arXiv subjects

M. Smidman

Publications and source records attributed to M. Smidman.

At least 37 records · Page 2Linked to original sources

arXiv2019

CaPtAs: a new noncentrosymmetric superconductor

We report the discovery of a new noncentrosymmetric superconductor CaPtAs. It crystallizes in a tetragonal structure (space group $I4_1md$, No.109), featuring three dimensional honeycomb networks of Pt-As and a much elongated $c$-axis ($a = b = 4.18 $ Å, and $c = 43.70 $ Å). The superconductivity of CaPtAs with $T_c$ = 1.47 K was characterized by means of electrical resistivity, specific heat, and ac magnetic susceptibility. The electronic specific heat $C_\mathrm{e}(T)/T$ shows evidence for a deviation from the behavior of a conventional BCS superconductor, and can be reasonably fitted by a $p$-wave model. The upper critical field $μ_0H_{c2}$ of CaPtAs exhibits a relatively large anisotropy, with an in-plane value of around 204 mT and an out-of-plane value of 148 mT. Density functional theory calculations indicate that the Pt-5$d$ and As-4$p$ orbitals mainly contribute to the density of states near the Fermi level, showing that the Pt-As honeycomb networks may significantly influence the superconducting properties.

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arXiv2019

Magnetic order in Nd$_2$PdSi$_3$ investigated using neutron scattering and muon spin relaxation

The rare-earth based ternary intermetallic compounds $R_2TX_3$ ($R$ = rare-earth, $T$ = transition-metal, $X$ = Si, Ge, Ga, In) have attracted considerable interest due to a wide range of interesting low temperature properties. Here we investigate the magnetic state of Nd$_{2}$PdSi$_{3}$ using neutron diffraction, muon spin relaxation ($μ$SR) and inelastic neutron scattering (INS). This compound appears anomalous among the $R_{2}$PdSi$_{3}$ series, since it was proposed to order ferromagnetically, whereas others in this series are antiferromagnets. Although some members of the $R_2TX_3$ series have been reported to form ordered superstructures, our data are well described by Nd$_{2}$PdSi$_{3}$ adopting the AlB$_2$-type structure with a single Nd site, and we do not find evidence for superlattice peaks in neutron diffraction. Our results confirm the onset of long range magnetic order below $T_0=17$~K, where the whole sample enters the ordered state. Neutron diffraction measurements establish the presence of a ferromagnetic component in this compound, as well as an antiferromagnetic one which has a propagation vector $\mathbf{k_2}=(1/2,1/2,1/4-δ)$ with a temperature dependent $δ\approx0.02-0.04$, and moments orientated exclusively along the $c$-axis. $μ$SR measurements suggest that these components coexist on a microscopic level, and therefore the magnetic structure of Nd$_{2}$PdSi$_{3}$ is predominantly ferromagnetic, with a sinusoidally modulated antiferromagnetic contribution which reaches a maximum amplitude at 11~K, and becomes smaller upon further decreasing the temperature. INS results show the presence of crystalline-electric field (CEF) excitations above $T_0$, and from our analysis we propose a CEF level scheme.

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arXiv2019

Magnetic field induced antiferromagnetic tricritical points in Ce$_2 $Sb and Ce$_2 $Bi

We present a detailed investigation of the physical properties of Ce$_2 $Sb and Ce$_2 $Bi single crystals, which undergo antiferromagnetic transitions at around 8.2 and 10 K respectively. When magnetic fields are applied parallel to the $c$ axis, metamagnetic transitions are observed at low temperatures, corresponding to a magnetic field-induced phase transition. It is found that the field-induced transition changes from second-order at higher temperatures, to first-order at low temperatures, suggesting the existence of tricritical points (TCPs) in both compounds. Since replacing Bi with Sb suppresses the TCP to lower temperatures and corresponds to a positive chemical pressure, these results suggest that applying pressure to Ce$_2$Sb may suppress the TCP to lower temperatures, potentially to zero temperature at a quantum tricritical point.

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arXiv2019

Enhancement of the effective mass at high magnetic fields in CeRhIn$_5$

The Kondo-lattice compound CeRhIn$_5$ displays a field-induced Fermi surface reconstruction at $B^*\approx30$ T, which occurs within the antiferromagnetic state, prior to the quantum critical point at $B_{c0}\approx50$ T. Here, in order to investigate the nature of the Fermi surface change, we measured the magnetostriction, specific heat, and magnetic torque of CeRhIn$_5$ across a wide range of magnetic fields. Our observations uncover the field-induced itineracy of the $4f$ electrons, where above $B_{\rm onset}\approx17$ T there is a significant enhancement of the Sommerfeld coefficient, and spin-dependent effective cyclotron masses determined from quantum oscillations. Upon crossing $B_{\rm onset}$, the temperature dependence of the specific heat also shows distinctly different behavior from that at low fields. Our results indicate that the Kondo coupling is remarkably robust upon increasing the magnetic field. This is ascribed to the delocalization of the $4f$ electrons at the Fermi surface reconstruction at $B^*$.

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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.

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arXiv2018

A brief review on muSR studies of unconventional Fe and Cr-based superconductors

Muon spin relaxation/rotation (muSR) is a vital technique for probing the superconducting gap structure, pairing symmetry and time reversal symmetry breaking, enabling an understanding of the mechanisms behind the unconventional superconductivity of cuprates and Fe-based high-temperature superconductors, which remain a puzzle. Very recently double layered Fe-based super-conductors having quasi-2D crystal structures and Cr-based superconductors with a quasi-1D structure have drawn considerable attention. Here we present a brief review of the characteristics of a few selected Fe- and Cr-based superconducting materials and highlight some of the major outstanding problems, with an emphasis on the superconducting pairing symmetries of these materials. We focus on muSR studies of the newly discovered superconductors ACa2Fe4As4F2(A = K, Rb, and Cs), ThFeAsN, and A2Cr3As3(A = K, Cs), which were used to determine the superconducting gap structures, the presence of spin fluctuations, and to search for time reversal symmetry breaking in the superconducting states. We also briefly discuss the results of muSR investigations of the superconductivity in hole and electron doped BaFe2As2.

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arXiv2018

Evidence for Weyl fermions in a canonical heavy-fermion semimetal YbPtBi

The manifestation of Weyl fermions in strongly correlated electron systems is of particular interest. We report evidence for Weyl fermions in the heavy fermion semimetal YbPtBi from electronic structure calculations, angle-resolved photoemission spectroscopy, magnetotransport and calorimetric measurements. At elevated temperatures where $4f$-electrons are localized, there are triply degenerate points, yielding Weyl nodes in applied magnetic fields. These are revealed by a contribution from the chiral anomaly in the magnetotransport, which at low temperatures becomes negligible due to the influence of electronic correlations. Instead, Weyl fermions are inferred from the topological Hall effect, which provides evidence for a Berry curvature, and a cubic temperature dependence of the specific heat, as expected from the linear dispersion near the Weyl nodes. The results suggest that YbPtBi is a Weyl heavy fermion semimetal, where the Kondo interaction renormalizes the bands hosting Weyl points. These findings open up an opportunity to explore the interplay between topology and strong electronic correlations.

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arXiv2018

Interplay between unconventional superconductivity and heavy-fermion quantum criticality: CeCu$_2$Si$_2$ versus YbRh$_2$Si$_2$

In this paper the low-temperature properties of two isostructural canonical heavy-fermion compounds are contrasted with regards to the interplay between antiferromagnetic (AF) quantum criticality and superconductivity. For CeCu$_2$Si$_2$, fully-gapped d-wave superconductivity forms in the vicinity of an itinerant three-dimensional heavy-fermion spin-density-wave (SDW) quantum critical point (QCP). Inelastic neutron scattering results highlight that both quantum critical SDW fluctuations as well as Mott-type fluctuations of local magnetic moments contribute to the formation of Cooper pairs in CeCu$_2$Si$_2$. In YbRh$_2$Si$_2$, superconductivity appears to be suppressed at $T\gtrsim~10$ mK by AF order ($T_N$ = 70 mK). Ultra-low temperature measurements reveal a hybrid order between nuclear and 4f-electronic spins, which is dominated by the Yb-derived nuclear spins, to develop at $T_A$ slightly above 2 mK. The hybrid order turns out to strongly compete with the primary 4f-electronic order and to push the material towards its QCP. Apparently, this paves the way for heavy-fermion superconductivity to form at $T_c$ = 2 mK. Like the pressure - induced QCP in CeRhIn$_5$, the magnetic field - induced one in YbRh$_2$Si$_2$ is of the local Kondo-destroying variety which corresponds to a Mott-type transition at zero temperature. Therefore, these materials form the link between the large family of about fifty low-$T$ unconventional heavy - fermion superconductors and other families of unconventional superconductors with higher $T_c$s, notably the doped Mott insulators of the cuprates, organic charge-transfer salts and some of the Fe-based superconductors. Our study suggests that heavy-fermion superconductivity near an AF QCP is a robust phenomenon.

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arXiv2018

Evidence for triplet superconductivity near an antiferromagnetic instability in CrAs

Superconductivity was recently observed in CrAs as the helimagnetic order is suppressed by applying pressure, suggesting possible unconventional superconductivity. To reveal the nature of the superconducting order parameter of CrAs, here we report the angular dependence of the upper critical field under pressure. Upon rotating the field by 360 degrees in the $bc$-plane, six maxima are observed in the upper critical field, where the oscillations have both six-fold and two-fold symmetric components. Our analysis suggests the presence of an unconventional odd-parity spin triplet state.

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arXiv2018

Fully-gapped superconductivity in single crystals of noncentrosymmetric Re$_6$Zr with broken time-reversal symmetry

The noncentrosymmetric superconductor Re$_6$Zr has attracted much interest due to the observation of broken time-reversal symmetry in the superconducting state. Here we report an investigation of the superconducting gap structure of Re$_6$Zr single crystals by measuring the magnetic penetration depth shift $Δλ(T)$ and electronic specific heat $C_e(T)$. $Δλ(T)$ exhibits an exponential temperature dependence behavior for $T~\ll~T_c$, which indicates a fully-open superconducting gap. Our analysis shows that a single gap $s$-wave model is sufficient to describe both the superfluid density $ρ_s(T)$ and $C_e(T)$ results, with a fitted gap magnitude larger than the weak coupling BCS value, providing evidence for fully-gapped superconductivity in Re$_6$Zr with moderate coupling.

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arXiv2018

Fully gapped $d$-wave superconductivity in CeCu$_2$Si$_2$

The nature of the pairing symmetry of the first heavy fermion superconductor CeCu$_2$Si$_2$ has recently become the subject of controversy. While CeCu$_2$Si$_2$ was generally believed to be a $d$-wave superconductor, recent low temperature specific heat measurements showed evidence for fully gapped superconductivity, contrary to the nodal behavior inferred from earlier results. Here we report London penetration depth measurements, which also reveal fully gapped behavior at very low temperatures. To explain these seemingly conflicting results, we propose a fully gapped $d+d$ band-mixing pairing state for CeCu$_2$Si$_2$, which yields very good fits to both the superfluid density and specific heat, as well as accounting for a sign change of the superconducting order parameter, as previously concluded from inelastic neutron scattering results.

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arXiv2018

Multigap superconductivity in RbCa2Fe4As4F2 investigated using muSR measurements

The superconducting properties of the recently discovered double Fe$_2$As$_2$ layered high-$T_c$ superconductor RbCa$_2$Fe$_4$As$_4$F$_2$ with $T_c\approx$ 30~K have been investigated using magnetization, heat capacity, transverse-field (TF) and zero-field (ZF) muon-spin rotation/relaxation ($μ$SR) measurements. Our low field magnetization measurements and heat capacity (C$_p$) reveal an onset of bulk superconductivity with $T_{\bf c}\sim$ 30.0(4) K. Furthermore, the heat capacity exhibits a jump at $T_{\bf c}$ of $Δ$C$_p$/$T_{\bf c}$=94.6 (mJ/mole-K$^2$) and no clear effect of applied magnetic fields was observed on C$_p$(T) up to 9 T between 2 K and 5 K. Our analysis of the TF-$μ$SR results shows that the temperature dependence of the magnetic penetration depth is better described by a two-gap model, either isotropic $s$+$s$-wave or $s$+$d$-wave than a single gap isotropic $s$-wave or $d$-wave model for the superconducting gap. The presence of two superconducting gaps in RbCa$_2$Fe$_4$As$_4$F$_2$ suggests a multiband nature of the superconductivity, which is consistent with the multigap superconductivity observed in other Fe-based superconductors, including ACa$_2$Fe$_4$As$_4$F$_2$ (A=K and Cs). Furthermore, from our TF-$μ$SR study we have estimated an in-plane penetration depth $λ_{\mathrm{ab}}$$(0)$ =231.5(3) nm, superconducting carrier density $n_s = 7.45 \times 10^{26}~ $m$^{-3}$, and carrier's effective-mass $m^*$ = 2.45\textit{m}$_{e}$. Our ZF $μ$SR measurements do not reveal a clear sign of time reversal symmetry breaking at $T_{\bf c}$, but the temperature dependent relaxation between 150 K and 1.2 K might indicate the presence of spin-fluctuations. The results of our present study have been compared with those reported for other Fe pnictide superconductors.

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arXiv2018

Nodal multigap superconductivity in KCa$_2$Fe$_4$As$_4$F$_2$

We find evidence that the newly discovered Fe-based superconductor KCa$_2$Fe$_4$As$_4$F$_2$ ($T_c~=~33.36(7)$~K) displays multigap superconductivity with line nodes. Transverse field muon spin rotation ($μ$SR) measurements show that the temperature dependence of the superfluid density does not have the expected behavior of a fully-gapped superconductor, due to the lack of saturation at low temperatures. Moreover, the data cannot be well fitted using either single band models or a multiband $s$-wave model, yet are well described by two-gap models with line nodes on either one or both of the gaps. Meanwhile the zero-field $μ$SR results indicate a lack of time reversal symmetry breaking in the superconducting state, but suggest the presence of magnetic fluctuations. These results demonstrate a different route for realizing nodal superconductivity in iron-based superconductors. Here the gap structure is drastically altered upon replacing one of the spacer layers, indicating the need to understand how the pairing state is tuned by changes of the asymmetry between the pnictogens located either side of the Fe planes.

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arXiv2017

Evidence for nodal superconductivity in a layered compound Ta$_4$Pd$_3$Te$_{16}$

We report an investigation of the London penetration depth $Δλ(T)$ on single crystals of the layered superconductor Ta$_4$Pd$_3$Te$_{16}$, where the crystal structure has quasi-one-dimensional characteristics. A linear temperature dependence of $Δλ(T)$ is observed for $T\ll T_c$, in contrast to the exponential decay of fully gapped superconductors. This indicates the existence of line nodes in the superconducting energy gap. A detailed analysis shows that the normalized superfluid density $ρ_s(T)$, which is converted from $Δλ(T)$, can be well described by a multigap scenario, with nodes in one of the superconducting gaps, providing clear evidence for nodal superconductivity in Ta$_4$Pd$_3$Te$_{16}$.

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arXiv2017

Possible Weyl fermions in the magnetic Kondo system CeSb

Materials where the electronic bands have unusual topologies allow for the realization of novel physics and have a wide range of potential applications. When two electronic bands with linear dispersions intersect at a point, the excitations could be described as Weyl fermions which are massless particles with a particular chirality. Here we report evidence for the presence of Weyl fermions in the ferromagnetic state of the low-carrier density, strongly correlated Kondo lattice system CeSb, from electronic structure calculations and angle-dependent magnetoresistance measurements. When the applied magnetic field is parallel to the electric current, a pronounced negative magnetoresistance is observed within the ferromagnetic state, which is destroyed upon slightly rotating the field away. These results give evidence for CeSb belonging to a new class of Kondo lattice materials with Weyl fermions in the ferromagnetic state.

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arXiv2017

Extremely large magnetoresistance and Fermi surface topology of PrSb

We report magnetotransport measurements of PrSb in high magnetic fields. Our results show that PrSb exhibits extremely large magnetoresistance(XMR) at low temperatures. Meanwhile angle-dependent magnetoresistance measurements were used to probe the Fermi surface via Shubnikov-de Haas (SdH) oscillations. The angular dependence of the frequencies of the $α$-branch indicate a two-dimensional character for this Fermi surface sheet, while the effective mass of this branch as a function of angle shows a four-fold signature. The evolution of the Fermi surface with field was also studied up to 32~T. An enlargement of the Fermi surface up to 14~T is observed, before the oscillation frequencies become constant at higher fields. Meanwhile our analysis of the residual Landau index from the high field data reveals a zero Berry phase and therefore trivial topology of the Fermi surface.

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arXiv2017

Probing the superconducting gap structure of (Li$_{1-x}$Fe$_x$)OHFeSe

We report measurements of the London penetration depth [$Δλ(T)$] of the recently discovered iron-based superconductor (Li$_{1-x}$Fe$_x$)OHFeSe, in order to characterize the nature of the superconducting gap structure. At low temperatures, $Δλ(T)$ displays nearly temperature independent behavior, indicating a fully open superconducting gap. We also analyze the superfluid density $ρ_s(T)$ which cannot be well accounted for by a single-gap isotropic $s$-wave model but are consistent with either two-gaps, a model for the orbital selective $s\timesτ_3$ state or anisotropic $s$-wave superconductivity.

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arXiv2017

Nodeless superconductivity and the peak effect in the quasi-skutterudites $\mathrm{Lu}_3\mathrm{Os}_4\mathrm{Ge}_{13}$ and $\mathrm{Y}_3\mathrm{Ru}_4\mathrm{Ge}_{13}$

We report an investigation of the superconducting states of $\mathrm{Lu}_3\mathrm{Os}_4\mathrm{Ge}_{13}$ and $\mathrm{Y}_3\mathrm{Ru}_4\mathrm{Ge}_{13}$ single crystals by measurements of the electrical resistivity, ac susceptibility and London penetration depth. The analysis of the penetration depth and the derived superfluid density indicates the presence of nodeless superconductivity and suggest that there are multiple superconducting gaps in both materials. Furthermore, ac susceptibility measurements of both compounds display the peak effect in the low temperature region of the $H-T$ phase diagram. This anomalous increase of the critical current with field gives an indication of a change of the arrangement of flux lines in the mixed state, as found in some of the isostructural stannide materials.

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