Search results for "Condensed Matter - Strongly Correlated Electrons"
showing 10 items of 322 documents
Orbital character variation of the Fermi surface and doping dependent changes of the dimensionality in BaFe2−xCoxAs2 from angle-resolved photoemissio…
2009
From a combination of high resolution angle-resolved photoemission spectroscopy and density functional calculations, we show that BaFe2As2 possesses essentially two-dimensional electronic states, with a strong change of orbital character of two of the Gamma-centered Fermi surfaces as a function of kz. Upon Co doping, the electronic states in the vicinity of the Fermi level take on increasingly three-dimensional character. Both the orbital variation with kz and the more three-dimensional nature of the doped compounds have important consequences for the nesting conditions and thus possibly also for the appearance of antiferromagnetic and superconducting phases.
Localised vibrations in superconducting YBCO revealed by ultra-fast optical coherent spectroscopy
2014
The interaction between phonons and high-energy excitations of electronic origin in cuprates and their role in the superconducting mechanisms is still controversial. Here we use coherent vibrational time-domain spectroscopy together with density functional and dynamical mean-field theory calculations to establish a direct link between the c-axis phonon modes and the in-plane electronic charge excitations in optimally doped YBCO. The non-equilibrium Raman tensor is measured by means of the broadband 'coherent-phonon' response in pump-probe experiments and is qualitatively described by our model using DFT in frozen phonon approximation plus single band DMFT to account for the electronic corre…
Giant resonant enhancement for photo-induced superconductivity in K$_3$C$_{60}$
2023
Photo-excitation at terahertz and mid-infrared frequencies has emerged as a new way to manipulate functionalities in quantum materials, in some cases creating non-equilibrium phases that have no equilibrium analogue. In K$_3$C$_{60}$, a metastable zero-resistance phase was documented with optical properties and pressure dependences compatible with non-equilibrium high temperature superconductivity. Here, we report the discovery of a dominant energy scale for this phenomenon, along with the demonstration of a giant increase in photo-susceptibility near 10 THz excitation frequency. At these drive frequencies a metastable superconducting-like phase is observed up to room temperature for fluenc…
Phase Diagram of Heavy Fermion Metal CeCoIn5
2006
We present a comprehensive analysis of the low temperature experimental H-T phase diagram of CeCoIn5. The main universal features of the diagram can be explained within the Fermi-liquid theory provided that quasiparticles form so called fermion-condensate state. We show that in this case the fluctuations accompanying an ordinary quantum critical point are strongly suppressed and cannot destroy the quasiparticles. Analyzing the phase diagram and giving predictions, we demonstrate that the electronic system of CeCoIn5 provides a unique opportunity to study the relationship between quasiparticles properties and non-Fermi liquid behavior.
Pressure induced collapse of the charge density wave and Higgs mode visibility in 2H-TaS$_2$
2018
The pressure evolution of the Raman active electronic excitations of the transition metal dichalcogenides 2H-TaS$_2$ is followed through the pressure phase diagram embedding incommensurate charge-density-wave and superconducting states. At high pressure, the charge-density-wave is found to collapse at 8.5~GPa. In the coexisting charge-density-wave and superconducting orders, we unravel a strong in-gap superconducting mode, attributed to a Higgs mode, coexisting with the expected incoherent Cooper-pair breaking signature. The latter remains in the pure superconducting state reached above 8.5~GPa. Our report constitutes the first observation of such Raman active Higgs mode since the longstand…
Superconductivity induced by structural reorganization in the electron-doped cuprate Nd$_{2-x}$Ce$_x$CuO$_4$
2020
Electron-doped and hole-doped superconducting cuprates exhibit a symmetric phase diagram as a function of doping. This symmetry is however only approximate. Indeed, electron-doped cuprates become superconductors only after a specific annealing process: This annealing affects the oxygen content by only a tiny amount, but has a dramatic impact on the electronic properties of the sample. Here we report the occurrence of superconductivity in oxygen-deficient Nd$_{2-x}$Ce$_x$CuO$_4$ thin films grown in an oxygen-free environment, after annealing in pure argon flow. As verified by x-ray diffraction, annealing induces an increase of the interlayer distance between CuO$_2$ planes in the crystal str…
Flat bands and the physics of strongly correlated Fermi systems
2018
Some materials can have the dispersionless parts in their electronic spectra. These parts are usually called flat bands and generate the corps of unusual physical properties of such materials. These flat bands are induced by the condensation of fermionic quasiparticles, being very similar to the Bose condensation. The difference is that fermions to condense, the Fermi surface should change its topology, leading to violation of time-reversal (T) and particle-hole (C) symmetries. Thus, the famous Landau theory of Fermi liquids does not work for the systems with fermion condensate (FC) so that several experimentally observable anomalies have not been explained so far. Here we use FC approach t…
Microwave Properties of Nb/PdNi/Nb Trilayers
2012
We combine wideband (1-20 GHz) Corbino disk and dielectric resonator (8.2 GHz) techniques to study the microwave properties in Nb/PdNi/Nb trilayers, grown by UHV dc magnetron sputtering, composed by Nb layers of nominal thickness $d_S$=15 nm, and a ferromagnetic PdNi layer of thickness $d_F$= 1, 2, 8 and 9 nm. We focus on the vortex state. Magnetic fields up to $H_{c2}$ were applied. The microwave resistivity at fixed $H/H_{c2}$ increases with $d_F$, eventually exceeding the Bardeen Stephen flux flow value.
The role of correlations in the high-pressure phase of FeSe
2011
This study addresses the structural and electronic properties of the NiAs- and MnP-type phases dominating in FeSe at high pressures. The analysis is performed using first-principle band structure calculations within the framework of the B3LYP hybrid exchange-correlation functional. Based on the volume-pressure relation deduced from the available experimental data, we optimize the form and internal coordinates of the unit cell, which agree reasonably well with experiment. In particular, the present calculations resolve the structural NiAs-MnP phase transition which occurs at about 10 GPa. Both structures are found to be semiconducting at low pressures and metallizing at about 80-90 GPa. Usin…
Multiple band crossings and Fermi surface topology: Role of double nonsymmorphic symmetries in MnP-type crystal structures
2019
We use relativistic ab-initio methods combined with model Hamiltonian approaches to analyze the normal-phase electronic and structural properties of the recently discovered WP superconductor. Remarkably, the outcomes of such study can be employed to set fundamental connections among WP and the CrAs and MnP superconductors belonging to the same space group. One of the key features of the resulting electronic structure is represented by the occurrence of multiple band crossings along specific high symmetry lines of the Brilloiun zone. In particular, we demonstrate that the eight-fold band degeneracy obtained along the SR path at (kx,ky)=(Pi,Pi) is due to inversion-time reversal invariance and…