Simultaneous low-temperature electrical resistivity and Hall effect measurements were performed on single-crystalline Bi2Se3 under applied pressures up to 50 GPa. As a function of pressure, superconductivity is observed to onset above 11 GPa with a transition temperature Tc and upper critical field Hc2 that both increase with pressure up to 30 GPa, where they reach maximum values of 7 K and 4 T, respectively. Upon further pressure increase, Tc remains anomalously constant up to the highest achieved pressure. Conversely, the carrier concentration increases continuously with pressure, including a tenfold increase over the pressure range where Tc remains constant. Together with a quasilinear temperature dependence of Hc2 that exceeds the orbital and Pauli limits, the anomalously stagnant pressure dependence of Tc points to an unconventional pressureinduced pairing state in Bi2Se3 that is unique among the superconducting topological insulators. PACS numbers:The interplay between superconductivity and topological insulator (TI) surface states has recently received enormous attention due to the observation of the long sought Majorana quasiparticle in InSb nanowires [1] and the promise of realizing topologically protected quantum computation [2]. Characterized by a nontrivial Z2 band topology with a bulk insulating energy gap that leads to a chiral metallic surface state with spin-momentum locking, TI surface states are analogous to the quantum Hall edge state and arise at the surface of a TI material due to the topological nature of the crossover between a nontrivial bulk insulating gap and the trivial insulating gap of the vacuum [3]. The use of the proximity effect [4][5][6][7] to induce superconductivity in Bi 2 Se 3 , the most well studied TI material to date, has had success in coupling these two states but suffers from the presence of bulk conducting states which require gating to realize true TI supercurrents [8].Theoretically, nontrivial surface Andreev bound states can be directly realized by opening a superconducting energy gap in a bulk conductor [9], which is why the quest for the topological superconductor is one of the most active areas in condensed-matter physics. Recently, superconductivity has been found in materials with topologically nontrivial band structures, such as in Cu x Bi 2 Se 3 [10-13] and YPtBi [14,15], providing not only intrinsic systems with which to study the interplay between superconductivity and TI states, but also the potential to realize a new class of odd-parity, unconventional superconductivity [9].The application of pressure has also uncovered superconductivity in several related materials, such as elemen- route to realizing topological superconductivity. In this study, we measure transport properties of Bi 2 Se 3 over an extended pressure range to investigate the ground state at ultrahigh pressures by using a designer diamond anvil cell capable of measuring both longitudinal and transverse resistivities up to 50 GPa. We observe the onset of a superconducting phase above 11 GPa...

The onset of antiferromagnetic order in URu 2 Si 2 has been studied via neutron diffraction in a helium pressure medium, which most closely approximates hydrostatic conditions. The antiferromagnetic critical pressure is 0.80 GPa, considerably higher than values previously reported. Complementary electrical resistivity measurements imply that the hidden-order-antiferromagnetic bicritical point falls between 1.3 and 1.5 GPa. Moreover, the redefined pressure-temperature phase diagram suggests that the superconducting and antiferromagnetic phase boundaries actually meet at a common critical pressure at zero temperature.

A persistent kink in the pressure dependence of the "hidden order" (HO) transition temperature of URu2−xRexSi2 is observed at a critical pressure Pc=15 kbar for 0 ≤ x ≤ 0.08. In URu2Si2, the kink at Pc is accompanied by the destruction of superconductivity; a change in the magnitude of a spin excitation gap, determined from electrical resistivity measurements; and a complete gapping of a portion of the Fermi surface (FS), inferred from a change in scattering and the competition between the HO state and superconductivity for FS fraction.PACS numbers: 75.30.Mb, 74.70.Tx, 81.30.Bx, 74.62.Fj Since its discovery over 20 years ago [1,2,3], the moderately heavy fermion compound URu 2 Si 2 has been the focus of many theoretical and experimental efforts designed to determine the elusive, hidden order parameter associated with the phase transition occurring at T 0 ≈ 17.5 K. The transition into this "hidden order" (HO) state is characterized by large anomalies (typical of magnetic ordering) in specific heat, electrical resistivity, thermal conductivity, and magnetization measurements [1,2,3,4,5,6,7]; however, only a small antiferromagnetic moment, insufficient to adequately explain the entropy released during the transition, was detected in low-temperature neutron diffraction experiments [8]. In addition to the puzzling order parameter of the HO state, URu 2 Si 2 undergoes a transition into an unconventional superconducting (SC) state, which coexists with weak antiferromagnetism (AFM), at T c ≈ 1.5 K. The potential interplay between the two ordered phases of URu 2 Si 2 as well as the nature of the HO state are underlying problems to our fundamental understanding of the properties of this compound.In an effort to explain the observed properties of URu 2 Si 2 , several microscopic models have been proposed [9,10,11,12,13,14,15]. In addition to the theoretical pursuits, many varied experimental techniques have been employed to confirm and/or constrain the proposed models; however, the experimental results fail to converge upon an encompassing microscopic description of the ordered states of URu 2 Si 2 , but do provide valuable insight when contextually analyzed. Low-temperature neutron diffraction measurements as a function of magnetic field provide evidence that the order parameter of the HO state must break time-reversal symmetry [16], and recent inelastic neutron scattering measurements reveal gapped spin excitations at incommensurate wavevectors [17]. Thermal transport measurements are consistent with the opening of a gap at the Fermi surface (FS), as previously suggested by optical conductivity and specific heat studies [3,18], depleting carriers and reducing electron-phonon scattering [5,6]. These exemplary measurements tend to converge upon a description of the HO state invoking the presence of a FS instability such as a spin density wave (SDW), further suggested by high-field measurements intimating the itinerant nature of the HO state [19].The application of pressure to URu 2 Si 2 further convolutes the discussion...

Resonant x-ray emission spectroscopy (RXES) was used to determine the pressure dependence of the f-electron occupancy in the Kondo insulator SmB6. Applied pressure reduces the f-occupancy, but surprisingly, the material maintains a significant divalent character up to a pressure of at least 35 GPa. Thus, the closure of the resistive activation energy gap and onset of magnetic order are not driven by stabilization of an integer valent state. Over the entire pressure range, the material maintains a remarkably stable intermediate valence that can in principle support a nontrivial band structure.

One of the most notorious non-Fermi-liquid properties of both archetypal heavy-fermion systems 1-4 and the high-T c copper oxide superconductors 5 is an electrical resistivity that evolves linearly (rather than quadratically) with temperature, T . In the heavy-fermion superconductor CeCoIn 5 (ref. 6), this linear behaviour was one of the first indications of the presence of a zero-temperature instability, or quantum critical point. Here, we report the observation of a unique control parameter of T -linear scattering in CeCoIn 5 , found through systematic chemical substitutions of both magnetic and non-magnetic rareearth, R, ions into the Ce sublattice. We find that the evolution of inelastic scattering in Ce 1−x R x CoIn 5 is strongly dependent on the f -electron configuration of the R ion, whereas two other key properties-Cooper-pair breaking and Kondo-lattice coherence-are not. Thus, T -linear resistivity in CeCoIn 5 is intimately related to the nature of incoherent scattering centres in the Kondo lattice, which provides insight into the anomalous scattering rate synonymous with quantum criticality 7 . Although recent theories 4,[8][9][10] provide possible routes to an explanation of T-linear resistivity-found in both f -electron systems (for example, Y 1−x U x Pd 3 (ref. [1][2][3][4]6 , its coexistence with conventional (T 2 ) Hall-angle scattering 11,12 and its inconsistency with oneparameter scaling 13 . Most recently, its observation over three decades of T at the field-tuned quantum critical point (QCP) of CeCoIn 5 has been linked to a violation of the Wiedemann-Franz law 14 , an indication that this scattering rate is associated with the failure of Fermi-liquid theory in its most basic form.Here, we present a rigorous study of the effects of rare-earth substitution on three closely related features of the exotic metal CeCoIn 5 : unconventional superconductivity, Kondo-lattice coherence and anomalous charge-carrier scattering. By diluting the Ce lattice within high-quality single-crystal specimens of Ce 1−x R x CoIn 5 with both non-magnetic (full or empty 4f -shell) and stable-4f -moment substituent ions of varying size and electronic configuration, we are able to inject both 'Kondo holes' (isoelectronic ions without magnetic moments) and strongly localized magnetic moments into the coherent Kondo lattice. This has allowed us to probe the spin exchange between the Ce 3+ localized magnetic moments and the spins of the conduction electrons involved in Cooper pairing, Kondo screening and anomalous transport in a controlled way, revealing a surprising contrast between the response of coherent phenomena and non-Fermi-liquid behaviour to this perturbation. Figure 1 shows the evolution of both the superconducting transition temperature T c (identified by the transition in resistivity, ρ) and Kondo-lattice coherence temperature T coh (identified by the maximum in ρ(T )) for all rare-earth substitutions made in Ce 1−x R x CoIn 5 through the complete range of concentrations where both features exist. As shown, the...

We report x-ray diffraction, electrical resistivity, and magnetoresistance measurements on Bi2Se3 under high pressure and low temperature conditions. Pressure induces profound changes in both the room temperature value of the electrical resistivity as well as the temperature dependence of the resistivity. Initially, pressure drives Bi2Se3 toward increasingly insulating behavior and then, at higher pressures, the sample appears to enter a fully metallic state coincident with a change in the crystal structure. Within the low pressure phase, Bi2Se3 exhibits an unusual field dependence of the transverse magnetoresistance Δρ(xx) that is positive at low fields and becomes negative at higher fields. Our results demonstrate that pressures below 8 GPa provide a non-chemical means to controllably reduce the bulk conductivity of Bi2Se3.

No abstract

We have performed several high-pressure resistivity experiments on the recently discovered superconductors La [O 0.89