arXiv subjects

T. Shiroka

Publications and source records attributed to T. Shiroka.

At least 37 records · Page 2Linked to original sources

arXiv2023

ZrOsSi: A $Z_2$ topological metal with a superconducting ground state

The silicide superconductors (Ta, Nb, Zr)OsSi are among the best candidate materials for investigating the interplay of topological order and superconductivity. Here, we investigate in detail the normal-state topological properties of (Ta, Nb, Zr)OsSi, focusing on ZrOsSi, by employing a combination of $^{29}$Si nuclear magnetic resonance (NMR) measurements and first-principles band-structure calculations. We show that, while (Ta, Nb)OsSi behave as almost ideal metals, characterized by weak electronic correlations and a relatively low density of states, the replacement of Ta (or Nb) with Zr expands the crystal lattice and shifts ZrOsSi towards an insulator. Our ab initio calculations indicate that ZrOsSi is a $Z_2$ topological metal with clear surface Dirac cones and properties similar to a doped strong topological insulator.

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arXiv2023

Neutron scattering and muon-spin spectroscopy studies of the magnetic triangular-lattice compounds $A_2$La$_2$NiW$_2$O$_{12}$ ($A$ = Sr, Ba)

We report on the geometrically frustrated two-dimensional triangular-lattice magnets $A_2$La$_2$NiW$_2$O$_{12}$ ($A$ = Sr, Ba) studied mostly by means of neutron powder diffraction (NPD) and muon-spin rotation and relaxation ($μ$SR) techniques. The chemical pressure induced by the Ba-for-Sr substitution suppresses the ferromagnetic (FM) transition from 6.3 K in the Ba-compound to 4.8 K in the Sr-compound. We find that the $R\bar{3}$ space group reproduces the NPD patterns better than the previously reported $R\bar{3}m$ space group. Both compounds adopt the same magnetic structure with a propagation vector $\boldsymbol{k} = (0, 0, 0)$, in which the Ni$^{2+}$ magnetic moments are aligned ferromagnetically along the $c$-axis. The zero-field {\textmu}SR results reveal two distinct internal fields (0.31 and 0.10 T), caused by the long-range ferromagnetic order. The small transverse muon-spin relaxation rates reflect the homogeneous internal field distribution in the ordered phase and, thus, further support the simple FM arrangement of the Ni$^{2+}$ moments. The small longitudinal muon-spin relaxation rates, in both the ferromagnetic- and paramagnetic states of A$_2$La$_2$NiW$_2$O$_{12}$, indicate that spin fluctuations are rather weak. Our results demonstrate that chemical pressure indeed changes the superexchange interactions in $A_2$La$_2$NiW$_2$O$_{12}$ compounds, with the FM interactions being dominant.

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arXiv2023

Fully-gapped superconductivity and topological aspects of the noncentrosymmetric TaReSi superconductor

We report a study of the noncentrosymmetric TaReSi superconductor by means of muon-spin rotation and relaxation ($μ$SR) technique, complemented by electronic band-structure calculations. Its superconductivity, with $T_c$ = 5.5 K and upper critical field $μ_0H_\mathrm{c2}(0)$ $\sim$ 3.4 T, was characterized via electrical-resistivity- and magnetic-susceptibility measurements. The temperature-dependent superfluid density, obtained from transverse-field $μ$SR, suggests a fully-gapped superconducting state in TaReSi, with an energy gap $Δ_0$ = 0.79 meV and a magnetic penetration depth $λ_0$ = 562 nm. The absence of a spontaneous magnetization below $T_c$, as confirmed by zero-field $μ$SR, indicates a preserved time-reversal symmetry in the superconducting state. The density of states near the Fermi level is dominated by the Ta- and Re-5$d$ orbitals, which account for the relatively large band splitting due to the antisymmetric spin-orbit coupling. In its normal state, TaReSi behaves as a three-dimensional Kramers nodal-line semimetal, characterized by an hourglass-shaped dispersion protected by glide reflection. By combining non\-triv\-i\-al electronic bands with intrinsic superconductivity, TaReSi is a promising material for investigating the topological aspects of noncentrosymmetric superconductors.

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arXiv2023

Fully-gapped superconductivity with preserved time-reversal symmetry in NiBi$_3$ single crystals

We report a study of NiBi$_3$ single crystals by means of electrical-resistivity-, magnetization-, and muon-spin rotation and relaxation ($μ$SR) measurements. As a single crystal, NiBi$_3$ adopts a needle-like shape and exhibits bulk superconductivity with $T_c \approx 4.1$ K. By applying magnetic fields parallel and perpendicular to the $b$-axis of NiBi$_3$, we establish that its lower- and upper critical fields, as well as the magnetic penetration depths show slightly different values, suggesting a weakly anisotropic superconductivity. In both cases, the zero-temperature upper critical fields are much smaller than the Pauli-limit value, indicating that the superconducting state is constrained by the orbital pair breaking. The temperature evolution of the superfluid density, obtained from transverse-field $μ$SR, reveals a fully-gapped superconductivity in NiBi$_3$, with a shared superconducting gap $Δ_0$ = 2.1 $k_\mathrm{B}$$T_c$ and magnetic penetration depths $λ_0$ = 223 and 210 nm for $H \parallel b$- and $H \perp b$, respectively. The lack of spontaneous fields below $T_c$ indicates that time-reversal symmetry is preserved in NiBi$_3$. The absence of a fast muon-spin relaxation and/or precession in the zero-field $μ$SR spectra definitely rules out any type of magnetic ordering in NiBi$_3$ single crystals. Overall, our investigation suggests that NiBi$_3$ behaves as a conventional $s$-type superconductor.

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arXiv2023

Nodeless superconductivity in the noncentrosymmetric ThIrSi compound

The ThIrSi superconductor, with $T_c = 6.5$ K, is expected to show unusual features in view of its noncentrosymmetric structure and the presence of heavy elements featuring a sizable spin-orbit coupling. Here, we report a comprehensive study of its electronic properties by means of local-probe techniques: muon-spin rotation and relaxation ({\textmu}SR) and nuclear magnetic resonance (NMR). Both the superfluid density $ρ_\mathrm{sc}(T)$ (determined via transverse-field {\textmu}SR) and the spin-lattice relaxation rate $T_1^{-1}(T)$ (determined via NMR) suggest a nodeless superconductivity. Furthermore, the absence of spontaneous magnetic fields below $T_c$, as evinced from zero-field {\textmu}SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of ThIrSi. Temperature-dependent upper critical fields as well as field-dependent superconducting muon-spin relaxations suggest the presence of multiple superconducting gaps in ThIrSi.

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arXiv2023

Designing the stripe-ordered cuprate phase diagram through uniaxial-stress

The ability to efficiently control charge and spin in the cuprate high-temperature superconductors is crucial for fundamental research and underpins technological development. Here, we explore the tunability of magnetism, superconductivity and crystal structure in the stripe phase of the cuprate La_2-xBa_xCuO_4, with x = 0.115 and 0.135, by employing temperature-dependent (down to 400 mK) muon-spin rotation and AC susceptibility, as well as X-ray scattering experiments under compressive uniaxial stress in the CuO_2 plane. A sixfold increase of the 3-dimensional (3D) superconducting critical temperature T_c and a full recovery of the 3D phase coherence is observed in both samples with the application of extremely low uniaxial stress of 0.1 GPa. This finding demonstrates the removal of the well-known 1/8-anomaly of cuprates by uniaxial stress. On the other hand, the spin-stripe order temperature as well as the magnetic fraction at 400 mK show only a modest decrease under stress. Moreover, the onset temperatures of 3D superconductivity and spin-stripe order are very similar in the large stress regime. However, a substantial decrease of the magnetic volume fraction and a full suppression of the low-temperature tetragonal structure is found at elevated temperatures, which is a necessary condition for the development of the 3D superconducting phase with optimal T_c. Our results evidence a remarkable cooperation between the long-range static spin-stripe order and the underlying crystalline order with the three-dimensional fully coherent superconductivity. Overall, these results suggest that the stripe- and the SC order may have a common physical mechanism.

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arXiv2022

Evidence of unconventional pairing in the quasi two-dimensional CuIr$_2$Te$_4$ superconductor

The CuIr$_{2-x}$Ru$_x$Te$_4$ superconductors (with a $T_c$ around 2.8 K) can host charge-density waves, whose onset and interplay with superconductivity are not well known at a microscopic level. Here, we report a comprehensive study of the $x$ = 0 and 0.05 cases, whose superconductivity was characterized via electrical-resistivity-, magnetization-, and heat-capacity measurements, while their microscopic superconducting properties were studied via muon-spin rotation and relaxation ($μ$SR). In CuIr$_{2-x}$Ru$_x$Te$_4$, both the temperature-dependent electronic specific heat and the superfluid density (determined via transverse-field $μ$SR) are best described by a two-gap (s+d)-wave model, comprising a nodeless gap and a gap with nodes. The multigap superconductivity is also supported by the temperature dependence of the upper critical field $H_\mathrm{c2}(T)$. However, under applied pressure, a charge-density-wave order starts to develop and, as a consequence, the superconductivity of CuIr$_2$Te$_4$ achieves a more conventional s-wave character. From a series of experiments, we provide ample evidence that the CuIr$_{2-x}$Ru$_x$Te$_4$ family belongs to the rare cases, where an unconventional superconducting pairing is found near a charge-density-wave quantum critical point.

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arXiv2022

Spin excitations in the quantum dipolar magnet Yb(BaBO$_3$)$_3$

We report results of magnetization, specific-heat and muon-spin relaxation measurements on single crystals of disorder-free Yb$^{3+}$ triangular lattice Yb(BaBO$_3$)$_3$. The magnetization experiments show anisotropic magnetic properties with Curie-Weiss temperatures $θ_{\perp}=-1.40$~K ($H \perp c$) and $θ_{\parallel}=-1.16$~K ($H \parallel c$) determined from low temperature data. The absence of both long-range antiferromagnetic order and spin freezing is confirmed down to 0.27 K at zero field. A two-level Schottky anomaly due to the opening of the ground-state Kramers doublet is observed from the low-temperature specific-heat measurements when the applied magnetic fields $μ_0H >0.7$~T. At zero field, the increase of both $C_{\rm mag}/T$ and the muon spin relaxation rate $λ$ below 1~K is due to the electronic spin excitations, which often exist in quantum magnets where dipole-dipole interaction creates an anisotropy of magnetic properties. The spin excitation is also supported by the unusual maximum of field dependence of $λ$ due to the field-induced increase of the density of excitations. We argue that dipolar interaction is dominant and induces the spin dynamics in the quantum magnet Yb(BaBO$_3$)$_3$.

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arXiv2022

Fully-gapped superconducting state in interstitial-carbon-doped Zr5Pt3

We report a comprehensive study of the Zr$_5$Pt$_3$C$_x$ superconductors, with interstitial carbon comprised between 0 and 0.3. At a macroscopic level, their superconductivity, with $T_c$ ranging from 4.5 to 6.3 K, was investigated via electrical-resistivity-, magnetic-susceptibility-, and specific-heat measurements. The upper critical fields $μ_0H_\mathrm{c2}$ $\sim$ 7 T were determined mostly from measurements of the electrical resistivity in applied magnetic fields. The microscopic electronic properties were investigated by means of muon-spin rotation and 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 electronic density of states at the Fermi level, dominated by the Zr 4$d$ orbitals. The low-temperature superfluid density, obtained via transverse-field $μ$SR, suggests a fully-gapped superconducting state in Zr$_5$Pt$_3$ and Zr$_5$Pt$_3$C$_{0.3}$, with a zero-temperature gap $Δ_0$ = 1.20 and 0.60 meV and a magnetic penetration depth $λ_0$ = 333 and 493 nm, respectively. The exponential dependence of the NMR relaxation rates below $T_c$ further supports a nodeless superconductivity. The absence of spontaneous magnetic fields below the onset of superconductivity, as determined from zero-field $μ$SR measurements, confirms a preserved time-reversal symmetry in the superconducting state of Zr$_5$Pt$_3$C$_x$. In contrast to a previous study, our $μ$SR and NMR results suggest a conventional superconductivity in the Zr$_5$Pt$_3$C$_x$ family, independent of the C content.

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arXiv2022

Evidence of fully-gapped superconductivity in NbReSi: A combined $μ$SR and NMR study

We report a comprehensive study of the noncentrosymmetric NbReSi superconductor by means of muon-spin rotation and relaxation ($μ$SR) and nuclear magnetic resonance (NMR) techniques. NbReSi is a bulk superconductor with $T_c = 6.5$ K, characterized by a large upper critical field, which exceeds the Pauli limit. Both the superfluid density $ρ_\mathrm{sc}(T)$ (determined via transverse-field $μ$SR) and the spin-lattice relaxation rate $T_1^{-1}(T)$ (determined via NMR) suggest a nodeless superconductivity (SC) in NbReSi. We also find signatures of multigap SC, here evidenced by the field-dependent muon-spin relaxation rate and the electronic specific-heat coefficient. The absence of spontaneous magnetic fields below $T_c$, as evinced from zero-field $μ$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of NbReSi. Finally, we discuss possible reasons for the unusually large upper critical field of NbReSi, most likely arising from its anisotropic crystal structure.

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arXiv2022

Nematic superconductivity in the topological semimetal CaSn$_{3}$

The superconducting behavior of the topological semimetal CaSn$_{3}$ was investigated by means of magnetotransport and muon spectroscopy $μ$SR measurements, both providing strong evidence of nematic behavior. Magnetotransport detects an anisotropic upper critical field, characterized by a twofold symmetry about $C_{4}$ axis, thus breaking the rotational symmetry of the underlying cubic lattice. Transverse-field $μ$SR data support such picture, with the muon depolarization rate depending strongly on the magnetic field direction, here applied along the [110] or [001] crystal directions. In the former case, the absence of any additional muon depolarization suggests an unconventional vortex lattice. In the latter case, a vortex lattice encompassing a sample volume of at least 52% indicates the bulk nature of CaSn$_{3}$ superconductivity. The resulting superfluid density in the (001) planes shows a gapped low-temperature behavior, with a superconducting gap value $Δ(0)\simeq 0.61(7)$ meV. Additional zero-field $μ$SR results indicate that the superconducting state is time-reversal-invariant. This fact and the breaking of rotational symmetry in a fully-gapped superconductor are consistent with an unconventional pairing state in a multi-dimensional representation, thus making CaSn$_{3}$ an important example of nematic superconductor.

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arXiv2022

μSR investigation of the Fe-doped Ca$_{3}$Ru$_{2}$O$_{7}$ polar metal

Ca$_{3}$Ru$_{2}$O$_{7}$ is a polar metal that belongs to the class of multiferroic magnetic materials. Here, tiny amounts of Fe doping in the Ru sites bring about dramatic changes in the electronic and magnetic properties and generate a complex H-T phase diagram. To date, not much is known about the ground state of such a system in the absence of magnetic field. By performing muon-spin spectroscopy ($μ$SR) measurements in 5% Fe-doped Ca$_{3}$Ru$_{2}$O$_{7}$ single crystals, we investigate its electronic properties at a local level. Transverse-field $μ$SR results indicate a very sharp normal-to-antiferromagnetic transition at T$_{N}$ = 79.7(1) K, with a width of only 1 K. Zero-field $μ$SR measurements in the magnetically ordered state allow us to determine the local fields B$_{i}$ at the muon implantation sites. By symmetry, muons stopping close to the RuO$_{2}$ planes detect only the weak nuclear dipolar fields, while those stopping next to apical oxygens sense magnetic fields as high as 150 mT. In remarkable agreement with the nominal Fe-doping, a $\sim$ 6% minority of the these muons feel slightly lower fields, reflecting a local magnetic frustration induced by iron ions. Finally, B$_{i}$ shows no significant changes across the metal-to-insulator transition, close to 40 K. We ascribe this surprising lack of sensitivity to the presence of crystal twinning.

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arXiv2022

$s$-wave superconductivity in the noncentrosymmetric W$_3$Al$_2$C superconductor: An NMR study

We report on a microscopic study of the noncentrosymmetric superconductor W$_3$Al$_2$C (with $T_c$ = 7.6 K), mostly by means of $^{27}$Al- and $^{13}$C nuclear magnetic resonance (NMR). Since in this material the density of states at the Fermi level is dominated by the tungsten's 5$d$ orbitals, we expect a sizeable spin-orbit coupling (SOC) effect. The normal-state electronic properties of W$_3$Al$_2$C resemble those of a standard metal, but with a Korringa product $1/(T_{1}T)$ significantly smaller than that of metallic Al, reflecting the marginal role played by $s$-electrons. In the superconducting state, we observe a reduction of the Knight shift and an exponential decrease of the NMR relaxation rate $1/T_1$, typical of $s$-wave superconductivity. This is further supported by the observation of a small but distinct coherence peak just below $T_c$ in the $^{13}$C NMR relaxation-rate, in agreement with the fully-gapped superconducting state inferred from the electronic specific-heat data well below $T_c$. The above features are compared to those of members of the same family, in particular, Mo$_3$Al$_2$C, often claimed to exhibit unconventional superconductivity. We discuss why, despite the enhanced SOC, W$_3$Al$_2$C does not show spin-triplet features in its superconducting state and consider the broader consequences of our results for noncentrosymmetric superconductors in general.

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arXiv2022

Spin order and fluctuations in the EuAl$_4$ and EuGa$_4$ topological antiferromagnets: A $μ$SR study

We report on systematic muon-spin rotation and relaxation ($μ$SR) studies of the magnetic properties of EuAl$_4$ and EuGa$_4$ single crystals at a microscopic level. Transverse-field $μ$SR measurements, spanning a wide temperature range (from 1.5 to 50 K), show clear bulk AFM transitions, with an almost 100% magnetic volume fraction in both cases. Zero-field $μ$SR measurements, covering both the AFM and the paramagnetic (PM) states, reveal internal magnetic fields $B_\mathrm{int}(0) = 0.33$ T and 0.89 T in EuAl$_4$ and EuGa$_4$, respectively. The transverse muon-spin relaxation rate $λ_\mathrm{T}$, a measure of the internal field distribution at the muon-stopping site, shows a contrasting behavior. In EuGa$_4$, it decreases with lowering the temperature, reaching its minimum at zero temperature, $λ_\mathrm{T}(0) = 0.71$ $μ$s$^{-1}$. In EuAl$_4$, it increases significantly below $T_\mathrm{N}$, to reach 58 $μ$s$^{-1}$ at 1.5 K, most likely reflecting the complex magnetic structure and the competing interactions in the AFM state of EuAl$_4$. In both compounds, the temperature-dependent longitudinal muon-spin relaxation $λ_\mathrm{L}(T)$, an indication of the rate of spin fluctuations, diverges near the onset of AFM order, followed by a significant drop at $T < T_\mathrm{N}$. In the AFM state, spin fluctuations are much stronger in EuAl$_4$ than in EuGa$_4$, while being comparable in the PM state. The evidence of robust spin fluctuations against the external magnetic fields provided by $μ$SR may offer new insights into the origin of the topological Hall effect and the possible magnetic skyrmions in the EuAl$_4$ and EuGa$_4$ compounds.

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arXiv2021

Giant magnetoresistance and topological Hall effect in the EuGa4 antiferromagnet

We report on systematic temperature- and magnetic field-dependent studies of the EuGa$_4$ binary compound, which crystallizes in a centrosymmetric tetragonal BaAl$_4$-type structure with space group $I4/mmm$. The electronic properties of EuGa$_4$ single crystals, with an antiferromagnetic (AFM) transition at $T_\mathrm{N} \sim 16.4$ K, were characterized via electrical resistivity and magnetization measurements. A giant nonsaturating magnetoresistance was observed at low temperatures, reaching $\sim 7 \times 10^4$ % at 2 K in a magnetic field of 9 T. In the AFM state, EuGa$_4$ undergoes a series of metamagnetic transitions in an applied magnetic field, clearly manifested in its field-dependent electrical resistivity. Below $T_\mathrm{N}$, in the $\sim$4-7 T field range, we observe also a clear hump-like anomaly in the Hall resistivity which is part of the anomalous Hall resistivity. We attribute such a hump-like feature to the topological Hall effect, usually occurring in noncentrosymmetric materials known to host topological spin textures (as e.g., magnetic skyrmions). Therefore, the family of materials with a tetragonal BaAl$_4$-type structure, to which EuGa$_4$ and EuAl$_4$ belong, seems to comprise suitable candidates on which one can study the interplay among correlated-electron phenomena (such as charge-density wave or exotic magnetism) with topological spin textures and topologically nontrivial bands.

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arXiv2021

Time-reversal symmetry breaking in Re-based superconductors: Recent developments

In the recent search for unconventional- and topological superconductivity, noncentrosymmetric superconductors (NCSCs) rank among the most promising candidate materials. Surprisingly, some of them -- especially those containing rhenium -- seem to exhibit also time-reversal symmetry (TRS) breaking in their superconducting state, while TRS is preserved in many other isostructural NCSCs. To date, a satisfactory explanation for such discrepant behavior, albeit crucial for understanding the unconventional superconductivity of these materials, is still missing. Here we review the most recent developments regarding the Re-based class, where the muon-spin relaxation ($μ$SR) technique plays a key role due to its high sensitivity to the weak internal fields associated with the TRS breaking phenomenon. We discuss different cases of Re-containing superconductors, comprising both centrosymmetric- and noncentrosymmetric crystal structures and ranging from pure rhenium, to Re$T$ ($T$ = 3$d$-5$d$ early transition metals), to the dilute-Re case of ReBe$_{22}$. $μ$SR results suggest that the rhenium presence and its amount are two key factors for the appearance and the extent of TRS breaking in Re-based superconductors. Besides summarizing the existing findings, we also put forward future research ideas regarding the exciting field of materials showing TRS breaking.

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arXiv2021

Nodeless superconductivity in the centro- and noncentrosymmetric rhenium-boron superconductors

We report a comprehensive study of the centrosymmetric Re$_3$B and noncentrosymmetric Re$_7$B$_3$ superconductors. At a macroscopic level, their bulk superconductivity (SC), with $T_c$ = 5.1 K (Re$_3$B) and 3.3 K (Re$_7$B$_3$), was characterized via electrical-resistivity, magnetization, and heat-capacity measurements, while their microscopic superconducting properties were investigated by means of muon-spin rotation/relaxation ($μ$SR). In both Re$_3$B and Re$_7$B$_3$ the low-$T$ zero-field electronic specific heat and the superfluid density (determined via tranverse-field $μ$SR) suggest a nodeless SC. Both compounds exhibit some features of multigap SC, as evidenced by temperature-dependent upper critical fields $H_\mathrm{c2}(T)$, as well as by electronic band-structure calculations. The absence of spontaneous magnetic fields below the onset of SC, as determined from zero-field $μ$SR measurements, indicates a preserved time-reversal symmetry in the superconducting state of both Re$_3$B and Re$_7$B$_3$. Our results suggest that a lack of inversion symmetry and the accompanying antisymmetric spin-orbit coupling effects are not essential for the occurrence of multigap SC in these rhenium-boron compounds.

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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.

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