The gas of elastic quantum strings in 2+1 dimensions: finite temperatures

The finite temperature physics of the gas of elastic quantum strings as introduced in J. Zaanen, Phys. Rev. Lett. 84, 753 is investigated. This model is inspired on the stripes in the high Tc superconductors. We analyze in detail how the kinetic interactions of the zero temperature quantum problem crossover into the entropic interactions of the high temperature limit.Comment: 14 pages, 2 figure

Orbital ordering in charge transfer insulators

We discuss a new mechanism of orbital ordering, which in charge transfer insulators is more important than the usual exchange interactions and which can make the very type of the ground state of a charge transfer insulator, i.e. its orbital and magnetic ordering, different from that of a Mott-Hubbard insulator. This purely electronic mechanism allows us to explain why orbitals in Jahn-Teller materials typically order at higher temperatures than spins, and to understand the type of orbital ordering in a number of materials, e.g. K_2CuF_4, without invoking the electron-lattice interaction.Comment: 4 pages, 2 figure

Charged domain walls as quantum strings living on a lattice

A generic lattice cut-off model is introduced describing the quantum meandering of a single cuprate stripe. The fixed point dynamics is derived, showing besides free string behavior a variety of partially quantum disordered phases, bearing relationships both with quantum spin-chains and surface statistical physics.Comment: 22 page, 17 figure

Strange metal electrodynamics across the phase diagram of Bi_2-<i>x</i>Pb_<i>x</i>Sr_2-<i>y</i>La_<i>y</i>CuO_6+<i>δ</i> cuprates

Unlocking the mystery of the strange metal state has become the focal point of high-Tcresearch, not because of its importance for superconductivity, but because it appears to represent a truly novel phase of matter dubbed "quantum supreme matter. " Detected originally through high magnetic field, transport experiments, signatures of this phase have now been uncovered with a variety of probes. Our high resolution optical data of the low-Tccuprate superconductor, Bi2-xPbxSr2-yLayCuO6+delta allows us to probe this phase over a large energy and temperature window. We demonstrate that the optical signatures of the strange metal phase persist throughout the phase diagram. The strange metal signatures in the optical conductivity are twofold: (i) a low energy Drude response with Drude width on the order of temperature and (ii) a high energy conformal tail with a doping dependent power-law exponent. While the Drude weight evolves monotonically throughout the entire doping range studied, the spectral weight contained in the high energy conformal tail appears to be doping and temperature independent. Our analysis further shows that the temperature dependence of the optical conductivity is completely determined by the Drude parameters. Our results indicate that there is no critical doping level inside the superconducting dome where the carrier density starts to change drastically and that the previously observed "return to normalcy " is a consequence of the increasing importance of the Drude component relative to the conformal tail with doping. Importantly, both the doping and temperature dependence of the resistivity are largely determined by the Drude width

Structural, electronic, and magneto-optical properties of YVO3_3

Optical and magneto-optical properties of YVO$_3$ single crystal were studied in FIR, visible, and UV regions. Two structural phase transitions at 75 K and 200 K were observed and established to be of the first and second order, respectively. The lattice has an orthorhombic $Pbnm$ symmetry both above 200 K as well as below 75 K, and is found to be dimerized monoclinic $Pb11$ in between. We identify YVO$_3$ as a Mott-Hubbard insulator with the optical gap of 1.6 eV. The electronic excitations in the visible spectrum are determined by three $d$-bands at 1.8, 2.4, and 3.3 eV, followed by the charge-transfer transitions at about 4 eV. The observed structure is in good agreement with LSDA+$U$ band structure calculations. By using ligand field considerations, we assigned these bands to the transitions to the $^4A_{2g}$, $^2E_{g} + ^2T_{1g}$, and $^2T_{2g}$ states. The strong temperature dependence of these bands is in agreement with the formation of orbital order. Despite the small net magnetic moment of 0.01 $\mu_B$ per vanadium, the Kerr effect of the order of $0.01^\circ$ was observed for all three $d$-bands in the magnetically ordered phase $T_{\text{N\'eel}}<116 K$. A surprisingly strong enhancement of the Kerr effect was found below 75 K, reaching a maximum of $0.1^\circ$. The effect is ascribed to the non-vanishing net orbital magnetic moment.Comment: Submitted to Phys. Rev.

Independent freezing of charge and spin dynamics in La1.5Sr0.5CoO4

We present elastic and quasielastic neutron scattering measurements characterizing peculiar short-range charge-orbital and spin order in the layered perovskite material La1.5Sr0.5CoO4. We find that below Tc~750 K holes introduced by Sr doping lose mobility and enter a statically ordered {\it charge glass} phase with loosely correlated checkerboard arrangement of empty and occupied d{3z2-r2} orbitals (Co3+ and Co2+). The dynamics of the resultant mixed spin system is governed by the anisotropic nature of the crystal-field Hamiltonian and the peculiar exchange pattern produced by the orbital order. It undergoes a {\it spin freezing} transition at much a lower temperature, Ts~30 K.Comment: 4 pages, 3 figures, Latex. Submitted to PR

Incoherent transport across the strange metal regime of highly overdoped cuprates

Strange metals possess highly unconventional transport characteristics, such as a linear-in-temperature ($T$) resistivity, an inverse Hall angle that varies as $T^2$ and a linear-in-field ($H$) magnetoresistance. Identifying the origin of these collective anomalies has proved profoundly challenging, even in materials such as the hole-doped cuprates that possess a simple band structure. The prevailing dogma is that strange metallicity in the cuprates is tied to a quantum critical point at a doping $p*$ inside the superconducting dome. Here, we study the high-field in-plane magnetoresistance of two superconducting cuprate families at doping levels beyond $p*$. At all dopings, the magnetoresistance exhibits quadrature scaling and becomes linear at high $H/T$ ratios. Moreover, its magnitude is found to be much larger than predicted by conventional theory and insensitive to both impurity scattering and magnetic field orientation. These observations, coupled with analysis of the zero-field and Hall resistivities, suggest that despite having a single band, the cuprate strange metal phase hosts two charge sectors, one containing coherent quasiparticles, the other scale-invariant `Planckian' dissipators.Comment: 15 pages plus 7 figures (including Supplementary Information

Ultrafast optical nonlinearity in quasi-one-dimensional Mott-insulator Sr2CuO3{\rm Sr_2CuO_3}

We report strong instantaneous photoinduced absorption (PA) in the quasi-one-dimensional Mott insulator ${\rm Sr_2CuO_3}$ in the IR spectral region. The observed PA is to an even-parity two-photon state that occurs immediately above the absorption edge. Theoretical calculations based on a two-band extended Hubbard model explains the experimental features and indicates that the strong two-photon absorption is due to a very large dipole-coupling between nearly degenerate one- and two-photon states. Room temperature picosecond recovery of the optical transparency suggests the strong potential of ${\rm Sr_2CuO_3}$ for all-optical switching.Comment: 10 pages, 4 figure

Angle-resolved photoemission study of MX-chain compound [Ni(chxn)2_2Br]Br2_2

We report on the results of angle-resolved photoemission experiments on a quasi-one-dimensional $MX$-chain compound [Ni(chxn)$_2$Br]Br$_2$ (chxn = 1$R$,2$R$-cyclohexanediamine), a one-dimensional Heisenberg system with $S=1/2$ and $J \sim 3600$ K, which shows a gigantic non-linear optical effect. A "band" having about 500 meV energy dispersion is found in the first half of the Brillouin zone $(0\le kb/\pi <1/2)$, but disappears at $kb / \pi \sim 1/2$. Two dispersive features, expected from the spin-charge separation, as have been observed in other quasi-one-dimensional systems like Sr$_2$CuO$_3$, are not detected. These characteristic features are well reproduced by the $d$-$p$ chain model calculations with a small charge-transfer energy $\Delta$ compared with that of one-dimensional Cu-O based compounds. We propose that this smaller $\Delta$ is the origin of the absence of clear spin- and charge-separation in the photoemission spectra and strong non-linear optical effect in [Ni(chxn)$_2$Br]Br$_2$.Comment: 4 pages, 3 figure