Condensed Matter

Real-space modulated Charge Density Waves (CDW) are an ubiquituous feature in many families of superconductors. In particular, how CDW relates to superconductivity is an active and open question that has recently gathered much interest since CDWs have been discovered in many cuprates superconductors. Here we show that disorder induced by proton irradiation is a full-fledged tuning parameter that can bring essential information to answer this question as it affects CDW and superconductivity with different and unequivocal mechanisms. Specifically, in the model CDW superconductor Lu 5 Ir 4 Si 10 that develops a 1D CDW below 77\,K and s-wave superconductivity below 4\,K, we show that disorder enhances the superconducting critical temperature T c and H c2 while it suppresses the CDW. Discussing how disorder affects both superconductivity and the CDW, we make a compelling case that superconductivity and CDW are competing for electronic density of states at the Fermi level in Lu 5 Ir 4 Si 10 , and we reconcile the results obtained via the more common tuning parameters of pressure and doping. Owing to its prototypical, 1D, Peierls type CDW and the s-wave, weak-coupling nature of its superconductivity, this irradiation study of Lu 5 Ir 4 Si 10 provides the basis to understand and extend such studies to the more complex cases of density waves and superconductivity coexistence in heavy fermions, Fe-based or cuprates superconductors.

We report a comprehensive Cu L 3 -edge resonant x-ray scattering study of two- and three-dimensional (2D and 3D) incommensurate charge correlations in single crystals of the underdoped high-temperature superconductor YBa 2 Cu 3 O 6.67 under uniaxial compression up to 1% along the two inequivalent Cu-O-Cu bond directions (a and b) in the CuO 2 planes. The pressure response of the 2D charge correlations is symmetric: pressure along a enhances correlations along b, and vice versa. Our results imply that the underlying order parameter is uniaxial. In contrast, 3D long-range charge order is only observed along b in response to compression along a. Spectroscopic resonant x-ray scattering measurements show that the 3D charge order resides exclusively in the CuO 2 planes and may thus be generic to the cuprates. We discuss implications of these results for models of electronic nematicity and for the interplay between charge order and superconductivity.

We show that unconventional nematic superconductors with multi-component order parameter in lattices with three-fold and six-fold rotational symmetries support a charge- 4e vestigial superconducting phase above T c . The charge- 4e state, which is a condensate of four-electron bound states that preserve the rotational symmetry of the lattice, is nearly degenerate with a competing vestigial nematic state, which is non-superconducting and breaks the rotational symmetry. This robust result is the consequence of a hidden discrete symmetry in the Ginzburg-Landau theory, which permutes quantities in the gauge sector and in the crystalline sector of the symmetry group. We argue that random strain generally favors the charge- 4e state over the nematic phase, as it acts as a random-mass to the former but as a random-field to the latter. Thus, we propose that two-dimensional inhomogeneous systems displaying nematic superconductivity, such as twisted bilayer graphene, provide a promising platform to realize the elusive charge- 4e superconducting phase.

We present an investigation into the optical properties of Nd 4 Ni 3 O 8 at different temperatures from 300 down to 5~K over a broad frequency range. The optical conductivity at 5~K is decomposed into IR-active phonons, a far-infrared band α , a mid-infrared band β , and a high-energy absorption edge. By comparing the measured optical conductivity to first-principles calculations and the optical response of other nickelates, we find that Nd 4 Ni 3 O 8 features evident charge-stripe fluctuations. The β band is attributed to electronic transitions between the gapped Ni- d x 2 ??y 2 bands due to fluctuating charge stripes, while the high-frequency absorption edge corresponds to the onset of transitions involving other high-energy bands. Furthermore, an analysis of the temperature-dependent optical spectral weight reveals a T 2 law, which is likely to originate from strong correlation effects.

Topological superconductors (SCs) are novel phases of matter with nontrivial bulk topology. They host at their boundaries and vortex cores zero-energy Majorana bound states, potentially useful in fault-tolerant quantum computation. Chiral SCs are particular examples of topological SCs with finite angular momentum Cooper pairs circulating around a unique chiral axis, thus spontaneously breaking time-reversal symmetry (TRS). They are rather scarce and usually feature triplet pairing: best studied examples in bulk materials are UPt3 and Sr2RuO4 proposed to be f-wave and p-wave SCs respectively, although many open questions still remain. Chiral triplet SCs are, however, topologically fragile with the gapless Majorana modes weakly protected against symmetry preserving perturbations in contrast to chiral singlet SCs. Using muon spin relaxation (muSR) measurements, here we report that the weakly correlated pnictide compound LaPt3P has the two key features of a chiral SC: spontaneous magnetic fields inside the superconducting state indicating broken TRS and low temperature linear behaviour in the superfluid density indicating line nodes in the order parameter. Using symmetry analysis, first principles band structure calculation and mean-field theory, we unambiguously establish that the superconducting ground state of LaPt3P is chiral d-wave singlet.

The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H-TaS 2 ) at ambient pressure, is boosted by applying hydrostatic pressures up to 30GPa, thereby inducing a typical dome-shaped superconducting phase. The ambient pressure CDW ground state which begins at TCDW = 76 K, with critically small Fermi surfaces, was found to be fully suppressed at Pc = 8.7GPa. Around Pc, we observe a superconducting dome with a maximum superconducting transition temperature Tc = 9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods and the corresponding lower critical field H c1 was deduced. While a d wave and single-gap BCS prediction cannot describe our H c1 experiments, the temperature dependence of the H c1 can be well described by a single-gap anisotropic s-wave order parameter.

The claim in Barkman et al [Physical Review Letters {\bf 122}, 165302 (2019)] that pair-density wave (PDW) superconductivity in magnetic field is more stable near surface than in bulk, is not supported by microscopic theory.

Superconductivity induced by a magnetic field near metamagnetism is a striking manifestation of magnetically-mediated superconducting pairing. After being observed in itinerant ferromagnets, this phenomenon was recently reported in the orthorhombic paramagnet UTe 2 . Under a magnetic field applied along the hard magnetization axis b, superconductivity is reinforced on approaching metamagnetism at μ 0 H m = 35 T, but it abruptly disappears beyond H m . On the contrary, field-induced superconductivity was reported beyond μ 0 H m = 40-50 T in a magnetic field tilted by ≃25−30° from b in the (b,c) plane. Here we explore the phase diagram of UTe2 under these two magnetic-field directions. Zero-resistance measurements permit to confirm unambiguously that superconductivity is established beyond Hm in the tilted-field direction. While superconductivity is locked exactly at fields either smaller (for a H || b), or larger (for H tilted by ≃27° from b to c), than Hm, the variations of the Fermi-liquid coefficient in the electrical resistivity and of the residual resistivity are surprisingly similar for the two field directions. The resemblance of the normal states for the two field directions puts constraints for theoretical models of superconductivity and implies that some subtle ingredients must be in play.

In polar-oxide interfaces, a certain number of monolayers (ML) is needed for conductivity to appear. This threshold for conductivity is explained by the accumulation of sufficient electric potential to initiate charge transfer to the interface. Here we study the (111) SrTiO3/LaAlO3 interface where a critical thickness of nine epitaxial LaAlO3 ML is required to turn the interface from insulating to conducting and even superconducting. We show that this critical thickness decreases to 3ML when depositing a cobalt over-layer (capping) and 6ML for platinum capping. The latter result contrasts with the (100) interface where platinum capping increases the critical thickness beyond that of the bare interface. These results suggest that the work function of the metallic capping plays an important role in both interfaces. Interestingly, for (111) SrTiO3/LaAlO3/Metal interfaces conductivity appears concomitantly with superconductivity in contrast with the SrTiO3/LaAlO3/Metal interface with LaAlO3 layer smaller than four ML (unit-cells), which are conducting but not superconducting. We suggest that this difference is related to the different sub-bands involved in conductivity for the (111) interfaces, comparing to the (100) interfaces. Our findings can be useful for superconducting devices made of such interfaces.

We develop a theory of conductivity of type-II superconductors in the flux flow regime taking into account random spatial fluctuations of the system parameters, such as the gap magnitude Δ (r) and the diffusion coefficient D(r). We find a contribution to the conductivity that is proportional to the inelastic relaxation time τ in , which is much longer than the elastic relaxation time. This new contribution is due to Debye-type relaxation, and it can be much larger than the conventional flux flow conductivity due to Bardeen and Stephen. The new contribution is expected to dominate in clean superconductors at low temperatures and in magnetic fields much smaller than H c2 .