High-speed measurement of rotational anisotropy nonlinear optical harmonic generation using position sensitive detection

We present a method of performing high-speed rotational anisotropy nonlinear optical harmonic generation experiments at rotational frequencies of several hertz by projecting the harmonic light reflected at different angles from a sample onto a stationary position sensitive detector. The high rotational speed of the technique, $10^3$ to $10^4$ times larger than existing methods, permits precise measurements of the crystallographic and electronic symmetries of samples by averaging over low frequency laser power, beam pointing, and pulse width fluctuations. We demonstrate the sensitivity of our technique by resolving the bulk four-fold rotational symmetry of GaAs about its [001] axis using second harmonic generation

A parity-breaking electronic nematic phase transition in the spin-orbit coupled metal Cd2_2Re2_2O7_7

Strong electron interactions can drive metallic systems toward a variety of well-known symmetry-broken phases, but the instabilities of correlated metals with strong spin-orbit coupling have only recently begun to be explored. We uncovered a multipolar nematic phase of matter in the metallic pyrochlore Cd$_2$Re$_2$O$_7$ using spatially resolved second-harmonic optical anisotropy measurements. Like previously discovered electronic nematic phases, this multipolar phase spontaneously breaks rotational symmetry while preserving translational invariance. However, it has the distinguishing property of being odd under spatial inversion, which is allowed only in the presence of spin-orbit coupling. By examining the critical behavior of the multipolar nematic order parameter, we show that it drives the thermal phase transition near 200 kelvin in Cd$_2$Re$_2$O$_7$ and induces a parity-breaking lattice distortion as a secondary order.Comment: 9 pages main text, 4 figures, 10 pages supplementary informatio

Antiferromagnetic correlations and impurity broadening of NMR linewidths in cuprate superconductors

We study a model of a d-wave superconductor with strong potential scatterers in the presence of antiferromagnetic correlations and apply it to experimental nuclear magnetic resonance (NMR) results on Zn impurities in the superconducting state of YBCO. We then focus on the contribution of impurity-induced paramagnetic moments, with Hubbard correlations in the host system accounted for in Hartree approximation. We show that local magnetism around individual impurities broadens the line, but quasiparticle interference between impurity states plays an important role in smearing out impurity satellite peaks. The model, together with estimates of vortex lattice effects, provides a semi-quantitative description of the impurity concentration dependence of the NMR line shape in the superconducting state, and gives a qualitative description of the temperature dependence of the line asymmetry. We argue that impurity-induced paramagnetism and resonant local density of states effects are both necessary to explain existing experiments.Comment: 15 pages, 23 figures, submitted to Phys. Rev.

Doping evolution and polar surface reconstruction of the infinite-layer cuprate Sr1−x_{1-x}Lax_{x}CuO2_{2}

We use angle-resolved photoemission spectroscopy to study the doping evolution of infinite-layer Sr$_{1-x}$La$_{x}$CuO$_{2}$ thin films grown by molecular-beam epitaxy. At low doping, the material exhibits a dispersive lower Hubbard band typical of the superconducting cuprate parent compounds. As carriers are added to the system, a continuous evolution from charge-transfer insulator to superconductor is observed, with the initial lower Hubbard band pinned well below the Fermi level and the development of a coherent low-energy band with electron doping. This two-component spectral function emphasizes the important role that strong local correlations play even at relatively high doping levels. Electron diffraction probes reveal a ${p(2\times2)}$ surface reconstruction of the material at low doping levels. Using a number of simple assumptions, we develop a model of this reconstruction based on the polar nature of the infinite-layer structure. Finally, we provide evidence for a thickness-controlled transition in ultrathin films of SrCuO$_2$ grown on nonpolar SrTiO$_3$, highlighting the diverse structural changes that can occur in polar complex oxide thin films

Nodeless superconductivity arising from strong (pi,pi) antiferromagnetism in the infinite-layer electron-doped cuprate Sr1-xLaxCuO2

The asymmetry between electron and hole doping remains one of the central issues in high-temperature cuprate superconductivity, but our understanding of the electron-doped cuprates has been hampered by apparent discrepancies between the only two known families: Re2-xCexCuO4 and A1-xLaxCuO2. Here we report in situ angle-resolved photoemission spectroscopy measurements of epitaxially-stabilized films of Sr1-xLaxCuO2 synthesized by oxide molecular-beam epitaxy. Our results reveal a strong coupling between electrons and (pi,pi) antiferromagnetism that induces a Fermi surface reconstruction which pushes the nodal states below the Fermi level. This removes the hole pocket near (pi/2,pi/2), realizing nodeless superconductivity without requiring a change in the symmetry of the order parameter and providing a universal understanding of all electron-doped cuprates

Tetrahedral Symmetry in Ground- and Low-Lying States of Exotic A ~ 110 Nuclei

Recent theoretical calculations predict a possible existence of nuclei with tetrahedral symmetry: more precisely, the mean-field hamiltonians of such nuclei are symmetric with respect to double point-group Td. In this paper, we focus on the neutron-rich Zirconium isotopes as an example and present realistic mean-field calculations which predict tetrahedral ground-state configurations in 108,110Zr and low-lying excited states of tetrahedral symmetry in a number of N > 66 isotopes. The motivations for focusing on these nuclei, as well as a discussion of the possible experimental signatures of tetrahedral symmetry are also presented.Comment: Accepted in Phys. Rev. C - Rapid Communication

Nonlinear and time-resolved optical study of the 112-type iron-based superconductor parent Ca_(1−x)La_xFeAs_2 across its structural phase transition

The newly discovered 112-type ferropnictide superconductors contain chains of As atoms that break the tetragonal symmetry between the ɑ and b axes. This feature eliminates the need for uniaxial strain that is usually required to stabilize large single domains in the electronic nematic state that exists in the vicinity of magnetic order in the iron-based superconductors. We report detailed structural symmetry measurements of 112-type Ca_(0.73)La_(0.27)FeAs_2 using rotational anisotropy optical second-harmonic generation. This technique is complementary to diffraction experiments and enables a precise determination of the point-group symmetry of a crystal. By combining our measurements with density functional theory calculations, we uncover a strong optical second-harmonic response of bulk electric dipole origin from the Fe and Ca 3d-derived states that enables us to assign C_2 as the crystallographic point group. This makes the 112-type materials high-temperature superconductors without a center of inversion, allowing for the possible mixing of singlet and triplet Cooper pairs in the superconducting state. We also perform pump-probe transient reflectivity experiments that reveal a 4.6-THz phonon mode associated with the out-of-plane motion of As atoms in the FeAs layers. We do not observe any suppression of the optical second-harmonic response or shift in the phonon frequency upon cooling through the reported monoclinic-to-triclinic transition at 58 K. This allows us to identify C_1 as the low-temperature crystallographic point group but suggests that structural changes induced by long-range magnetic order are subtle and do not significantly affect electronic states near the Fermi level

Formation of the coherent heavy fermion liquid at the 'hidden order' transition in URu2Si2

In this article we present high-resolution angle-resolved photoemission (ARPES) spectra of the heavy-fermion superconductor URu$_2$Si$_2$. Measurements as a function of both excitation energy and temperature allow us to disentangle a variety of spectral features, revealing the evolution of the low energy electronic structure across the hidden order transition. Already above the hidden order transition our measurements reveal the existence of weakly dispersive states below the Fermi level that exhibit a large scattering rate. Upon entering the hidden order phase, these states transform into a coherent heavy fermion liquid that hybridizes with the conduction bands.Comment: 5 pages, 4 figure

Impact of aerobic and resistance exercise combination on physical self-perceptions and self-esteem in women with obesity with one-year follow-up

The effectiveness of an exercise intervention including both aerobics and resistance training components in improving physical self-perceptions and global self-esteem (GSE) in women with obesity was examined. An experimental design with a one-year follow-up was used. Women with obesity (n = 72) participated in a structured exercise programme for 12 weeks after being randomised into a control and an exercise group. Exercise self-efficacy, body attractiveness, physical strength, sport competence, physical condition, physical self-worth, and GSE were measured at pre-intervention, early intervention, mid-intervention, immediately after the intervention, and five times following programme termination at 1, 3, 6, 9, and 12 months, respectively. Analyses of covariance revealed exercise effects for all of the dependent variables except for body attractiveness. Generally, exercise effects lasted between 6 and 12 months. A 12-week physical exercise programme including aerobic and resistance training components has the potential to improve physical self-perceptions and self-esteem in women with obesity