Pressure-Induced Exciton Formation and Superconductivity in Platinum-Based Mineral Sperrylite.

We report a comprehensive study of Sperrylite (PtAs), the main platinum source in natural minerals, as a function of applied pressures up to 150 GPa. While no structural phase transition is detected from pressure-dependent X-ray measurements, the unit cell volume shrinks monotonically with pressure following the third-order Birch-Murnaghan equation of state. The mildly semiconducting behavior found in pure synthesized crystals at ambient pressures becomes more insulating upon increasing the applied pressure before metalizing at higher pressures, giving way to the appearance of an abrupt decrease in resistance near 3 K at pressures above 92 GPa consistent with the onset of a superconducing phase.

Orphan high field superconductivity in non-superconducting uranium ditelluride.

Reentrant superconductivity is an uncommon phenomenon in which the destructive effects of magnetic field on superconductivity are mitigated, allowing a zero-resistance state to survive under conditions that would otherwise destroy it. Typically, the reentrant superconducting region derives from a zero-field parent superconducting phase. Here, we show that in UTe crystals extreme applied magnetic fields give rise to an unprecedented high-field superconductor that lacks a zero-field antecedent.

Resonant Ultrasound Spectroscopy for Irregularly Shaped Samples and Its Application to Uranium Ditelluride.

Resonant ultrasound spectroscopy (RUS) is a powerful technique for measuring the full elastic tensor of a given material in a single experiment. Previously, this technique was practically limited to regularly shaped samples such as rectangular parallelepipeds, spheres, and cylinders [W. M.

Potential Lifshitz transition at optimal substitution in nematic pnictide BaSrNiAs.

BaNiAs is a structural analog of the pnictide superconductor BaFeAs, which, like the iron-based superconductors, hosts a variety of ordered phases including charge density waves (CDWs), electronic nematicity, and superconductivity. Upon isovalent Sr substitution on the Ba site, the charge and nematic orders are suppressed, followed by a sixfold enhancement of the superconducting transition temperature (). To understand the mechanisms responsible for enhancement of , we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the BaSrNiAs series, which agree well with our density functional theory (DFT) calculations throughout the substitution range.

A review of UTeat high magnetic fields.

Uranium ditelluride (UTe) is recognized as a host material to unconventional spin-triplet superconductivity, but it also exhibits a wealth of additional unusual behavior at high magnetic fields. One of the most prominent signatures of the unconventional superconductivity is a large and anisotropic upper critical field that exceeds the paramagnetic limit. This superconductivity survives to 35 T and is bounded by a discontinuous magnetic transition, which itself is also field-direction-dependent.

Detection of a pair density wave state in UTe.

Spin-triplet topological superconductors should exhibit many unprecedented electronic properties, including fractionalized electronic states relevant to quantum information processing. Although UTe may embody such bulk topological superconductivity, its superconductive order parameter Δ(k) remains unknown. Many diverse forms for Δ(k) are physically possible in such heavy fermion materials.

Visualizing the atomic-scale origin of metallic behavior in Kondo insulators.

A Kondo lattice is often electrically insulating at low temperatures. However, several recent experiments have detected signatures of bulk metallicity within this Kondo insulating phase. In this study, we visualized the real-space charge landscape within a Kondo lattice with atomic resolution using a scanning tunneling microscope.

Observation of the Superconducting Proximity Effect from Surface States in SmB_{6}/YB_{6} Thin Film Heterostructures via Terahertz Spectroscopy.

The ac conduction of epitaxially grown SmB_{6} thin films and superconducting heterostructures of SmB_{6}/YB_{6} are investigated via time-domain terahertz spectroscopy. A two-channel model of thickness-dependent bulk states and thickness-independent surface states accurately describes the measured conductance of bare SmB_{6} thin films, demonstrating the presence of surface states in SmB_{6}. While the observed reductions in the simultaneously measured superconducting gap, transition temperature, and superfluid density of SmB_{6}/YB_{6} heterostructures relative to bare YB_{6} indicate the penetration of proximity-induced superconductivity into the SmB_{6} overlayer; the corresponding SmB_{6}-thickness independence between different heterostructures indicates that the induced superconductivity is predominantly confined to the interface surface state of the SmB_{6}.

Observation of a Flat and Extended Surface State in a Topological Semimetal.

A flat band structure in momentum space is considered key for the realization of novel phenomena. A topological flat band, also known as a drumhead state, is an ideal platform to drive new exotic topological quantum phases. Using angle-resolved photoemission spectroscopy experiments, we reveal the emergence of a highly localized surface state in a topological semimetal BaAl4 and provide its full energy and momentum space topology.

Comparison of Two Different Synthesis Methods of Single Crystals of Superconducting Uranium Ditelluride.

Single crystal specimens of the actinide compound uranium ditelluride, UTe2, are of great importance to the study and characterization of its dramatic unconventional superconductivity, believed to entail spin-triplet electron pairing. A variety in the superconducting properties of UTe2 reported in the literature indicates that discrepancies between synthesis methods yield crystals with different superconducting properties, including the absence of superconductivity entirely. This protocol describes a process to synthesize crystals that exhibit superconductivity via chemical vapor transport, which has consistently exhibited a superconducting critical temperature of 1.

Multiple Charge Density Waves and Superconductivity Nucleation at Antiphase Domain Walls in the Nematic Pnictide Ba_{1-x}Sr_{x}Ni_{2}As_{2}.

How superconductivity interacts with charge or nematic order is one of the great unresolved issues at the center of research in quantum materials. Ba_{1-x}Sr_{x}Ni_{2}As_{2} (BSNA) is a charge ordered pnictide superconductor recently shown to exhibit a sixfold enhancement of superconductivity due to nematic fluctuations near a quantum phase transition (at x_{c}=0.7) [1].

Sixfold enhancement of superconductivity in a tunable electronic nematic system.

The electronic nematic phase-in which electronic degrees of freedom lower the crystal rotational symmetry-is commonly observed in high-temperature superconductors. However, understanding the role of nematicity and nematic fluctuations in Cooper pairing is often made more complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system that is not magnetic, and show that the enhancement is directly born out of strong nematic fluctuations associated with a quantum phase transition.

Optical signatures of multifold fermions in the chiral topological semimetal CoSi.

We report the optical conductivity in high-quality crystals of the chiral topological semimetal CoSi, which hosts exotic quasiparticles known as multifold fermions. We find that the optical response is separated into several distinct regions as a function of frequency, each dominated by different types of quasiparticles. The low-frequency intraband response is captured by a narrow Drude peak from a high-mobility electron pocket of double Weyl quasiparticles, and the temperature dependence of the spectral weight is consistent with its Fermi velocity.

Effect of chemical substitution on the skyrmion phase in CuOSeO.

Magnetic skyrmions have been the focus of intense research due to their unique qualities which result from their topological protections. Previous work on CuOSeO, the only known insulating multiferroic skyrmion material, has shown that chemical substitution alters the skyrmion phase. We chemically substitute Zn, Ag, and S into powdered CuOSeO to study the effect on the magnetic phase diagram.

Enhancement and reentrance of spin triplet superconductivity in UTe under pressure.

Spin triplet superconductivity in the Kondo lattice UTe appears to be associated with spin fluctuations originating from incipient ferromagnetic order. Here we show clear evidence of twofold enhancement of superconductivity under pressure, which discontinuously transitions to magnetic order, likely of ferromagnetic nature, at higher pressures. The application of a magnetic field tunes the system back across a first-order phase boundary.

Low Energy Band Structure and Symmetries of UTe_{2} from Angle-Resolved Photoemission Spectroscopy.

The compound UTe_{2} has recently been shown to realize spin triplet superconductivity from a nonmagnetic normal state. This has sparked intense research activity, including theoretical analyses that suggest the superconducting order parameter to be topologically nontrivial. However, the underlying electronic band structure is a critical factor for these analyses, and remains poorly understood.

Long-range magnetic order in hydroxide-layer-doped (Li Fe Mn OD)FeSe.

The (Li Fe OH)FeSe superconductor has been suspected of exhibiting long-range magnetic ordering due to Fe substitution in the LiOH layer. However, no direct observation such as magnetic reflection from neutron diffraction has been reported. Here, we use a chemical design strategy to manipulate the doping level of transition metals in the LiOH layer to tune the magnetic properties of the (Li Fe Mn OD)FeSe system.

Point-node gap structure of the spin-triplet superconductor UTe.

Low-temperature electrical and thermal transport, magnetic penetration depth, and heat capacity measurements were performed on single crystals of the actinide superconductor UTe to determine the structure of the superconducting energy gap. Heat transport measurements performed with currents directed along both crystallographic and axes reveal a vanishingly small residual fermionic component of the thermal conductivity. The magnetic field dependence of the residual term follows a rapid, quasilinear increase consistent with the presence of nodal quasiparticles, rising toward the -axis upper critical field where the Wiedemann-Franz law is recovered.

Nearly ferromagnetic spin-triplet superconductivity.

Spin-triplet superconductors potentially host topological excitations that are of interest for quantum information processing. We report the discovery of spin-triplet superconductivity in UTe, featuring a transition temperature of 1.6 kelvin and a very large and anisotropic upper critical field exceeding 40 teslas.

Perfect Andreev reflection due to the Klein paradox in a topological superconducting state.

In 1928, Dirac proposed a wave equation to describe relativistic electrons. Shortly afterwards, Klein solved a simple potential step problem for the Dirac equation and encountered an apparent paradox: the potential barrier becomes transparent when its height is larger than the electron energy. For massless particles, backscattering is completely forbidden in Klein tunnelling, leading to perfect transmission through any potential barrier.

Unconventional Charge Density Wave Order in the Pnictide Superconductor Ba(Ni_{1-x}Co_{x})_{2}As_{2}.

Ba(Ni_{1-x}Co_{x})_{2}As_{2} is a structural homologue of the pnictide high temperature superconductor, Ba(Fe_{1-x}Co_{x})_{2}As_{2}, in which the Fe atoms are replaced by Ni. Superconductivity is highly suppressed in this system, reaching a maximum T_{c}=2.3  K, compared to 24 K in its iron-based cousin, and the origin of this T_{c} suppression is not known.

Law and Disorder: Special Stacking Units-Building the Intergrowth CeCoGe.

A new structure type of composition CeCoGe was grown out of a molten Sn flux. CeCoGe crystallizes in the orthorhombic space group Cmcm, with highly anisotropic lattice parameters of a = 4.3293(5) Å, b = 55.