Inhomogeneous high temperature melting and decoupling of charge density waves in spin-triplet superconductor UTe2.

Charge, spin and Cooper-pair density waves have now been widely detected in exotic superconductors. Understanding how these density waves emerge - and become suppressed by external parameters - is a key research direction in condensed matter physics. Here we study the temperature and magnetic-field evolution of charge density waves in the rare spin-triplet superconductor candidate UTe2 using scanning tunneling microscopy/spectroscopy. We reveal that charge modulations composed of three different wave vectors gradually weaken in a spatially inhomogeneous manner, while persisting to surprisingly high temperatures of 10-12 K. We also reveal an unexpected decoupling of the three-component charge density wave state. Our observations match closely to the temperature scale potentially related to short-range magnetic correlations, providing a possible connection between density waves observed by surface probes and intrinsic bulk features. Importantly, charge density wave modulations become suppressed with magnetic field both below and above superconducting Tc in a comparable manner. Our work points towards an intimate connection between hidden magnetic correlations and the origin of the unusual charge density waves in UTe2.

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 UTe2 crystals extreme applied magnetic fields give rise to an unprecedented high-field superconductor that lacks a zero-field antecedent. This high-field orphan superconductivity exists at angles offset between 29o and 42o from the crystallographic b to c axes with applied fields between 37 T and 52 T. The stability of field-induced orphan superconductivity presented in this work defies both empirical precedent and theoretical explanation and demonstrates that high-field superconductivity can exist in an otherwise non-superconducting material.

A review of UTe2at high magnetic fields.

Uranium ditelluride (UTe2) 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. A different, reentrant superconducting phase emerges only on the high-field side of the magnetic transition, in a range of angles between the crystallographicbandcaxes. This review discusses the current state of knowledge of these high-field phases, the high-field behavior of the heavy fermion normal state, and other phases that are stabilized by applied pressure.

Tl2Ir2O7: A Pauli Paramagnetic Metal, Proximal to a Metal Insulator Transition.

A polycrystalline sample of Tl2Ir2O7 was synthesized by high-pressure and high-temperature methods. Tl2Ir2O7 crystallizes in the cubic pyrochlore structure with space group Fd3̅m (No. 227). The Ir4+ oxidation state is confirmed by Ir-L3 X-ray absorption near-edge spectroscopy. Combined temperature-dependent magnetic susceptibility, resistivity, specific heat, and DFT+DMFT calculation data show that Tl2Ir2O7 is a Pauli paramagnetic metal, but it is close to a metal-insulator transition. The effective ionic size of Tl3+ is much smaller than that of Pr3+ in metallic Pr2Ir2O7; hence, Tl2Ir2O7 would be expected to be insulating according to the established phase diagram of the pyrochlore iridate compounds, A3+2Ir4+2O7. Our experimental and theoretical studies indicate that Tl2Ir2O7 is uniquely different from the current A3+2Ir4+2O7 phase diagram. This uniqueness is attributed primarily to the electronic configuration difference between Tl3+ and rare-earth ions, which plays a substantial role in determining the Ir-O-Ir bond angle, and the corresponding electrical and magnetic properties.

High-Pressure Synthesis of Double Perovskite Ba2NiIrO6: In Search of a Ferromagnetic Insulator.

Double perovskite oxides with d8-d3 electronic configurations are expected to be ferromagnetic from the Goodenough-Kanamori rules, such as ferromagnetic La2NiMnO6. In search of new ferromagnetic insulators, double perovskite Ba2NiIrO6 was successfully synthesized by high-pressure and high-temperature methods (8 GPa and 1573 K). Ba2NiIrO6 crystallizes in a cubic double perovskite structure (space group: Fm3̅m), with an ordered arrangement of NiO6 and IrO6 octahedra. X-ray absorption near-edge spectroscopy confirms the nominal Ni(II) and Ir(VI) valence states. Ba2NiIrO6 displays an antiferromagnetic order at 51 K. The positive Weiss temperature, however, indicates that ferromagnetic interactions are dominant. Isothermal magnetization curves at low temperatures support a field-induced spin-flop transition.

Ambient and High Pressure CuNiSb2: Metal-Ordered and Metal-Disordered NiAs-Type Derivative Pnictides.

The mineral Zlatogorite, CuNiSb2, was synthesized in the laboratory for the first time by annealing elements at ambient pressure (CuNiSb2-AP). Rietveld refinement of synchrotron powder X-ray diffraction data indicates that CuNiSb2-AP crystallizes in the NiAs-derived structure (P3m1, #164) with Cu and Ni ordering. The structure consists of alternate NiSb6 and CuSb6 octahedral layers via face-sharing. The formation of such structure instead of metal disordered NiAs-type structure (P63/mmc, #194) is validated by the lower energy of the ordered phase by first-principle calculations. Interatomic crystal orbital Hamilton population, electron localization function, and charge density analysis reveal strong Ni-Sb, Cu-Sb, and Cu-Ni bonding and long weak Sb-Sb interactions in CuNiSb2-AP. The magnetic measurement indicates that CuNiSb2-AP is Pauli paramagnetic. First-principle calculations and experimental electrical resistivity measurements reveal that CuNiSb2-AP is a metal. The low Seebeck coefficient and large thermal conductivity suggest that CuNiSb2 is not a potential thermoelectric material. Single crystals were grown by chemical vapor transport. The high pressure sample (CuNiSb2-8 GPa) was prepared by pressing CuNiSb2-AP at 700 °C and 8 GPa. However, the structures of single crystal and CuNiSb2-8 GPa are best fit with a disordered metal structure in the P3m1 space group, corroborated by transmission electron microscopy.

High-Pressure, High-Temperature Synthesis and Characterization of Polar and Magnetic LuCrWO6.

A new polar and magnetic oxide, LuCrWO6, was synthesized under high pressure (6 GPa) and high temperature (1673 K). LuCrWO6 is isostructural with the previously reported polar YCrWO6 (SG: Pna21, no. 33). The ordering of CrO6 and WO6 octahedra in the edge-shared dimers induce the polar structure. The effective size of rare earth, Ln cation does not seem to affect the symmetry of LnCrWO6. Second harmonic generation measurements of LuCrWO6 confirmed the noncentrosymmetric character and strong piezoelectric domains are observed from piezoresponse force microscopy at room temperature. LuCrWO6 exhibits antiferromagnetic behavior, TN, of ∼18 K with a Weiss temperature of -30.7 K.

Nonmetallic metal toward a pressure-induced bad-metal state in two-dimensional Cu3LiRu2O6.

The novel two-dimensional honeycomb layered Cu3LiRu2O6 exhibits Pauli-like paramagnetic and Mott variable range hopping semiconduction behaviors, which contradict the large specific-heat Sommerfeld coefficient for metals, and indicate a possible spin-excitation induced nonmetallic metal. This nonmetallic feature can be significantly suppressed by pressure toward producing a bad-metal state, as reflected by the temperature-dependent resistivity response up to 35 GPa.

High-Pressure Synthesis and Ferrimagnetism of Ni3TeO6-Type Mn2ScMO6 (M = Nb, Ta).

The corundum-related oxides Mn2ScNbO6 and Mn2ScTaO6 were synthesized at high pressure and high temperature (6 GPa and 1475 K). Analysis of the synchrotron powder X-ray diffraction shows that Mn2ScNbO6 and Mn2ScTaO6 crystallize in Ni3TeO6-type noncentrosymmetric crystal structures with space group R3. The asymmetric crystal structure was confirmed by second harmonic generation measurement. X-ray absorption near-edge spectroscopies indicate formal valence states of Mn2+2Sc3+Nb5+O6 and Mn2+2Sc3+Ta5+O6, also supported by the calculated bond valence sums. Both samples are electrically insulating. Magnetic measurements indicate that Mn2ScNbO6 and Mn2ScTaO6 order ferrimagnetically at 53 and 50 K, respectively, and Mn2ScTaO6 is found to have a field-induced magnetic transition.

LaMn3Rh4O12: An Antiferromagnetic Quadruple Perovskite Synthesized at High Pressure.

A quadruple perovskite LaMn3Rh4O12 with A' = Mn and B = 4d transition metal was synthesized at high pressure (8 GPa) and temperature (1423 K) for the first time. Room temperature powder X-ray diffraction indicates that LaMn3Rh4O12 forms in cubic symmetry (Im3̅, a = 7.4997(1) Å). X-ray absorption near-edge spectroscopy shows predominantly Mn3+ and Rh3+ oxidation states. An antiferromagnetic transition at TN ∼ 41 K is corroborated by specific heat measurements. The resistivity measurements indicate a three-dimensional Mott variable-range hopping conduction mechanism between 300 and 160 K.

Mn2CoReO6: a robust multisublattice antiferromagnetic perovskite with small A-site cations.

Mn2CoReO6, the fourth known magnetic transition-metal-only double perovskite oxide (space group P21/n) was synthesized at high pressure and temperature (8 GPa, 1350 °C). Large structural distortions are induced by the small A-site Mn2+ cations. Mn2CoReO6 exhibits complex magnetic properties with a robust antiferromagnetic order (TN = 94 K) involving all cation sublattices.

High-Pressure Synthesis of Lu2NiIrO6 with Ferrimagnetism and Large Coercivity.

Double-perovskite Lu2NiIrO6 was synthesized at high pressure (6 GPa) and high temperature (1300 °C). Synchrotron powder X-ray diffraction indicates that its structure is a monoclinic double perovskite (space group P21/ n) with a small, 11% Ni/Ir antisite disorder. X-ray absorption near-edge spectroscopy measurements established Ni2+ and Ir4+ formal oxidation states. Magnetic studies indicate a ferrimagnetic transition at 207 K. The low-temperature magnetization curve of Lu2NiIrO6 features broad hysteresis with a coercive field as high as 48 kOe. These results encourage the search for hard magnets in the class of 3d/5d double-perovskite oxides.