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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Unconventional chiral charge order in kagome superconductor KV 3 Sb 5

Intertwining quantum order and non-trivial topology is at the frontier of condensed matter physics. A charge-density-wave-like order with orbital currents has been proposed for achieving the quantum anomalous Hall effect in topological materials and for the hidden phase in cuprate high-temperature superconductors. However, the experimental realization of such an order is challenging. Here we use high-resolution scanning tunnelling microscopy to discover an unconventional chiral charge order in a kagome material, KV 3 Sb 5 , with both a topological band structure and a superconducting ground state. Through both topography and spectroscopic imaging, we observe a robust 2 × 2 superlattice. Spectroscopically, an energy gap opens at the Fermi level, across which the 2 × 2 charge modulation exhibits an intensity reversal in real space, signalling charge ordering. At the impurity-pinning-free region, the strength of intrinsic charge modulations further exhibits chiral anisotropy with unusual magnetic field response. Therefore theoretical analysis of our experiments suggests a tantalizing unconventional chiral charge density wave in the frustrated kagome lattice, which can not only lead to a large anomalous Hall effect with orbital magnetism, but also be a precursor of unconventional superconductivity.

36 MATERIALS SCIENCE↗
DOE OSTI2025

Unconventional gapping behaviour in a kagome superconductor

Determining the types of superconducting order in quantum materials is a challenge. This is especially true when several degrees of freedom contribute to the fermiology and when superconductivity competes with other symmetry-breaking orders. One example is the kagome-lattice superconductor CsV 3 Sb 5 , in which multiband superconductivity coexists with a charge order that substantially reduces the space group symmetries of the compound. Here we demonstrate the presence of two superconducting regimes in CsV 3 Sb 5 that are characterized by distinct transport and thermodynamic properties. Our results reveal a substantial quasiparticle weight in a high-temperature regime. At lower temperatures, this weight is removed through the formation of a second gap. We argue that the band with a gap opening at lower temperatures continues to host low-energy quasiparticles, possibly due to the nodal structure of the gap. Taken together, our results present evidence for band-selective superconductivity with uncoupled superconducting gaps.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1989

Design And Fabrication Of Superconductors

Report reviews selected aspects of technology of superconductors, reflecting progress of recent decades ending in year 1982. Provides fairly comprehensive discussion of design and fabrication of conventionally and unconventionally processed composite superconductors made of alloys based on Ti/Nb and on A15 compounds. Review illustrated with 44 line drawings and 29 photographs and lists 245 references.

Collings, E. W.↗
DOE OSTI2020

Time-reversal invariant and fully gapped unconventional superconducting state in the bulk of the topological compound Nb 0.25 Bi 2 Se 3

Recently, the niobium (Nb) doped topological insulator Bi 2 Se 3 , in which the finite magnetic moments of the Nb atoms are intercalated in the van der Waals gap between the Bi 2 Se 3 layers, has been shown to exhibit both superconductivity with $T_c$ ≃ 3 K and topological surface states. Here we report on muon spin rotation experiments probing the temperature and field dependence of effective magnetic penetration depth $λ_{\text{eff}}(T)$ in the layered topological superconductor candidate Nb 0.25 Bi 2 Se 3 . In this work the exponential temperature dependence of $λ^{–2}_{\text{eff}}(T)$ at low temperatures suggests a fully gapped superconducting state in the bulk with the superconducting transition temperature $T_c$ = 2.9 K and the gap to $T_c$ ratio 2Δ /$k_BT_c$ = 3.95(19). We also reveal that the ratio Tc/λ–2eff is comparable to those of unconventional superconductors, which hints at an unconventional pairing mechanism. Furthermore, time-reversal symmetry breaking was excluded in the superconducting state with sensitive zero-field μSR experiments. We hope the present results will stimulate theoretical investigations to obtain a microscopic understanding of the relation between superconductivity and the topologically nontrivial electronic structure of Nb 0.25 Bi 2 Se 3 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Two-component nematic superconductivity in 4Hb-TaS 2

Most superconductors have an isotropic, single component order parameter and are well described by the standard (BCS) theory for superconductivity. Unconventional, multiple-component superconductors are exceptionally rare and are much less understood. Here, we combine scanning tunneling microscopy and angle-resolved macroscopic transport for studying the candidate chiral superconductor, 4Hb-TaS 2 . We reveal quasi-periodic one-dimensional modulations in the tunneling conductance accompanied by two-fold symmetric superconducting critical field. The strong modulation of the in-plane critical field, H c2 , points to a nematic, unconventional order parameter. However, the imaged vortex core is isotropic at low temperatures. We suggest a model that reconciles this apparent discrepancy and takes into account previously observed spontaneous time-reversal symmetry breaking at low temperatures. The model describes a competition between a dominating chiral superconducting order parameter and a nematic one. The latter emerges close to the normal phase. Our results strongly support the existence of two-component superconductivity in 4Hb-TaS 2 and can provide valuable insights into other systems with coexistent charge order and superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Absence of spin susceptibility decrease in a bulk organic superconductor with triangular lattice

The study of non-s-wave unconventional superconductivities in strongly correlated electron systems has been a central issue in condensed matter physics for more than 30 years. In such unconventional superconductivities, d-wave Cooper pairing with antiparallel spins has been often observed in various quasi-two-dimensional (quasi-2D) bulk systems. Interestingly, many theories predicted that the triangular lattice causes the d-wave pairing to be unstable and may lead to more exotic pairing such as parallel spin (spin-triplet) pairing. Here we focus on a bulk organic triangular-lattice system in which superconductivity emerges near a nonmagnetic Mott insulating phase. We demonstrate, by using low-power nuclear magnetic resonance (NMR) measurements, that the spin susceptibility of the superconducting state retains the normal state value even deep in the superconducting state. This result indicates the possibility that the material exhibits spin-triplet superconductivity. Our finding will bring insights also into understanding the 2D materials with triangular moiré superlattices that are considered also to show unconventional superconductivities near Mott-like insulating states.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Analysis of the ghost and mirror fields in the Nernst signal induced by superconducting fluctuations

In this work, we present a complete analysis of the Nernst signal due to superconducting fluctuations in a large variety of superconductors from conventional to unconventional ones. A closed analytical expression of the fluctuation contribution to the Nernst signal is obtained in a large range of temperature and magnetic field. We apply this expression directly to experimental measurements of the Nernst signal in Nb x Si 1-x thin films and URu 2 Si 2 superconductors. Both magnetic field and temperature dependence of the available data are fitted with very good accuracy using only two fitting parameters, the superconducting temperature T c0 and the upper critical field H c2 (0). The obtained values agree very well with experimentally obtained values. We also extract the ghost lines (the maximum of the Nernst signal for constant temperature or magnetic field) from the complete expression and also compare it to several experimentally obtained curves. Our approach predicts a linear temperature dependence for the ghost critical field well above T c0 . Within the errors of the experimental data, this linearity is indeed observed in many superconductors far from T c0 .

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Muon spin rotation and relaxation in Pr 1 - x Nd x Os 4 Sb 12 : Superconductivity and magnetism in Pr-rich alloys

The Pr-rich end 0 ≤ x ≤ 0.25 of the alloy series Pr 1-x Nd x ⁢Os 4 ⁢Sb 12 has been studied using muon spin rotation and relaxation. The end compound PrOs 4 ⁢Sb 12 is an unconventional heavy-fermion superconductor, which exhibits a spontaneous magnetic field associated with broken time-reversal symmetry (TRS) in the superconducting phase. Further, no such field is observed in the Nd-doped alloys for x ≥ 0.05, indicating that TRS has been restored. The superfluid density from the vortex-lattice field distribution is insensitive to Nd concentration for x ≲ 0.2, indicating that TRS restoration does not have a strong effect on the superconducting state. No Nd 3+ static magnetism, ordered or disordered, is found down to the lowest temperatures of measurement.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2021

Charge density waves in cuprate superconductors beyond the critical doping

The unconventional normal-state properties of the cuprates are often discussed in terms of emergent electronic order that onsets below a putative critical doping of x c ≈ 0.19. Charge density wave (CDW) correlations represent one such order; however, experimental evidence for such order generally spans a limited range of doping that falls short of the critical value x c , leading to questions regarding its essential relevance. Here, we use X-ray diffraction to demonstrate that CDW correlations in La 2–x Sr x CuO 4 persist up to a doping of at least x = 0.21. The correlations show strong changes through the superconducting transition, but no obvious discontinuity through x c ≈ 0.19, despite changes in Fermi surface topology and electronic transport at this doping. These results demonstrate the interaction between CDWs and superconductivity even in overdoped cuprates and prompt a reconsideration of the role of CDW correlations in the high-temperature cuprate phase diagram.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Possible unconventional pairing in ( Ca , Sr ) 3 ( Ir , Rh ) 4 Sn 13 superconductors revealed by controlling disorder

Here we study the evolution of temperature-dependent resistivity with added pointlike disorder induced by 2.5 MeV electron irradiation in stoichiometric compositions of the “3-4-13” stannides, (Ca,Sr) 3 (Ir,Rh) 4 Sn 13 . Three of these cubic compounds exhibit a proposed microscopic coexistence of charge density wave (CDW) order and superconductivity (SC), while Ca 3 Rh 4 Sn 13 does not develop CDW order. As expected, the CDW transition temperature T CDW is universally suppressed by irradiation in all three compositions. The superconducting transition temperature, T c , behaves in a more complex manner. In Sr 3 Rh 4 Sn 13 , it increases initially in a way consistent with a direct competition of CDW and SC, but quickly saturates at higher irradiation doses. In the other three compounds, T c is monotonically suppressed by irradiation. The strongest suppression is found in Ca 3 Rh 4 Sn 13 , which does not have CDW order. We further examine this composition by measuring the London penetration depth λ(T), from which we derive the superfluid density. The result unambiguously points to a weak-coupling, full single gap, isotropic superconducting state. Therefore we must explain two seemingly incompatible experimental observations: a single isotropic superconducting gap and a significant suppression of T c by nonmagnetic disorder. We conduct a quantitative theoretical analysis based on a generalized Anderson theorem which points to an unconventional multiband s +– -pairing state where the sign of the order parameter is different on one (or a small subset) of the smaller Fermi surface sheets but remains isotropic and overall fully gapped.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Unconventional spectral signature of $T_c$ in a pure d -wave superconductor

We report in conventional superconductors, the phase transition into a zero-resistance and perfectly diamagnetic state is accompanied by a jump in the specific heat and the opening of a spectral gap. In the high-transition-temperature (high-T c ) cuprates, although the transport, magnetic and thermodynamic signatures of T c have been known since the 1980s, the spectroscopic singularity associated with the transition remains unknown. Here we resolve this long-standing puzzle with a high-precision angle-resolved photoemission spectroscopy (ARPES) study on overdoped (Bi,Pb) 2 Sr 2 CaCu 2 O 8+δ (Bi2212). We first probe the momentum-resolved electronic specific heat via spectroscopy and reproduce the specific heat peak at T c , completing the missing link for a holistic description of superconductivity. Then, by studying the full momentum, energy and temperature evolution of the spectra, we reveal that this thermodynamic anomaly arises from the singular growth of in-gap spectral intensity across T c . Furthermore, we observe that the temperature evolution of in-gap intensity is highly anisotropic in the momentum space, and the gap itself obeys both the d-wave functional form and particle–hole symmetry. These findings support the scenario that the superconducting transition is driven by phase fluctuations. They also serve as an anchor point for understanding the Fermi arc and pseudogap phenomena in underdoped cuprates.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2020

High-temperature short-range order in Mn 3 RhSi

Conventional phase transitions are well understood in terms of the order parameter, based on the Landau–Ginzburg–Wilson theory. However, unconventional magnetic orders have been observed in clean systems such as MnSi. The unconventional magnetic orders of conduction electrons in the metallic phase has been observed for high-temperature superconductors and heavy fermion compounds. However, these unconventional magnetic orders have been limited to relatively low temperatures as quantum phase transitions. Here high-temperature magnetic short-range order is observed as one of the unconventional magnetic orders at temperatures up to 720 K in a noncentrosymmetric intermetallic antiferromagnet Mn 3 RhSi with a well-ordered lattice. The magnetic Mn ions form a hyperkagome network of corner-sharing triangles, where the spins are geometrically frustrated. The spin network is equivalent to that of a spin liquid and non-Fermi-liquid material, β-Mn. Our observation indicates that a metallic phase with magnetic short-range order exists at high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Evidence for chiral superconductivity on a silicon surface

Tin adatoms on a Si(111) substrate with a one-third monolayer coverage form a two-dimensional triangular lattice with one unpaired electron per site. These electrons order into an antiferromagnetic Mott-insulating state, but doping the Sn layer with holes creates a two-dimensional conductor that becomes superconducting at low temperatures. Although the pairing symmetry of the superconducting state is currently unknown, the combination of repulsive interactions and frustration inherent in the triangular adatom lattice opens up the possibility of a chiral order parameter. Here we study the superconducting state of Sn/Si(111) using scanning tunneling microscopy, scanning tunneling spectroscopy and quasiparticle interference imaging. Here we find evidence for a doping-dependent superconducting critical temperature with a fully gapped order parameter, the presence of time-reversal symmetry breaking and a strong enhancement in zero-bias conductance near the edges of the superconducting domains. Although each individual piece of evidence could have a more mundane interpretation, our combined results suggest the possibility that Sn/Si(111) is an unconventional chiral d-wave superconductor.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Nodal superconductivity in miassite Rh 17 S 15

Solid state chemistry has produced a plethora of materials with properties not found in nature. For example, high-temperature superconductivity in cuprates is drastically different from the superconductivity of naturally occurring metals and alloys and is frequently referred to as unconventional. Unconventional superconductivity is also found in other synthetic compounds, such as iron-based and heavy-fermion superconductors. Here, we report compelling evidence of unconventional nodal superconductivity in synthetic samples of Rh 17 S 15 (T c = 5.4 K), which is also found in nature as the mineral miassite. We investigated the temperature-dependent variation of the London penetration depth Δλ(T) and the disorder evolution of the critical superconducting temperature T c and the upper critical field H c2 (T) in single crystalline Rh 17 S 15 . We found a T - linear temperature variation of Δλ(T) below 0.3T c , which is consistent with the presence of nodal lines in the superconducting gap of Rh 17 S 15 . The nodal character of the superconducting state is supported by the observed suppression of T c and H c2 (T) in samples with a controlled level of non-magnetic disorder introduced by 2.5 MeV electron irradiation. We propose a nodal sign-changing superconducting gap in the A 1g irreducible representation, which preserves the cubic symmetry of the crystal and is in excellent agreement with the derived superfluid density. To the best of our knowledge, this establishes miassite as the only mineral known so far that reveals unconventional superconductivity in its clean synthetic form, though it is unlikely that it is present in natural crystals because of unavoidable impurities that quickly destroy nodal superconductivity.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Time-reversal symmetry-breaking charge order in a kagome superconductor

The kagome lattice 1 , which is the most prominent structural motif in quantum physics, benefits from inherent non-trivial geometry so that it can host diverse quantum phases, ranging from spin-liquid phases, to topological matter, to intertwined orders 2,3,4,5,6,7,8 and, most rarely, to unconventional superconductivity 6,9 . Recently, charge sensitive probes have indicated that the kagome superconductors AV 3 Sb 5 (A = K, Rb, Cs) 9,10,11 exhibit unconventional chiral charge order 12,13,14,15,16,17,18,19, which is analogous to the long-sought-after quantum order in the Haldane model 20 or Varma model 21 . However, direct evidence for the time-reversal symmetry breaking of the charge order remains elusive. Here we use muon spin relaxation to probe the kagome charge order and superconductivity in KV 3 Sb 5 . We observe a noticeable enhancement of the internal field width sensed by the muon ensemble, which takes place just below the charge ordering temperature and persists into the superconducting state. Notably, the muon spin relaxation rate below the charge ordering temperature is substantially enhanced by applying an external magnetic field. We further show the multigap nature of superconductivity in KV 3 Sb 5 and that the T c /λ$^{-2}_{ab}$ ratio (where T c is the superconducting transition temperature and λ ab is the magnetic penetration depth in the kagome plane) is comparable to those of unconventional high-temperature superconductors. Our results point to time-reversal symmetry-breaking charge order intertwining with unconventional superconductivity in the correlated kagome lattice.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Ubiquitous enhancement of nematic fluctuations across the phase diagram of iron based superconductors probed by the Nernst effect

The role of nematic fluctuations for unconventional superconductivity has been the subject of intense discussions for many years. In iron-based superconductors, the most established probe for electronic-nematic fluctuations, i.e. the elastoresistivity seems to imply that superconductivity is reinforced by electronic-nematic fluctuations, since the elastoresistivity amplitude peaks at or close to optimal T c . However, on the over-doped side of the superconducting dome, the diminishing elastoresistivity suggests a negligible importance in the mechanism of superconductivity. Here we introduce the Nernst coefficient as a genuine probe for electronic nematic fluctuations, and we show that the amplitude of the Nernst coefficient tracks the superconducting dome of two prototype families of iron-based superconductors, namely Rh-doped BaFe 2 As 2 and Co-doped LaFeAsO. Our data thus provide fresh evidence that in these systems, nematic fluctuations foster the superconductivity throughout the phase diagram.

36 MATERIALS SCIENCE↗