14 research outputs found
Sharp Raman Anomalies and Broken Adiabaticity at a Pressure Induced Transition from Band to Topological Insulator in Sb2Se3
The nontrivial electronic topology of a topological insulator is thus far
known to display signatures in a robust metallic state at the surface. Here, we
establish vibrational anomalies in Raman spectra of the bulk that signify
changes in electronic topology: an E2 g phonon softens unusually and its
linewidth exhibits an asymmetric peak at the pressure induced electronic
topological transition (ETT) in Sb2Se3 crystal. Our first-principles
calculations confirm the electronic transition from band to topological
insulating state with reversal of parity of electronic bands passing through a
metallic state at the ETT, but do not capture the phonon anomalies which
involve breakdown of adiabatic approximation due to strongly coupled dynamics
of phonons and electrons. Treating this within a four-band model of topological
insulators, we elucidate how nonadiabatic renormalization of phonons
constitutes readily measurable bulk signatures of an ETT, which will facilitate
efforts to develop topological insulators by modifying a band insulator
Discovery of highly spin-polarized conducting surface states in the strong spin-orbit coupling semiconductor SbSe
Majority of the AB type chalcogenide systems with strong spin-orbit
coupling, like BiSe, BiTe and SbTe etc., are
topological insulators. One important exception is SbSe, where a
topological non-trivial phase was argued to be possible under ambient
conditions, but such a phase could be detected to exist only under pressure. In
this Letter, we show that like BiSe, SbSe, displays generation
of highly spin-polarized current under mesoscopic superconducting point
contacts as measured by point contact Andreev reflection spectroscopy. In
addition, we observe a large negative and anisotropic magnetoresistance in
SbSe, when the field is rotated in the basal plane. However, unlike in
BiSe, in case of SbSe a prominent quasiparticle interference
(QPI) pattern around the defects could be obtained in STM conductance imaging.
Thus, our experiments indicate that SbSe is a regular band insulator
under ambient conditions, but due to it's high spin-orbit coupling, non-trivial
spin-texture exists on the surface and the system could be on the verge of a
topological insulator phase.Comment: 5 pages, 4 figures, supplemental material not include
Charge Density Wave Order in the Topological Insulator
Hexagonally deformed Fermi surfaces and strong nesting, found in topological
insulators (TIs) such as and over the past decade, have
led to several predictions of possible Density Wave order in these systems.
Recent evidence for strong Fermi nesting in superconducting and
has led to further speculation about the importance of charge
order in the context of unconventional superconductivity. Here, we report what
we believe is the first direct observation of Charge Density Wave (CDW) order
in . Our results include the observation of a 140K
metal-insulator-metal transition in resistivity as a function of temperature.
We corroborate this with nuclear magnetic resonance (NMR) studies of the
spin-lattice relaxation rate of the nucleus, which
also displays a transition at 140K. Additionally, we use electron diffraction
to reveal a periodic lattice distortion (PLD) in , together with
diffuse charge order between and . This
diffuse scattering points toward the presence of an incommensurate charge
density wave (I-CDW) above room temperature, which locks into a CDW upon
cooling below . We also observe two additional transitions in
near 200K and 15K. The transition at 200K appears to display
some anisotropy with the direction of applied magnetic field. In this report,
we focus on the CDW transition at 140K. We include some speculation of the two
other transitions observed at 15K and 200K by NMR, also revealed here for the
first time.Comment: 16 pages, 5 figure
Two-dimensional Vortices in Superconductors
Superconductors have two key characteristics. They expel magnetic field and
they conduct electrical current with zero resistance. However, both properties
are compromised in high magnetic fields which can penetrate the material and
create a mixed state of quantized vortices. The vortices move in response to an
electrical current dissipating energy which destroys the zero resistance
state\cite{And64}. One of the central problems for applications of high
temperature superconductivity is the stabilization of vortices to ensure zero
electrical resistance. We find that vortices in the anisotropic superconductor
BiSrCaCuO (Bi-2212) have a phase transition from
a liquid state, which is inherently unstable, to a two-dimensional vortex
solid. We show that at high field the transition temperature is independent of
magnetic field, as was predicted theoretically for the melting of an ideal
two-dimensional vortex lattice\cite{Fis80,Gla91}. Our results indicate that the
stable solid phase can be reached at any field as may be necessary for
applications involving superconducting magnets\cite{Has04,Sca04,COHMAG}. The
vortex solid is disordered, as suggested by previous studies at lower
fields\cite{Lee93,Cub93}. But its evolution with increasing magnetic field
displays unexpected threshold behavior that needs further investigation.Comment: 5 pages and 4 figures. submitted to Nature Physic
Synthesis of YBa<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB> in air through complex precursor formation and fine-particle techniques
We report the successful synthesis in air of the superconductor YBa<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB> (1-2-4) without the need of high oxygen pressure or an alkali-carbonate based catalyst. The technique involves formation, in solution, of a fine-particle-complex based precursor. The 1-2-4 phase then forms in air with a single firing of 30 min at 900°C and a T<SUB>c</SUB> near 80 K. We also report some results of attempts to prepare 1-2-4 using precursors from sol-gel and spray drying
Influence of magnetic impurities on current transport in epitaxial thin films of Y<SUB>1</SUB>Ba<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7-δ</SUB>
The critical current density (Jc) and the nature of its temperature dependence is examined for the first time for the case of Fe doped epitaxial thin films of Y1Ba2Cu3O7-δ. These Jc systematics are analysed in terms of flux pinning at Twin boundary (TB) planes, tunneling across such planes and influence of magnetic impurities on superconducting gap parameter via Cooper pair breaking
Thin and ultra-thin epitaxial films of YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7−δ</SUB> deposited on LiNbO<SUB>3</SUB> substrates by pulsed excimer laser ablation
We present here detailed transport, magnetic, structural and morphological studies on thin and ultra-thin films of superconducting YBa<SUB>2</SUB>Cu<SUB>3</SUB>O<SUB>7−δ</SUB> (1-2-3) deposited on single crystalline substrates of LiNbO<SUB>3</SUB>. The films were deposited using the technique of excimer laser ablation and have been found to have their c-axis oriented perpendicular to the plane of the film. We report results for films of thickness 500 Å, 1000 Å, 1600 Å and 6000 Å. Superconducting transition temperatures (T<SUB>c</SUB>) have been studied by DC-resistivity, AC- and DC-magnetization. Critical current densities (J<SUB>c</SUB>) from transport measurements are found to be in excess of 10<SUP>5</SUP> A/cm<SUP>2</SUP> at 77 K. The temperature dependence of J<SUB>c</SUB> is also examined and the flux pinning energy is estimated within the flux creep picture. High field hysteresis loop studies are in agreement with the above results and yield meaningful indications of critical fields H<SUB>c</SUB> and irreversibility fields H<SUB>r</SUB>. A study of the evolution of film morphology with increase of film thickness is also reported and it reflects on the growth mechanism of 1-2-3 on LiNbO<SUB>3</SUB>
STM investigation of BSCCO 2212 and borocarbide materials
We present spectroscopic Scanning Tunneling Microscope (STM) measurements performed at 4.2 K on BSCCO 2212 single crystals and Y, Lu and Er based borocarbide compounds. The conductance versus voltage spectra on BSCCO 2212 reveal a reproducible dip feature near einfo:eu-repo/semantics/publishe
Epitaxial thin films of YBa<SUB>2</SUB>Cu<SUB>4</SUB>O<SUB>8</SUB> superconductor deposited by laser ablation of solution derived complex targets
We report here the successful fabrication of single phase epitaxial thin films of the superconductor YBa2Cu4O8 (1-2-4) deposited on monocrystalline (100) substrates of yttriastabilized ZrO2 using the technique of pulsed-laser ablation. The targets of 1-2-4 were fabricated through a novel low temperature synthesis technique whereby it was possible to synthesize reacted 1-2-4 pellets without the application of high oxygen pressure. In this report, the target material was composed of an unreacted solution derived Y-Ba-Cu (1-2-4)-complex. The films have superconducting transition temperatures varying from 70-80K and critical current densities higher than 5×106 A/cm2 below 60K. X-ray diffractograms indicate that the films are oriented with their c-axis perpendicular to the surface of the substrate