Pressure effects on the superconducting transition innH-CaAlSi

Abstract

We present a combined experimental and theoretical study of the effects of pressure on Tc of the hexagonal layered superconductors nH -CaAlSi (n = 1, 5, 6), where nH labels the different stacking variants that were recently discovered. Experimentally, the pressure dependence of Tc has been investigated by measuring the magnetic susceptibility of single crystals up to 10 kbar. In contrast to previous results on polycrystalline samples, single crystals with different stacking sequences display different pressure dependences of Tc. 1H-CaAlSi shows a decrease of Tc with pressure, whereas 5H and 6H-CaAlSi exhibit an increase of Tc with pressure. Ab-initio calculations for 1H, 5H and 6H -CaAlSi reveal that an ultrasoft phonon branch associated to out-of-plane vibrations of the Al-Si layers softens with pressure, leading to a structural instability at high pressures. For 1H-CaAlSi the softening is not sufficient to cause an increase of Tc, which is consistent with the present experiments, but adverse to previous reports. For 5H and 6H the softening provides the mechanism to understand the observed increase of Tc with pressure. Calculations for hypothetical 2H and 3H stacking variants reveal qualitative and quantitative differences.

Crystal structure, electronic structure, and vibrational properties ofMAlSiH(M=Ca,Sr,Ba)<

“…By this, coupling strength is lost and T c reduces accordingly. 31 Our phonon dispersions for MAlSi are in good agreement with previous calculations, 9,32,33 apart from the fact that soft-mode frequencies obtained were imaginary for CaAlSi and too low for SrAlSi and BaAlSi. A recent study of 1H-CaAlSi ͑the polymorph with the simplest structure͒ achieved excellent agreement between experimental phonon dispersion obtained by inelastic x-ray scattering and DFT calculations using linear response in combination with the mixed pseudopotential method.…”

“…The Brillouin-zone integration was performed over a 14ϫ 14 ϫ 14 Monkhorst-Pack grid. 36 The result was that the mode remains stable even along the A-L direction, which agrees with the result obtained by Giantomassi et al 9 and also Heid et al 31 and Boeri et al 33 We consider the stable lowfrequency mode physically more reasonable, but for the sake of methodological consistency we will present the VASPdirect method result in Sec. III C.…”

“…We noticed, however, that the frequency of the soft mode is highly susceptible to c / a ratio variations. Further, Huang et al 32 and Boeri et al 33 showed that the low-frequency mode for CaAlSi becomes softer and an imaginary frequency occurs at a large volume of the Brillouin zone when pressure is increased. We also performed ABINIT computations 34,35 of the phonon dispersion for CaAlSi using the Perdew-Wang-Ceperley-Alder functional to approximate the exchangecorrelation and Troullier-Martins pseudopotentials.…”

Resonant generation of coherent phonons in a superconductor by ultrafast optical pump pulses

“…One interesting class of examples are superconductors that are close to a structural transition that is driven by a soft optical phonon, i.e., e.g., CaC 6 , 40,41 or CaAlSi. 42,43 The coherent phonon oscillations are experimentally observable, for example, as periodic modulations in time-resolved reflectivity measurements. Driving a superconductor into the regime of nonadiabatic coherent dynamics requires ultrashort laser pulses with frequencies of the order of the superconducting gap, i.e., in the terahertz regime.…”

Ab initio investigation of superconductivity in orthorhombic MgPtSi

“…Since the discovery of superconductivity in MgB 2 with a transition temperature T c of 39.7 K [1], countless studies [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] of this as well as related compounds [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32], have been carried out to establish the origin of superconductivity in them. It is well known that the high temperature superconductor MgB 2 [1] has the layered hexagonal AlB 2 type structure, where Boron atoms form a honeycomb network, consisting of graphite-like sheets separated by hexagonal layers of Mg atoms.…”