Enhancing the superconducting transition temperature of the heavy fermion compound CeIrIn5 in the absence of spin correlations

We report on a pressure- (P-)induced evolution of superconductivity and spin correlations in CeIrIn(5) via the (115)In nuclear-spin-lattice-relaxation rate measurements. We find that applying pressure suppresses dramatically the antiferromagnetic fluctuations that are strong at ambient pressure. At P = 2.1 GPa, T(c) increases to T(c) = 0.8 K, which is twice T(c) (P = 0 GPa), in the background of Fermi-liquid state. This is in sharp contrast to the previous case in which a negative, chemical pressure (replacing Ir with Rh) enhances magnetic interaction and increases T(c). Our results suggest that multiple mechanisms work to produce superconductivity in the same compound CeIrIn(5).     

Superconducting phase diagram of Li metal in nearly hydrostatic pressures up to 67 GPa

The dependence of the superconducting transition temperature T(c) on nearly hydrostatic pressure has been determined to 67 GPa in an ac susceptibility measurement for a Li sample embedded in helium pressure medium. With increasing pressure, superconductivity appears at 5.47 K for 20.3 GPa, T(c) rising rapidly to approximately 14 K at 30 GPa. The T(c)(P) dependence to 67 GPa differs significantly from that observed in previous studies where no pressure medium was used. Evidence is given that superconductivity in Li competes with symmetry breaking structural phase transitions which occur near 20, 30, and 62 GPa. In the pressure range 20-30 GPa, T(c) is found to decrease rapidly in a dc magnetic field, the first evidence that Li is a type I superconductor.     

压力对HgBa2Ca2Cu3O8+y超导转变温度的影响

 利用低温超高压装置，测量了Hg系样品HgBa₂Ca₂Cu₃O_8+y（Hg-1223）超导转变温度T_c在压力作用下的增强效应。压力最高达7.8 GPa，超导起始转变温度常压下为130 K，加压到5.4 GPa时获得最高温度为140 K。在5.4 GPa以下获得dT_c/dp为1.85 K/GPa。用压力作用下氧原子位置的改变使载流子浓度提高和CuO₂面间的耦合作用来解释高温超导的压力效应。     

Anomalous pressure dependence of the superconducting transition temperature of beta-pyrochlore AOs2O6 oxides

High-pressure effects on the superconducting transitions of beta-pyrochlore oxide superconductors AOs(2)O(6) (A = Cs,Rb,K) are studied by measuring resistivity under high pressures up to 10 GPa. The superconducting transition temperature T(c) first increases with increasing pressure in every compound and then exhibits a broad maximum at 7.6 K (6 GPa), 8.2 K (2 GPa), and 10 K (0.6 GPa) for A = Cs, Rb, and K, respectively. Finally, the superconductivity is suppressed completely at a critical pressure near 7 GPa and 6 GPa for A = Rb and K and probably above 10 GPa for A = Cs. Characteristic changes in the coefficient A of the T(2) term in resistivity and residual resistivity are observed, both of which are synchronized with the corresponding change in T(c).     

Pressure evolution of the EFG and magnetic hyperfine field in the layered antiferromagnet FeI2

When FeI₂ is subjected to pressures of up to 20 GPa, a change of approximately 20% occurs in the unit cell volume.⁵⁷Fe Mössbauer spectroscopy (MS) in a diamondanvil cell has been used to monitor the pressure evolution of the hyperfine interaction parameters of this layered antiferromagnetic insulator. The pressure dependence of the quadrupole splittingQS at 296 K exhibits a maximum at 12 GPa and the saturation magnetic hyperfine fieldH ₀ increases from 7.4 T at ambient pressure to 12 T at 18 GPa. A qualitative analysis identifies the pressure evolution ofQS with changes in the trigonal component of the crystal field splitting. The pressure variation ofH ₀ is attributed to an increase in the average value of the 3d charge density distribution.     

Pressure-induced superconductor-insulator transition in the spinel compound CuRh2S4

We performed resistivity measurements in CuRh2S4 under quasihydrostatic pressure of up to 8.0 GPa, and found a pressure-induced superconductor-insulator transition. Initially, with increasing pressure, the superconducting transition temperature T(c) increases from 4.7 K at ambient pressure to 6.4 K at 4.0 GPa, but decreases at higher pressures. With further compression, superconductivity in CuRh2S4 disappears abruptly at a critical pressure P(SI) between 5.0 and 5.6 GPa, when it becomes an insulator.     

50 GPa下NiO等温状态方程的高压X光研究

 本文采用在位的（in situ）高压X光衍射方法研究了近50 GPa和室温下三方结构NiO的等温压缩行为，并用Murnaghan状态方程对实验值进行了最小二乘法拟合，得到的NiO室温状态方程的相应参量分别为：B₀=223 GPa，B₀'=4.21。在室温压力范围内没有观察到第一类结构相变。NiO在六方指标下的轴比c/a随压力的变化在实验压力范围内可用c/a=2.450~1.569×10^-3（GPa）近似描述。     

Enhancement of superconductivity and evidence of structural instability in intercalated graphite CaC6 under high pressure

We measured the temperature dependent resistivity, varrho(T), of the intercalated graphite superconductor CaC6 as a function of pressure up to 16 GPa. We found a large linear increase of critical temperature, Tc, from the ambient pressure value 11.5 K up to 15.1 K, the largest value for intercalated graphite, at 7.5 GPa. At approximately 8 GPa, a jump of varrho and a sudden drop of Tc down to approximately 5 K indicates the occurrence of a phase transition. Our data analysis suggests that a pressure-induced phonon softening related to an in-plane Ca phonon mode is responsible for the Tc increase and that higher pressures greater, similar8 GPa lead to a structural transition into a new phase with a low Tc less, similar3 K.     

Metallic state in cubic FeGe beyond its quantum phase transition

We report on results of electrical resistivity and structural investigations on the cubic modification of FeGe under high pressure. The long-wavelength helical order (T(C) = 280 K) is suppressed at a critical pressure p(c) approximately 19 GPa. An anomaly at T(X)(p) and strong deviations from a Fermi-liquid behavior in a wide pressure range above p(c) suggest that the suppression of T(C) disagrees with the standard notion of a quantum critical phase transition. The metallic ground state persisting at high pressure can be described by band-structure calculations if zero-point motion is included. The shortest FeGe interatomic distance display discontinuous changes in the pressure dependence close to the T(C)(p) phase line.     

Heat Capacity Under Pressure of CeCoIn 5

Among heavy-fermion (HF) superconductors, CeCoIn 5 exhibits a record high value of T c =2.3 K at ambient pressure [1]. CeCoIn 5 belongs to a new class of HF-superconductors that crystallize in the tetragonal HoCoGa 5 -structure. This structure can be regarded as alternating layers of CeIn 3 and CoIn 2 . Bulk CeIn 3 undergoes a transition from an antiferromagnetic (AFM) state at ambient pressure ( T N =10.2 K) to a superconducting state with very low T C =0.15 K at a critical pressure p c =2.8 GPa [2] at which long range magnetic order vanishes. It is, therefore, regarded as a possible candidate for magnetically mediated superconductivity (SC). We report on measurements of the heat capacity of CeCoIn 5 at hydrostatic pressures p h 1.5 GPa. While T c increases with increasing pressure, the effective mass of the quasi-particles m eff decreases, as indicated by the ratio C / T | T c . As a working hypothesis based on theories of a nearly antiferromagnetic Fermi-liquid (NAFFL), this may be interpreted as the stabilization of the superconducting state by an increase of the characteristic spin fluctuation temperature T_{SF} (T_{SF}\propto k_F^2/m_{\rm eff}).     

Pressure-induced phase transition(s) in KMnF3 and the importance of the excess volume for phase transitions in perovskite structures

We report a pressure-dependent investigation of KMnF(3) by x-ray diffraction up to 30 GPa. The results are discussed in the framework of Landau theory and in relation to the isostructural phase transition in SrTiO(3). The phase transition temperature near 186 K in KMnF(3) shifts to room temperature at a critical pressure of P(c) = 3.4 GPa; the pressure dependence of the transition point follows ΔP(c)/ΔT(c) = 0.0315 GPa K(-1). The transition becomes second order under high pressure, close to the tricritical point. The phase transition is determined by the rotation of MnF(6) octahedra with their simultaneous expansion along the rotation axis. The rotation angle was found to increase to 10.5° at 24 GPa. An additional anomaly was observed at higher pressure around 25 GPa, suggesting a further phase transition.     

Phase inhomogeneity of the itinerant ferromagnet MnSi at high pressures

The pressure induced quantum phase transition of the weakly ferromagnetic metal MnSi is studied using zero-field 29Si NMR spectroscopy and relaxation. Below P(*) approximately 1.2 GPa, the intensity of the signal and the nuclear spin-lattice relaxation are independent of pressure, even though the amplitude of the magnetization drops by 20% from the ambient-pressure amplitude. For P>P(*), the decreasing intensity within the experimentally detectable bandwidth signals the onset of an inhomogeneous phase that persists to the highest pressure measured, P>/=1.75 GPa, which is well beyond the known critical pressure P(c)=1.46 GPa. Implications for the non-Fermi liquid behavior observed for P>P(c) are discussed.     

Pressue Dependence of the Ferroelectric-Paraelectric Transition Temperature in KNBO 3 . Comparision with the Ice VII-VIII Transition

New results on the phase diagram of KNbO 3 determined, from 10 to 650 K up to 30 GPa, with Raman scattering are given. On increasing pressure the Curie temperature T c was found to shift down regularly from 700 K, at atmospheric pressure, to 20 K at ～25 GPa. Up to 20 GPa, the pressure dependence of T c follows a classical regime described by the Ising model where the dipolar interaction follow a i 2 /v law, where i is the off-center displacement and v the unit cell volume. This behaviour found also for the ice VII-VIII is very likely encountered in a number of (anti)ferroelectric-paraelectric transformations involving positional disorder.     

压力对有机半导体均苯四甲酸晶体结构转变和电子性质的影响

研究高压条件下均苯四甲酸(C₁₀H₆O₈)材料的结构和性质对探索有机半导体材料的应用有积极意义.基于密度泛函理论的第一性原理赝势平面波方法,开展了0-300 GPa压强下C₁₀H₆O₈晶体的结构、电子和光学性质的研究.晶格常数在压强20 GPa和150 GPa下出现了明显跳变,且原子之间随着压强变化反复地出现成键/断键现象,表明压强可诱导晶体结构变化.电子结构的性质表明,0 GPa的C₁₀H₆O₈晶体是带隙为3.1 eV的直接带隙半导体,而压强增加到150 GPa时,带隙突变为0 eV,表明了晶体由半导体转变为导体.当压强为160 GPa时,晶体又变成了能隙约为1eV的间接带隙半导体,这可能是费米能级附近仅受O-2p轨道电子影响所导致.通过对C₁₀H₆O₈晶体介电函数的分析,再次验证了晶体在150 GPa时发生了结构相变.同时...     

Predicted superconductive properties of lithium under pressure

A superconducting state of lithium has not been found at ambient pressure, but the present theoretical work shows that high values of the critical temperature, T(c), may be expected for some high-pressure phases. Ab initio electronic structure calculations are used to calculate the electron-phonon coupling in a "rigid-muffin-tin approximation," and estimates using McMillan's formula suggest that under increasing pressure T(c) in fcc-Li may reach 50--70 K before transitions occur to the rhombohedral (hR1-Li) and subsequently to the cI16-Li phase near 40 GPa. In cI16-Li T(c) may reach a maximum in the range 60--80 K.     

High-pressure phase transition of Bi2Fe4O9

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.     

Compression of silver in a diamond anvil cell: Pressure dependences of strength and grain size from X-ray diffraction data

Two silver samples, coarse grained (c-Ag, grain size 300±30 nm) and nanocrystalline (n-Ag, grain size 55±6 nm), are compressed in a diamond anvil cell in separate experiments. The pressure is increased in steps of ∼3 GPa and the diffraction pattern recorded at each pressure. The grain size and compressive strength are determined from the analysis of the diffraction line-widths. The grain size of c-Ag decreases rapidly from 300±30 nm at ambient pressure to 40±8 nm at 15 GPa, and then gradually to 20±3 nm at 40 GPa. After pressure release to ambient condition, the grain size is 25±4 nm. The strength at ambient pressure is 0.18±0.05 GPa and increases to 1.0±0.3 GPa at 40 GPa. The grain size of n-Ag decreases from 55±6 nm at ambient pressure to 17±4 nm at 15 GPa and to 14±3 nm at 55 GPa. After release of pressure to ambient condition, the grain size is 50±7 nm. The strength increases from 0.51±0.07 GPa at ambient pressure to 3.5±0.4 GPa at 55 GPa. The strength is found to vary as the inverse of the square-root of the grain size. The results of the present measurements agree well with the grain-size dependence of strength derived from the hardness versus grain size data at ambient pressure available in the literature.     

Melting curve of MgO from first-principles simulations

First-principles calculations based on density functional theory, both with the local density approximation (LDA) and with generalized gradient corrections (GGA), have been used to simulate solid and liquid MgO in direct coexistence in the range of pressure 0 < or = p < or = 135 GPa. The calculated LDA zero pressure melting temperature is T(LDA)m = 3110 +/- 50 K, in good agreement with the experimental data. The GGA zero pressure melting temperature T(GGA)m = 2575 +/- 100 K is significantly lower than the LDA one, but the difference between the GGA and the LDA is greatly reduced at high pressure. The LDA zero pressure melting slope is dT/dp approximately 100 K/GPa, which is more than 3 times higher than the currently available experimental one from Zerr and Boehler [Nature (London) 371, 506 (1994)]. At the core mantle boundary pressure of 135 GPa MgO melts at Tm = 8140 +/- 150 K.     

Transition from the antiferromagnetic to a nonmagnetic state in FeBO3 under high pressure

A ⁵⁷FeBO₃ single crystal is studied by the nuclear forward scattering (NFS) method. The NFS time spectra from ⁵⁷Fe nuclei are recorded at room temperature under high pressures up to 50 GPa in a diamond anvil cell. In the pressure interval 0<p<44 GPa, the magnetic field H ^Fe at the ⁵⁷Fe nuclei is found to increase nonlinearly, reaching a maximum value of 48.1 T at p=44 GPa. As the pressure increases further and reaches the point p=46 GPa, the field H ^Fe abruptly drops to zero, indicating that a transition from the antiferromagnetic to a non-magnetic state occurs in the crystal. In the pressure interval 0<p<46 GPa, the magnetic moments of the iron ions lie in the (111) basal plane of the crystal. Several possible mechanisms of magnetic collapse are discussed.     

Pressure induced effects on the Fermi surface of superconducting 2H-NbSe2

The pressure dependence of the critical temperature T(c) and upper critical field H(c2)(T) has been measured up to 19 GPa in the layered superconducting material 2H-NbSe2. T(c)(P) has a maximum at 10.5 GPa, well above the pressure for the suppression of the charge density wave (CDW) order. Using an effective two-band model to fit H(c2)(T), we obtain the pressure dependence of the anisotropy in the electron-phonon coupling and Fermi velocities, which reveals the peculiar interplay between CDW order, Fermi surface complexity, and superconductivity in this system.