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Register a new userPressure dependence of the superconducting transition temperature in C$_6$Yb and C$_6$Ca
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We have studied the evolution, with hydrostatic pressure, of the recently discovered superconductivity in the graphite intercalation compounds C$_6$Yb and C$_6$Ca. We present pressure-temperature phase diagrams, for both superconductors, established by electrical transport and magnetization measurements. In the range 0-1.2 GPa the superconducting transition temperature increases linearly with pressure in both materials with $dT_c/dP = +0.39 K/GPa$ and $dT_c/dP = +0.50 K/GPa$ for C$_6$Yb and C$_6$Ca respectively. The transition temperature in C$_6$Yb, which has beenmeasured up to 2.3 GPa, reaches a peak at around 1.8 GPa and then starts to drop. We also discuss how this pressure dependence may be explained within a plasmon pairing mechanism.
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We derive analytic expressions for the critical temperatures of the superconducting (SC) and pseudogap (PG) transitions of the high-Tc cuprates as a function of doping. These are in excellent agreement with the experimental data both for single-layered materials such as LSCO, Bi2201 and Hg1201 and multi-layered ones, such as Bi2212, Bi2223, Hg1212 and Hg1223. Optimal doping occurs when the chemical potential vanishes, thus leading to an universal expression for the optimal SC transition temperatures. This allows for the obtainment of a quantitative description of the growth of such temperatures with the number of layers, N, which accurately applies to the $Bi$, $Hg$ and $Tl$ families of cuprates. We study the pressure dependence of the SC transition temperatures, obtaining excellent agreement with the experimental data for different materials and dopings. These results are obtained from an effective Hamiltonian for the itinerant oxygen holes, which includes both the electric repulsion between them and their magnetic interactions with the localized copper ions. We show that the former interaction is responsible for the SC and the latter, for the PG phases, the phase diagram of cuprates resulting from the competition of both. The Hamiltonian is defined on a bipartite oxygen lattice, which results from the fact that only the $p_x$ and $p_y$ oxygen orbitals alternatively hybridize with the $3d$ copper orbitals. From this, we can provide an unified explanation for the $d_{x^2-y^2}$ symmetry of both the SC and PG order parameters and obtain the Fermi pockets observed in ARPES experiments.
Electronic spectra of C$_6$H are measured in the $18,950-21,100$ cm$^{-1}$ domain using cavity ring-down spectroscopy of a supersonically expanding hydrocarbon plasma. In total, 19 (sub)bands of C$_6$H are presented, all probing the vibrational manifold of the B$^2Pi$ electronically excited state. The assignments are guided by electronic spectra available from matrix isolation work, isotopic substitution experiments (yielding also spectra for $^{13}$C$_6$H and C$_6$D), predictions from ab initio calculations as well as rotational fitting and vibrational contour simulations using the available ground state parameters as obtained from microwave experiments. Besides the $0_0^0$ origin band, three non-degenerate stretching vibrations along the linear backbone of the C$_6$H molecule are assigned: the $ u_6$ mode associated with the C-C bond vibration and the $ u_4$ and $ u_3$ modes associated with C$equiv$C triple bonds. For the two lowest $ u_{11}$ and $ u_{10}$ bending modes, a Renner-Teller analysis is performed identifying the $mu^2Sigma$($ u_{11}$) and both $mu^2Sigma$($ u_{10}$) and $kappa^2Sigma$($ u_{10}$) components. In addition, two higher lying bending modes are observed, which are tentatively assigned as $mu^2Sigma$($ u_9$) and $mu^2Sigma$($ u_8$) levels. In the excitation region below the first non-degenerate vibration ($ u_6$), some $^2Pi-^{2}Pi$ transitions are observed that are assigned as even combination modes of low-lying bending vibrations. The same holds for a $^2Pi-^{2}Pi$ transition found above the $ u_6$ level. From these spectroscopic data and the vibronic analysis a comprehensive energy level diagram for the B$^2Pi$ state of C$_6$H is derived and presented.
The superconducting and normal state characteristics of yttrium hexaboride (YB$_6$) have been investigated for the single crystals with a transition temperatures $T_c$ ranging between 6 K and 7.6 K. The extracted set of microscopic parameters [the coherence length $xi$(0) $sim$ 320$div$340 ${AA}$, the penetration depth $lambda$(0) $sim$ 1100$div$1600 ${AA}$ and the mean free path of charge carriers $l$ = 31$div$58 ${AA}$, the Ginzburg-Landau-Maki parameters $kappa$$_{1,2}$(0) $sim$ 3.3$div$4.8 and the superconducting gap $Delta$(0) $sim$ 10.3$div$14.8 K] confirms the type II superconductivity in dirty limit ($xi$$gg$ $l$) with a medium to strong electron-phonon interaction (the electron-phonon interaction constant $lambda_{e-ph}$ = 0.93$div$0.96) and $s$-type pairing of charge carriers in this compound [2$Delta$(0)$/k_BT_c$ $approx$ 4]. The comparative analysis of charge transport (resistivity, Hall and Seebeck coefficients) and thermodynamic (heat capacity, magnetization) properties in the normal state in YB$_6$ allowed to detect a transition into the cage-glass state at $T^*$ $sim$ 50 K with a static disorder in the arrangement of the Y$^{3+}$ ions. We argue that the significant $T_c$ variations in the YB$_6$ single crystals are determined by two main factors: (i) the superconductivity enhancement is related with the increase of the number of isolated vacancies, both at yttrium and boron sites, which leads to the development of an instability in the hexaboride lattice; (ii) the $T_c$ depression is additionally stimulated by the spin polarization of conduction electrons emerged and enhanced by the magnetic field in the vicinity of defect complexes in the YB$_6$ matrix.
We have investigated the temperature dependence of the H || c flux line lattice structural phase transition from square to hexagonal symmetry, in the tetragonal superconductor LuNi_2B_2C (T_c = 16.6 K). At temperatures below 10 K the transition onset field, H_2(T), is only weakly temperature dependent. Above 10 K, H_2(T) rises sharply, bending away from the upper critical field. This contradicts theoretical predictions of H_2(T) merging with the upper critical field, and suggests that just below the H_c2(T)-curve the flux line lattice might be hexagonal.
Pressure-stabilized hydrides are a new rapidly growing class of high-temperature superconductors which is believed to be described within the conventional phonon-mediated mechanism of coupling. Here we report the synthesis of yttrium hexahydride Im3m-YH$_6$ that demonstrates the superconducting transition with T$_c$ = 224 K at 166 GPa, much lower than the theoretically predicted (>270 K). The measured upper critical magnetic field B$_c$$_2$(0) of YH$_6$ was found to be 116-158 T, which is 2-2.5 times larger than the calculated value. A pronounced shift of T$_c$ in yttrium deuteride YD$_6$ with the isotope coefficient 0.4 supports the phonon-assisted superconductivity. Current-voltage measurements showed that the critical current I$_c$ and its density J$_c$ may exceed 1.75 A and 3500 A/mm$^2$ at 0 K, respectively, which is comparable with the parameters of commercial superconductors, such as NbTi and YBCO. The superconducting density functional theory (SCDFT) and anharmonic calculations suggest unusually large impact of the Coulomb repulsion in this compound. The results indicate notable departures of the superconducting properties of the discovered YH$_6$ from the conventional Migdal-Eliashberg and Bardeen-Cooper-Schrieffer theories.
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