We have conducted temperature dependent Angle Resolved Photoemission Spectroscopy (ARPES) study of the electronic structure of n-, p- type PbTe, PbSe and PbS, which are pre- mier thermoelectric materials. Our ARPES measurements on them provide direct
evidence for the light hole upper valence bands (UVBs) and the so-called heavy hole lower valence bands (LVBs), and an unusual temperature dependent relative movement between their band maxima leading to a monotonic decrease in the energy separation between LVBs and UVBs with increase in temperature. This enables convergence of these valence bands and consequently, an effective increase in the valley degeneracy in PbQ at higher temperatures, which has long been speculated to be the driving factor behind their extraordinary thermoelectric performance.
The energy gap for electronic excitations is one of the most important characteristics of the superconducting state, as it directly refects the pairing of electrons. In the copper-oxide high temperature superconductors (HTSCs), a strongly anisotropic
energy gap, which vanishes along high symmetry directions, is a clear manifestation of the d-wave symmetry of the pairing. There is, however, a dramatic change in the form of the gap anisotropy with reduced carrier concentration (underdoping). Although the vanishing of the gap along the diagonal to the square Cu-O bond directions is robust, the doping dependence of the large gap along the Cu-O directions suggests that its origin might be different from pairing. It is thus tempting to associate the large gap with a second order parameter distinct from superconductivity. We use angle-resolved photoemission spectroscopy (ARPES) to show that the two-gap behavior, and the destruction of well defined electronic excitations, are not universal features of HTSCs, and depend sensitively on how the underdoped materials are prepared. Depending on cation substitution, underdoped samples either show two-gap behavior or not. In contrast, many other characteristics of HTSCs, such as the domelike dependence of Tc on doping, long-lived excitations along the diagonals to the Cu-O bonds, energy gap at the antinode (crossing of the underlying Fermi surface and the (pi, 0)-(pi, pi) line) decreasing monotonically with doping, while persisting above Tc (the pseudogap), are present in all samples, irrespective of whether they exhibit two-gap behavior or not. Our results imply that universal aspects of high Tc superconductivity are relatively insensitive to differences in the electronic states along the Cu-O bond directions.
Non-equilibrium self-organized patterns formed by particles interacting through competing range interaction are driven over a substrate by an external force. We show that, with increasing driving force, the pre-existed static patterns evolve into dyn
amic patterns either via disordered phase or depinned patterns, or via the formation of non-equilibrium stripes. Strikingly, the stripes are formed either in the direction of the driving force or in the transverse direction, depending on the pinning strength. The revealed dynamical patterns are summarized in a dynamical phase diagram.
The pressure-dependent relation between Eu valence and lattice structure in model compound EuO is studied with synchrotron-based x-ray spectroscopic and diffraction techniques. Contrary to expectation, a 7% volume collapse at $approx$ 45 GPa is accom
panied by a reentrant Eu valence transition into a $emph{lower}$ valence state. In addition to highlighting the need for probing both structure and electronic states directly when valence information is sought in mixed-valent systems, the results also show that widely used bond-valence methods fail to quantitatively describe the complex electronic valence behavior of EuO under pressure.
Resistively Detected Nuclear Magnetic Resonance (RD-NMR) has been used to investigate a two-subband electron system in a regime where quantum Hall pseudo-spin ferromagnetic (QHPF) states are prominently developed. It reveals that the easy-axis QHPF s
tate around the total filling factor $ u =4 $ can be detected by the RD-NMR measurement. Approaching one of the Landau level (LL) crossing points, the RD-NMR signal strength and the nuclear spin relaxation rate $1/T_{1}$ enhance significantly, a signature of low energy spin excitations. However, the RD-NMR signal at another identical LL crossing point is surprisingly missing which presents a puzzle.
Temperature dependence of the longitudinal and Hall resistance is studied in the regime of localization-delocalization transition. We carry out measurements of a scaling exponent $kappa$ in the Landau level mixing region at several filling factors. T
he localization exponent $gamma$ is extracted using an approach based on the variable range hopping theory. The values of $gamma$ and $kappa$ are found to be universal, independent of filling factor in our sample. We can conclude that although Landau level mixing can change the degeneracy of a quantum Hall state, the value of the scaling exponent remains the same for a given sample that contains a fixed disorder profile.
In condensed matter physics, the study of electronic states with SU(N) symmetry has attracted considerable and growing attention in recent years, as systems with such a symmetry can often have a spontaneous symmetry-breaking effect giving rise to a n
ovel ground state. For example, pseudospin quantum Hall ferromagnet of broken SU(2) symmetry has been realized by bringing two Landau levels close to degeneracy in a bilayer quantum Hall system. In the past several years, the exploration of collective states in other multi-component quantum Hall systems has emerged. Here we show the conventional pseudospin quantum Hall ferromagnetic states with broken SU(2) symmetry collapsed rapidly into an unexpected state with broken SU(4) symmetry, by in-plane magnetic field in a two-subband GaAs/AlGaAs two-dimensional electron system at filling factor around $ u=4$. Within a narrow tilting range angle of 0.5 degrees, the activation energy increases as much as 12 K. While the origin of this puzzling observation remains to be exploited, we discuss the possibility of a long-sought pairing state of electrons with a four-fold degeneracy.
