Evidence For Unconventional Superconductivity Research Articles

Evidence of unconventional superconductivity on the surface of the nodal semimetal CaAg1−xPdxP

Surface states of topological materials provide extreme electronic states for unconventional superconducting states. CaAg1−xPdxP is an ideal candidate for a nodal-line Dirac semimetal with drumhead surface states and no additional bulk bands. Here, we report that CaAg1−xPdxP has surface states that exhibit unconventional superconductivity (SC) around 1.5 K. Extremely sharp magnetoresistance, tuned by surface-sensitive gating, determines the surface origin of the ultrahigh-mobility “electrons.” The Pd-doping elevates the Fermi level towards the surface states, and as a result, the critical temperature (Tc) is increased up to 1.7 K from 1.2 K for undoped CaAgP. Furthermore, a soft point-contact study at the surface of Pd-doped CaAgP proved the emergence of unconventional SC on the surface. We observed the bell-shaped conductance spectra, a hallmark of the unconventional SC. Ultrahigh mobility carriers derived from the surface flat bands generate a new class of unconventional SC.

Evidence for unconventional superconductivity and nontrivial topology in PdTe

PdTe is a superconductor with Tc ~ 4.25 K. Recently, evidence for bulk-nodal and surface-nodeless gap features has been reported in PdTe. Here, we investigate the physical properties of PdTe in both the normal and superconducting states via specific heat and magnetic torque measurements and first-principles calculations. Below Tc, the electronic specific heat initially decreases in T3 behavior (1.5 K < T < Tc) then exponentially decays. Using the two-band model, the superconducting specific heat can be well described with two energy gaps: one is 0.372 meV and another 1.93 meV. The calculated bulk band structure consists of two electron bands (α and β) and two hole bands (γ and η) at the Fermi level. Experimental detection of the de Haas-van Alphen (dHvA) oscillations allows us to identify four frequencies (Fα = 65 T, Fβ = 658 T, Fγ = 1154 T, and Fη = 1867 T for H // a), consistent with theoretical predictions. Nontrivial α and β bands are further identified via both calculations and the angle dependence of the dHvA oscillations. Our results suggest that PdTe is a candidate for unconventional superconductivity.

Evidence for unconventional superconductivity in twisted trilayer graphene.

Magic-angle twisted trilayer graphene (MATTG) has emerged as a moiré material that exhibits strong electronic correlations and unconventional superconductivity1,2. However, local spectroscopic studies of this system are still lacking. Here we perform high-resolution scanning tunnelling microscopy and spectroscopy of MATTG that reveal extensive regions of atomic reconstruction favouring mirror-symmetric stacking. In these regions, we observe symmetry-breaking electronic transitions and doping-dependent band-structure deformations similar to those in magic-angle bilayers, as expected theoretically given the commonality of flat bands3,4. Most notably in a density window spanning two to three holes per moiré unit cell, the spectroscopic signatures of superconductivity are manifest as pronounced dips in the tunnelling conductance at the Fermi level accompanied by coherence peaks that become gradually suppressed at elevated temperatures and magnetic fields. The observed evolution of the conductance with doping is consistent with a gate-tunable transition from a gapped superconductor to a nodal superconductor, which is theoretically compatible with a sharp transition from a Bardeen-Cooper-Schrieffer superconductor to a Bose-Einstein-condensation superconductor with a nodal order parameter. Within this doping window, we also detect peak-dip-hump structures that suggest that superconductivity is driven by strong coupling to bosonic modes of MATTG. Our results will enable further understanding of superconductivity and correlated states in graphene-based moiré structures beyond twisted bilayers5.

Evidence for unconventional superconductivity in twisted bilayer graphene.

The emergence of superconductivity and correlated insulators in magic-angle twisted bilayer graphene (MATBG) has raised the intriguing possibility that its pairing mechanism is distinct from that of conventional superconductors1-4, as described by the Bardeen-Cooper-Schrieffer (BCS) theory. However, recent studies have shown that superconductivity persists even when Coulomb interactions are partially screened5,6. This suggests that pairing in MATBG might be conventional in nature and a consequence of the large density of states of its flat bands. Here we combine tunnelling and Andreev reflection spectroscopy with a scanning tunnelling microscope to observe several key experimental signatures of unconventional superconductivity in MATBG. We show that the tunnelling spectra below the transition temperature Tc are inconsistent with those of a conventional s-wave superconductor, but rather resemble those of a nodal superconductor with an anisotropic pairing mechanism. We observe a large discrepancy between the tunnelling gap ΔT, which far exceeds the mean-field BCS ratio (with 2ΔT/kBTc ~ 25), and the gap ΔAR extracted from Andreev reflection spectroscopy (2ΔAR/kBTc ~ 6). The tunnelling gap persists even when superconductivity is suppressed, indicating its emergence from a pseudogap phase. Moreover, the pseudogap and superconductivity are both absent when MATBG is aligned with hexagonal boron nitride. These findings and other observations reported here provide a preponderance of evidence for a non-BCS mechanism for superconductivity in MATBG.

Fate of the Hebel-Slichter peak in superconductors with strong antiferromagnetic fluctuations

We show that magnetic fluctuations can destroy the Hebel-Slichter peak in conventional superconductors. The Hebel-Slichter peak has previously been expected to survive even in the presence of strong electronic interactions. However, we show that antiferromagnetic fluctuations suppress the peak at $\bf{q}=0$ in the imaginary part of the magnetic susceptibility, $\chi_{+-}''\left(\bf{q},\omega\right)$, which causes the Hebel-Slichter peak. This is of general interest as in many materials superconductivity is found near a magnetically ordered phase, and the absence of a Hebel-Slichter peak is taken as evidence of unconventional superconductivity in these systems. For example, no Hebel-Slichter peak is observed in the $\kappa$-(BEDT-TTF)$_2X$ organic superconductors but heat capacity measurements have been taken to indicate $s$-wave superconductivity. If antiferromagnetic fluctuations destroy the putative Hebel-Slichter peak in organic superconductors then the peak should be restored by applying a pressure, which is known to suppress antiferromagnetic correlations in these materials.

Evidence of unconventional superconductivity in the Ni-doped NbB2 system

In this paper, a comprehensive study of the effects of Ni-doping on structural, electrical, thermal and magnetic properties of the NbB2 is presented. Low amounts (≤10%) of Ni substitution on Nb sites cause structural distortions and induce drastic changes in the physical properties, such as the emergence of a bulk superconducting state with anomalous behaviors in the critical fields (lower and upper) and in the specific heat. Ni-doping at the 9 at.% level, for instance, is able to increase the critical temperature (Tc) in stoichiometric NbB2 (<1.3 K) to approximately 6.0 K. Bulk superconductivity is confirmed by magnetization, electronic transport, and specific heat measurements. Both Hc1 and Hc2 critical fields exhibit a linear dependence with reduced temperature (T/Tc), and the specific heat deviates remarkably from the conventional exponential temperature dependence of the single-band BCS theory. These findings suggest multiband superconductivity in the composition range from 0.01 ≤ x ≤ 0.10 (Nb1‒xNixB2).

Evidence for unconventional superconductivity in half-Heusler YPdBi and TbPdBi compounds revealed by London penetration depth measurements

The half-Heusler compounds YPdBi and TbPdBi, while having similar band structure, exhibit different magnetic properties. YPdBi is a diamagnet, while TbPdBi shows antiferromagnetic order below 5.5 K. Both are superconductors with T${_c}\approx$1 K for YPdBi and T${_c}\approx$1.75 K for TbPdBi. Such a contrast in properties between these two compounds opens a question about the effects of band structure or magnetic correlations on superconductivity. Using the combination of a tunnel diode oscillator (TDO) and a commercial dilution refrigerator, we measured temperature dependent magnetic penetration depth $\Delta\lambda(T)$ in single crystals of YPdBi and TbPdBi, down to temperatures as low as 0.1K. We found that penetration depths of both compounds do not show exponential temperature dependence and saturation at low temperatures, as expected for conventional BCS superconductors. Instead, in both compounds, the penetration depth can be described by a power law $\Delta\lambda(T) = A\times T^{n}$. The coefficient A was found to be about 50$\%$ smaller in TbPdBi, but the exponents are very similar, $n = 2.76\pm 0.04$ in YPdBi and $n = 2.6\pm 0.3$ in TbPdBi, respectively. Our results suggest unconventional superconductivity in both YPdBi and TbPdBi.

Non-Ising-like two-dimensional superconductivity in a bulk single crystal

Both Nb$_3$Pd$_x$Se$_7$ and Ta$_4$Pd$_3$Te$_{16}$ crystallize in a monoclinic point group while exhibiting superconducting transition temperatures as high as $T_c\sim 3.5$ and $\sim 4.7 $ K, respectively. Disorder was claimed to lead to the extremely large upper critical fields ($H_{c2}$) observed in related compounds. Despite the presence of disorder and heavier elements, $H_{c2}$s in Ta$_4$Pd$_3$Te$_{16}$ are found to be considerably smaller than those of Nb$_3$Pd$_x$Se$_7$ while displaying an anomalous, non-saturating linear dependence on temperature $T$ for fields along all three crystallographic axes. In contrast, crystals of the latter compound displaying the highest $T_c$s display $H_{c2}\propto (1-T/T_c)^{1/2}$, which in monolayers of transition metal dichalcogenides is claimed to be evidence for an Ising paired superconducting state resulting from strong spin-orbit coupling. This anomalous $T$-dependence indicates that the superconducting state of Nb$_3$Pd$_x$Se$_7$ is quasi-two-dimensional in nature. This is further supported by a nearly divergent anisotropy in upper-critical fields, i.e. $\gamma= H_{c2}^{b}/H_{c2}^{a^{\prime}}$, upon approaching $T_c$. Hence, in Nb$_3$Pd$_x$Se$_7$ the increase of $T_c$ correlates with a marked reduction in electronic dimensionality as observed, for example, in intercalated FeSe. For the Nb compound, Density functional theory (DFT) calculations indicate that an increase in the external field produces an anisotropic orbital response, with especially strong polarization at the Pd sites when the field is perpendicular to their square planar environment. Therefore, DFT suggests the field-induced pinning of the spin to the lattice as a possible mechanism for decoupling the superconducting planes. Overall, our observations represent further evidence for unconventional superconductivity in the Pd chalcogenides.

A dash of salt is superconducting

In an experimental tour de force involving ultrahigh vacuum and low-temperature scanning probe techniques, researchers have created organic nanostructures that show evidence of unconventional superconductivity.

Extendeds±scenario for the nuclear spin-lattice relaxation rate in superconducting pnictides

Recently, several measurements of the nuclear spin-lattice relaxation rate ${T}_{1}^{\ensuremath{-}1}$ in the superconducting Fe pnictides have been reported. These measurements generally show no coherence peak below ${T}_{c}$ and indicate a low-temperature power-law behavior, the characteristics commonly taken as evidence of unconventional superconductivity with lines of nodes crossing the Fermi surface. In this work we show that (i) the lack of a coherence peak is fully consistent with the previously proposed nodeless extended ${s}_{\ifmmode\pm\else\textpm\fi{}}$-wave symmetry of the order parameter (whether in the clean or dirty limit) and (ii) the low-temperature power-law behavior can be also explained in the framework of the same model but requires going beyond the Born limit.

Field and Temperature Dependence of the Superfluid Density inLaFeAsO1−xFxSuperconductors: A Muon Spin Relaxation Study

We present zero field and transverse field muon spin relaxation experiments on the recently discovered Fe-based superconductor LaFeAsO1-xFx (x=0.075 and x=0.1). The temperature dependence of the deduced superfluid density is consistent with a BCS s-wave or a dirty d-wave gap function, while the field dependence strongly evidences unconventional superconductivity. We obtain the in-plane penetration depth of lambda ab(0)=254(2) nm for x=0.1 and lambda ab(0)=364(8) nm for x=0.075. Further evidence for unconventional superconductivity is provided by the ratio of Tc versus the superfluid density, which is close to the Uemura line of high-Tc cuprates.

Josephson effect of S(Nb)–N(Cu)–S′ (heavy-fermion, superconductor) junctions

The Josephson effect of S(Nb)–N(Cu)-S′(HFS: heavy-fermion superconductor) junctions has been investigated for polycrystalline CeCu 2.2Si 2, UBe 13 and single-crystalline UPt 3. The temperature dependence of J c which provides evidence for unconventional superconductivity has been observed when UPt 3 is used as HFS.

Superconductivity in Low-Dimensional Organic Compounds

Superconductivity in organic materials is debated controversially because the experimental situation is not yet conclusive. For one-dimensional Bechgaard salts, TMTSF, as well as for two-dimensional BEDT-TTF salts, there is some evidence of unconventional superconductivity. However, a large number of experimental results point in the direction of standard behavior. Some aspects of this discussion are presented.

The Unconventional Superconductivity of Sr2RuO4

The authors review the evidence for unconventional superconductivity in Sr{sub 2}RuO{sub 4}; the pairing in this material is certainly non-s-wave, and most of the current observations can be accounted for by a particular p-wave state, which is doubly degenerate and breaks time-reversal symmetry. The observations of the flux lattice by small-angle neutron scattering contribute to this picture. The square flux lattice and the intensity of the diffracted signal indicate that superconductivity resides primarily on the {gamma} sheet of the Fermi surface, which is derived from the Ru--d{sub xy} orbitals. This is in agreement with the orbital-dependent p-wave scenario. Furthermore, the shape of the flux lines derived from these measurements is strongly indicative of a two-component order parameter.

Unconventional superconductivity in exotic metals

This is a brief review on how the order parameter symmetry of exotic superconductors may be probed experimentally. Some variants of experimental techniques employed in searches for evidence of unconventional superconductivity in unusual metals are discussed. Many independent results indicate that the gap in the electronic excitation spectrum of these superconductors has nodes in the form of lines or points on the Fermi surface.

Evidence for unconventional superconductivity in UPd2Al3 thin films

The superconducting energy gap of UPd2Al3 thin films was investigated by quasiparticle tunneling spectroscopy. Planar cross junctions were prepared by MBE thin-film deposition, photolithography and ion-beam etching completed by a lift-off process. The measured differential conductance dI/dV clearly revealed the tunneling density of states. At temperatures below 1K we identified a BCS-like differential tunneling density of states along the c-axis with 2Δ (0K)/kBTc≈3.8. At higher temperatures a crossover toa V-shaped differential conductance develops. We propose this to be evidence for nodes in the superconducting energy gap. Measurements were performed at various magnetic fields and temperatures between 0.3K and Tc≈2K. The experimental observations are discussed in the framework of unconventional superconductivity.

Evidence for unconventional superconductivity in YBa 2Cu 3O 6.9

We report measurements of the dynamic resistance of a dc-SQUID formed by two contacts between two different edges of a single crystal of YBa 2Cu 3O 6.9 and a Nb block. The phase of the field induced R D oscillations was compared with those of two adjacent SQUIDs, each formed by two NbNb contacts. We find a phase difference of 160±30° between the oscillations of the composite SQUID and those of the SQUIDs formed by NbNb junctions. A possible explanation for this reproducible effect is that the cuprate superconductor has an unconventional order parameter.

Microwave surface impedance of kappa -(BEDT-TTF)2Cu(NCS)2, where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene: Evidence for unconventional superconductivity.

High-sensitivity microwave-impedance measurements performed on the quasi-two-dimensional organic superconductor κ-(BEDT-TTF) 2 Cu(NCS) 2 give sound evidence for an unconventional type of superconductivity. The penetration depth λ(T) is in excellent agreement with recent muon-spin-relaxation measurements which are consistent with anisotropic pairings with lines of nodes in the energy gap. The real part of the conductivity a σ 1 shows a peaked structure which is free of coherence factor effects and results from a competition between two temperature dependences, the penetration depth and the inelastic relaxation time of the normal fluid

Evidence for unconventional superconductivity in single crystals of the antiferromagnetic heavy-electron compound URu 2Si 2

Specific heat C( T) and upper critical field H c2( T) measurements have been performed on two single crystal specimens (denoted A and B) of the antiferromagnetic heavy-electron superconductor URu 2Si 2. Specific heat measurements on both single crystals reveal two distinct jumps, indicative of two superconducting phases. This is reminiscent of the antiferromagnetic heavy-electron compound UPt 3 in which multiple superconducting transitions have been observed and attributed to coupled antiferromagnetic and multicomponent superconducting order parameters, although two superconducting phases associated with two different states of the crystal cannot be ruled out. The relative magnitudes of the two specific heat jumps observed in crystal B suggest that the two superconducting phases occupy nearly equal volume fractions of the crystal. At temperatures below the lower jump, the specific heat of both crystals can be described by C 3( T) = γ s (0) T+ AT 3 with γ 3(0) ≈ 0.5 γ n (0), where γ n(0) is the value of the normal-state electronic specific heat coefficient γ n( T), extrapolated to T = 0. The two critical temperatures inferred from the specific heat jumps in crystal B, measured in magnetic fields H between 0 and 15kOe applied parallel to the c-axis, have a similar dependence on H. Resistive measurements of H c2( T) on specimens from crystal A between 0 and 60 kOe reveal a kink near 2 kOe for H⌈ c and strong positive curvature in H c2( T) below ∼3 kOe for H⌈ a.

Evidence for unconventional superconductivity in UPt 3 : Splitting of the specific heat anomaly at T c

Abstract The specific heats of two samples of UPt3 have been measured in the vicinity of Tc. Both samples show specific heat anomalies sharper than any previously observed and two clearly resolved maxima. The results are interpreted as evidence of splitting of the transition and unconventional pairing.