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Register a new userUncovering a pressure-tuned electronic transition in BiSrYCu2O8 using Raman scattering and x-ray diffraction
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We report pressure tuned Raman and x-ray diffraction data of Bi1.98Sr2.06Y0.68Cu2O8 revealing a critical pressure at 21 GPa with anomalies in six physical quantities: electronic Raman background, electron-phonon coupling, spectral weight transfer from high to low frequency, density dependent behaviour of phonon and magnon frequencies, and a compressibility change in the c-axis. For the first time in a cuprate, mobile charge carriers, lattice, and magnetism all show anomalies at a distinct critical pressure in the same experimental setting. Furthermore, the Raman spectral changes are similar to that seen traversing the superconducting dome with doping, suggesting that the critical pressure at 21 GPa is related to the much discussed critical point at optimal doping.
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We report the evolution of charge density wave states under pressure for two NbS3 phases triclinic (phase I) and monoclinic (phase II) at room temperature. Raman and X-ray diffraction (XRD) techniques are applied. The x-ray studies on the monoclinic phase under pressure show a compression of the lattice at different rates below and above 7 GPa but without a change in space group symmetry. The Raman spectra of the two phases evolve similarly with pressure; all peaks almost disappear in the 6-8 GPa range, indicating a transition from an insulating to a metallic state, and peaks at new positions appear above 8 GPa. The results suggest suppression of the ambient charge-density waves and their subsequent recovery with new orderings above 8 GPa.
Cubic boron phosphide BP has been studied in situ by X-ray diffraction and Raman scattering up to 55 GPa at 300 K in a diamond anvil cell. The bulk modulus of B0 = 174(2) GPa has been established, which is in excellent agreement with our ab initio calculations. The data on Raman shift as a function of pressure, combined with equation-of-state data, allowed us to estimate the Gruneisen parameters of the TO and LO modes of zinc-blende structure, {gamma}GTO = 1.16 and {gamma}GLO = 1.04, just like in the case of other AIIIBV diamond-like phases, for which {gamma}GTO > {gamma}GLO = 1. We also established that the pressure dependence of the effective electro-optical constant {alpha} is responsible for a strong change in relative intensities of the TO and LO modes from ITO/ILO ~0.25 at 0.1 MPa to ITO/ILO ~2.5 at 45 GPa, for which we also find excellent agreement between experiment and theory.
Two magnon excitations and the nodal spin density wave (SDW) gap were observed in BaFe2As2 by Raman scattering. Below the SDW transition temperature (TSDW) nodal SDW gap opens together with new excitations in reconstructed electronic states. The two-magnon peak remains above TSDW and moreover the energy increases a little. The change from the long-range ordered state to the short-range correlated state is compared to the cuprate superconductors.
We formulate a theory for the polarization-dependence of the electronic (pair-breaking) Raman response for the recently discovered non-centrosymmetric superconductors in the clean limit at zero temperature. Possible applications include the systems CePt$_3$Si and Li$_2$Pd$_x$Pt$_{3-x}$B which reflect the two important classes of the involved spin-orbit coupling. We provide analytical expressions for the Raman vertices for these two classes and calculate the polarization dependence of the electronic spectra. We predict a two-peak structure and different power laws with respect to the unknown relative magnitude of the singlet and triplet contributions to the superconducting order parameter, revealing a large variety of characteristic fingerprints of the underlying condensate.
We present Raman experiments on underdoped and overdoped Bi2Sr2CaCu2O(8+d) (Bi-2212) single crystals. We reveal the pseudogap in the electronic Raman spectra in the B1g and B2g geometries. In these geometries we probe respectively, the antinodal (AN) and nodal (N) regions corresponding to the principal axes and the diagonal of the Brillouin zone. The pseudogap appears in underdoped regime and manifests itself in the B1g spectra by a strong depletion of the low energy electronic continuum as the temperature decreases. We define a temperature T* below which the depletion appears and the pseudogap energy, omegaPG the energy at which the depeletion closes. The pseudogap is also present in the B2g spectra but the depletion opens at higher energy than in the B1g spectra. We observe the creation of new electronic states inside the depletion as we enter the superconducting phase. This leads us to conclude (as proposed by S. Sakai et al.) that the pseudogap has a different structure than the superconducting gap and competes with it. We show that the nodal quasiparticle dynamic is very robust and almost insensitive to the pseudogap phase contrary to the antinodal quasiparticle dynamic. We finally reveal, in contrast to what it is usually admitted,an increase of the nodal quasiparticle spectral weight with underdoping. We interpret this result as the consequence of a possible Fermi surface disturbances in the doping range p=0.1-0.2.
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