Nature Of Low-lying Excitations Research Articles

Effects of Quantum Spin-1/2Impurities on the Magnetic Properties of Zigzag Spin Chains

We investigate the effect of Co^{2+} (spin-1/2) impurities on the magnetic ground state and low-lying spin excitations of the quasione-dimensional spin-1/2 antiferromagnet SrCuO_{2} by means of neutron scattering, muon spin spectroscopy, and bulk (ac and dc) magnetic susceptibilities. We found that dilute Co doping induces an Ising-like anisotropy and enhances the magnetic ordering temperature rather significantly, but preserves the gapless nature of the spin excitations. These results are in apparent contradiction with the recent studies of Ni (spin-1) doped SrCuO_{2}. Low-temperature magnetic behavior of the Co-doped zigzag chains in SrCuO_{2} reveals the presence of a weak geometrical spin frustration.

Inelastic Neutron Scattering on160Gd

The nature of low-lying excitations, Kπ=0+ bands in deformed nuclei remain enigmatic in the field, especially in relationship to quadrupole vibrations. One method of characterizing these states beyond excitation energies is through measurements of absolute transition probabilities. In the rare earth region of deformation, there are five stable Gd isotopes, 154Gd, 156Gd, and 158Gd have been studied to obtain B(E2) values, a fourth, 160Gd is the focus of this work. We have examined 160Gd with the (n, n′γ) reaction and neutron energies up to 3.0 MeV to confirm known 0+ states.

Slicing the cuprate Fermi surface to reveal underlying order

A complete theoretical picture of high-Tc superconductivity in the cuprates is still lacking despite more than two decades of research. One of the central puzzles in these materials is related to the nature of the low-lying electronic excitations that seems to differ radically between the underdoped and overdoped regimes that define the edges of the superconducting dome. While there is little doubt that overdoped cuprates are bad metals with a large Fermi surface, the nature of low-lying excitations in underdoped cuprates is a matter of intensive debate. The recent discovery of quantum oscillations (QO) in the electrical resistance of the low-disorder ortho-II YBa2Cu3O6.5[1] under a magnetic field strong enough to suppress superconductivity has opened up a new direction of research in the high Tc area and re-inserted the concept of a Fermi liquid (FL) into the underdoped region, where it was long thought that fermionic coherence is completely lost. The issue of QO has been the subject of intensive debates in the literature over the last three years (see Refs. [1–5, 7–10], to name a few). The debates will surely intensify because of new quantum oscillation experiments that show more periods than before, allowing the detection of multiple oscillation frequencies, and a new interpretation of this data presented in an article in Physical Review B by Suchitra E. Sebastian and her international team of colleagues from Cambridge and Oxford universities in the UK, Los Alamos Laboratory in the US, the University of British Columbia and the Canadian Institute for Advanced Research, both in Canada, and Max-Planck-Institut fur Feskorperforschung in Stuttgart, Germany [11]. According to the standard Lifshitz-Kosevich theory [12], quantum oscillations in a FL originate from Landau quantization of closed orbits in a magnetic field. A period F of QO is related to an extreme cross section of the Fermi surface (FS) by F=nΦ0, where n is the carrier density enclosed by the FS, and Φ0 is the magnetic flux quantum. QO measurements in the overdoped Tl2Ba2CuO6+δ[6] revealed a large hole FS, consistent with photoemission [13] and with the Luttinger count for free electrons. In sharp contrast, Fermi surfaces extracted from QO measurements in underdoped YBCO are small pockets, which collectively account for only few percent of the area of the Brillouin zone (BZ). Such small pockets imply that the FS undergoes a substantial reconstruction between overdoped and underdoped regimes. FS reconstruction and small pockets are expected when the system develops a density-wave order in either spin or charge channel (spin-density wave (SDW) [14] or d-density wave (DDW) [7], respectively). In both cases, however, the reconstructed FS contains both electron and hole pockets, at least when the order parameter is not too large. The measurements of QO revealed multiple oscillation frequencies, several of them as satellites of the major peak at F∼ 530 T[3, 5]. This peak was originally identified [5] as coming from an electron pocket on the basis of required consistency with the observed negative sign of the Hall coefficient, and satellites were attributed to bilayer splitting and warping of an electron FS. Sebastian et al. have offered another interpretation of the QO data [11]. They argue that the major peak corresponds to the hole pocket located near (π/2,π/2) (the α pocket in their notations), while the two satellites at 460 T and 602 T correspond to electron γ pockets near (0,π) and symmetry-related points (see Fig.1). They further associate the high-frequency oscillation at 1650 T, which they identified earlier [3], with another, larger hole β pocket, and argue that their (α,β,γ) pocket structure is quantitatively consistent with the FS for an incommensurate SDW state. The agreement with the data becomes even better once one includes the ortho-II potential due to oxygen ordering [8].

Single-particle anomalous excitations of Gutzwiller-projected BCS superconductors and Bogoliubov quasiparticle characteristics

Low-lying one-particle anomalous excitations are studied for Gutzwiller projected strongly correlated BCS states. It is found that the one-particle anomalous excitations are highly coherent, and the numerically calculated spectrum can be reproduced quantitatively by a renormalized BCS theory, thus strongly indicating that the nature of low-lying excitations described by the projected BCS states is essentially understood within a renormalized Bogoliubov quasi-particle picture. This finding resembles a well known fact that a Gutzwiller projected Fermi gas is a Fermi liquid. The present results are consistent with numerically exact calculations of the two-dimensional t-J model as well as recent photoemission experiments on high-T_{\rm C} cuprate superconductors.