79 research outputs found
Low-Energy Surface States in the Normal State of -PdBi2 Superconductor
Topological superconductors as characterized by Majorana surface states has
been actively searched for their significance in fundamental science and
technological implication. The large spin-orbit coupling in Bi-Pd binaries has
stimulated extensive investigations on the topological surface states in these
superconducting compounds. Here we report a study of normal-state electronic
structure in a centrosymmetric -PdBi2 within density functional theory
calculations. By investigating the electronic structure from the bulk to slab
geometries in this system, we predict for the first time that -PdBi2
can host orbital-dependent and asymmetric Rashba surface states near the Fermi
energy. This study suggests that -PdBi2 will be a good candidate to
explore the relationship between superconductivity and topology in condensed
matter physics
Theory of ultrafast quasiparticle dynamics in high-temperature superconductors: Pump fluence dependence
We present a theory for the time-resolved optical spectroscopy of
high-temperature superconductors at high excitation densities with strongly
anisotropic electron-phonon coupling. A signature of the strong coupling
between the out-of-plane, out-of-phase O buckling mode () and
electronic states near the antinode is observed as a higher-energy peak in the
time-resolved optical conductivity and Raman spectra, while no evidence of the
strong coupling between the in-plane Cu-O breathing mode and nodal electronic
states is observed. More interestingly, it is observed that under appropriate
conditions of pump fluence, this signature exhibits a re-entrant behavior with
time delay, following the fate of the superconducting condensate.Comment: 5 pages, 3 embedded eps figures, to appear in PR
Optical Properties of Organometallic Perovskite: An ab initio Study using Relativistic GW Correction and Bethe-Salpeter Equation
In the development of highly efficient photovoltaic cells, solid perovskite
systems have demonstrated unprecedented promise, with the figure of merit
exceeding nineteen percent of efficiency. In this paper, we investigate the
optical and vibrational properties of organometallic cubic perovskite
CH3NH3PbI3 using first-principles calculations. For accurate theoretical
description, we go beyond conventional density functional theory (DFT), and
calculated optical conductivity using relativist quasi-particle (GW)
correction. Incorporating these many-body effects, we further solve
Bethe-Salpeter equations (BSE) for excitons, and found enhanced optical
conductivity near the gap edge. Due to the presence of organic methylammonium
cations near the center of the perovskite cell, the system is sensitive to low
energy vibrational modes. We estimate the phonon modes of CH3NH3PbI3 using
small displacement approach, and further calculate the infrared absorption (IR)
spectra. Qualitatively, our calculations of low-energy phonon frequencies are
in good agreement with our terahertz measurements. Therefore, for both energy
scales (around 2 eV and 0-20 meV), our calculations reveal the importance of
many-body effects and their contributions to the desirable optical properties
in the cubic organometallic perovskites system.Comment: 5 pages, 4 figure
Induced Ferromagnetism at BiFeO3/YBa2Cu3O7 Interfaces
Transition metal oxides (TMOs) exhibit many emergent phenomena ranging from
high-temperature superconductivity and giant magnetoresistance to magnetism and
ferroelectricity. In addition, when TMOs are interfaced with each other, new
functionalities can arise, which are absent in individual components. Here, we
report results from first-principles calculations on the magnetism at the
BiFeO3/YBa2Cu3O7 interfaces. By comparing the total energy for various magnetic
spin configurations inside BiFeO3, we are able to show that a metallic
ferromagnetism is induced near the interface. We further develop an interface
exchange-coupling model and place the extracted exchange coupling interaction
strengths, from the first-principles calculations, into a resultant generic
phase diagram. Our conclusion of interfacial ferromagnetism is confirmed by the
presence of a hysteresis loop in field-dependent magnetization data. The
emergence of interfacial ferromagnetism should have implications to electronic
and transport properties.Comment: 13 pages, 4 figure
Coupling Between An Optical Phonon and the Kondo Effect
We explore the ultra-fast optical response of Yb_{14}MnSb_{11}, providing
further evidence that this Zintl compound is the first ferromagnetic,
under-screened Kondo lattice. These experiments also provide the first
demonstration of coupling between an optical phonon mode and the Kondo effect.Comment: 4 Pages, 3 Figures, submitted to Phys. Rev. Let
Penetration depth study of LaOsSb: Multiband s-wave superconductivity
We measured the magnetic penetration depth in single crystals of
LaOsSb (=0.74 K) down to 85 mK using a tunnel diode
oscillator technique. The observed low-temperature exponential dependence
indicates a s-wave gap. Fitting the low temperature data to BCS s-wave
expression gives the zero temperature gap value which is significantly smaller than the BCS value of 1.76. In
addition, the normalized superfluid density shows an unusually long
suppression near , and are best fit by a two-band s-wave model.Comment: 5 pages, 2 figure
Observation of Competing Order in a High- Superconductor with Femtosecond Optical Pulses
We present studies of the photoexcited quasiparticle dynamics in
TlBaCaCuO (Tl-2223) using femtosecond optical
techniques. Deep into the superconducting state (below 40 K), a dramatic change
occurs in the temporal dynamics associated with photoexcited quasiparticles
rejoining the condensate. This is suggestive of entry into a coexistence phase
which, as our analysis reveals, opens a gap in the density of states (in
addition to the superconducting gap), and furthermore, competes with
superconductivity resulting in a depression of the superconducting gap.Comment: 5 pages, 3 figure
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