanced numerical calculations showing that even moderate rung coupling dramatically simplifies the phase diagram below half-saturation magnetization by stabilizing a field-induced chiral phase. Surprisingly for a one-dimensional model, this phase and its response to Dzyaloshinskii-Moriya (DM) interactions adhere to classical expectations. While explaining the behavior at the highest accessible magnetic fields, our results imply a different origin for the solitonic phases occurring at lower fields in $\text{BiCu}_2\text{PO}_6$. An exciting possibility is that the known, DM-mediated coupling between chirality and crystal lattice gives rise to a new kind of spin-Peierls instability.

cond-mat.str-el↗

arXiv2019

Muon spin rotation measurements on RbEuFe$_{4}$As$_{4}$ under pressure

We report muon spin rotation and magnetization measurements on the magnetic superconductor RbEuFe$_{4}$As$_{4}$ under hydrostatic pressures up to 3.8 GPa. At ambient pressure, RbEuFe$_{4}$As$_{4}$ exhibits a superconducting transition at $T_{c} \approx$ 36.5 K and a magnetic transition at $T_{m} \approx$ 15 K below which the magnetic and the superconducting order coexist. With increasing pressure, $T_{c}$ decreases while $T_{m}$ and the ordered Eu magnetic moment increase. In contrast to iron-based superconductors with ordering Fe moments, the size of the ordered Eu moment is not proportional to $T_{m}$. The muon spin rotation signal is dominated by the magnetic response impeding the determination of the superconducting properties.

cond-mat.supr-con↗

arXiv2019

Coupled Magnetic and Superconducting Transitions in Sr$_{2}$VO$_{3}$FeAs Under Pressure

We report muon spin rotation ($μ$SR) and magnetization measurements on superconducting Sr$_{2}$VO$_{3}$FeAs under pressure. At ambient pressure, Sr$_{2}$VO$_{3}$FeAs undergoes an antiferromagnetic transition of the V moments at $T_{N}$ and becomes superconducting at $T_{c}<T_{N}$. As a function of pressure, $T_{N}$ initially decreases while $T_{c}$ increases. Surprisingly, once $T_{N}\approx T_{c}$ at 0.6~GPa, $T_{N}$ reverses its trend and increases together with $T_{c}$ suggesting that the static V magnetism is a prerequisite for superconductivity. We explain this cooperative coupling by a possible localization of the V 3$d$ states below the magnetic transition which enables the nesting of the Fermi surface necessary for superconductivity.

cond-mat.supr-con↗

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

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

Room-temperature structural phase transition in the quasi-2D spin-1/2 Heisenberg antiferromagnet Cu(pz)$_2$(ClO$_4$)$_2$

Cu(pz)$_2$(ClO$_4$)$_2$ (with pz denoting pyrazine C$_4$H$_4$N$_2$) is a two-dimensional spin-1/2 square-lattice antiferromagnet with $T_{\mathrm{N}}$ = 4.24 K. Due to a persisting focus on the low-temperature magnetic properties, its room-temperature structural and physical properties caught no attention up to now. Here we report a study of the structural features of Cu(pz)$_2$(ClO$_4$)$_2$ in the paramagnetic phase, up to 330 K. By employing magnetization, specific heat, $^{35}$Cl nuclear magnetic resonance, and neutron diffraction measurements, we provide evidence of a second-order phase transition at $T^{\star}$ = 294 K, not reported before. The absence of a magnetic ordering across $T^{\star}$ in the magnetization data, yet the presence of a sizable anomaly in the specific heat, suggest a structural order-to-disorder type transition. NMR and neutron-diffraction data corroborate our conjecture, by revealing subtle angular distortions of the pyrazine rings and of ClO$^-_4$ counteranion tetrahedra, shown to adopt a configuration of higher symmetry above the transition temperature.

cond-mat.mtrl-sci↗

arXiv2019

Structure and superconductivity in the binary Re$_{1-x}$Mo$_x$ alloys

The binary Re$_{1-x}$Mo$_x$ alloys, known to cover the full range of solid solutions, were successfully synthesized and their crystal structures and physical properties investigated via powder x-ray diffraction, electrical resistivity, magnetic susceptibility, and heat capacity. By varying the Re/Mo ratio we explore the full Re$_{1-x}$Mo$_x$ binary phase diagram, in all its four different solid phases: hcp-Mg ($P6_3/mmc$), $α$-Mn ($I\overline{4}3m$), $β$-CrFe ($P4_2/mnm$), and bcc-W ($Im\overline{3}m$), of which the second is non-centrosymmetric with the rest being centrosymmetric. All Re$_{1-x}$Mo$_x$ alloys are superconductors, whose critical temperatures exhibit a peculiar phase diagram, characterized by three different superconducting regions. In most alloys the $T_c$ is almost an order of magnitude higher than in pure Re and Mo. Low-temperature electronic specific-heat data evidence a fully-gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling across the series. Considering that several $α$-Mn-type Re$T$ alloys ($T$ = transition metal) show time-reversal symmetry breaking (TRSB) in the superconducting state, while TRS is preserved in the isostructural Mg$_{10}$Ir$_{19}$B$_{16}$ or Nb$_{0.5}$Os$_{0.5}$, the Re$_{1-x}$Mo$_x$ alloys represent another suitable system for studying the interplay of space-inversion, gauge, and time-reversal symmetries in future experiments expected to probe TRSB in the Re$T$ family.

cond-mat.supr-con↗

arXiv2018

From order to randomness: Onset and evolution of the random-singlet state in bond-disordered BaCu$_2$(Si$_{1-x}$Ge$_x$)$_2$O$_7$ spin-chain compounds

Heisenberg-type spin-chain materials have been extensively studied over the years, yet not much is known about their behavior in the presence of disorder. Starting from BaCu$_2$Si$_2$O$_7$, a typical spin-1/2 chain system, we investigate a series of compounds with different degrees of bond disorder, where the systematic replacement of Si with Ge results in a re-modulation of the Cu$^{2+}$ exchange interactions. By combining magnetometry measurements with nuclear magnetic resonance studies we follow the evolution of the disorder-related properties from the well-ordered BaCu$_2$Si$_2$O$_7$ to the maximally disordered BaCu$_2$SiGeO$_7$. Our data indicate that already a weak degree of disorder of only 5% Ge, apart from reducing the 3D magnetic ordering temperature $T_\mathrm{N}$ quite effectively, induces a qualitatively different state in the paramagnetic regime. At maximum disorder our data indicate that this state may be identified with the theoretically predicted random singlet (RS) state. With decreasing disorder the extension of the RS regime at temperatures above $T_\mathrm{N}$ is reduced, yet its influence is clearly manifest, particularly in the features of NMR relaxation data.

cond-mat.str-el↗

arXiv2018

Time-reversal symmetry breaking in Re-based superconductors

To trace the origin of time-reversal symmetry breaking (TRSB) in Re-based superconductors, we performed comparative muon-spin rotation/relaxation ($μ$SR) studies of superconducting noncentrosymmetric Re$_{0.82}$Nb$_{0.18}$ ($T_c = 8.8$ K) and centrosymmetric Re ($T_c = 2.7$ K). In Re$_{0.82}$Nb$_{0.18}$, the low temperature superfluid density and the electronic specific heat evidence a fully-gapped superconducting state, whose enhanced gap magnitude and specific-heat discontinuity suggest a moderately strong electron-phonon coupling. In both Re$_{0.82}$Nb$_{0.18}$ and pure Re, the spontaneous magnetic fields revealed by zero-field $μ$SR below $T_c$ indicate time-reversal symmetry breaking and thus unconventional superconductivity. The concomitant occurrence of TRSB in centrosymmetric Re and noncentrosymmetric Re$T$ ($T$ = transition metal), yet its preservation in the isostructural noncentrosymmetric superconductors Mg$_{10}$Ir$_{19}$B$_{16}$ and Nb$_{0.5}$Os$_{0.5}$, strongly suggests that the local electronic structure of Re is crucial for understanding the TRSB superconducting state in Re and Re$T$. We discuss the superconducting order parameter symmetries that are compatible with the observations.

cond-mat.supr-con↗