Quantum critical point in a periodic Anderson model

We investigate the symmetric Periodic Anderson Model (PAM) on a three-dimensional cubic lattice with nearest-neighbor hopping and hybridization matrix elements. Using Gutzwiller's variational method and the Hubbard-III approximation (which corresponds to the exact solution of an appropriate Falicov-Kimball model in infinite dimensions) we demonstrate the existence of a quantum critical point at zero temperature. Below a critical value $V_c$ of the hybridization (or above a critical interaction $U_c$) the system is an {\em insulator} in Gutzwiller's and a {\em semi-metal} in Hubbard's approach, whereas above $V_c$ (below $U_c$) it behaves like a metal in both approximations. These predictions are compared with the density of states of the $d$- and $f$-bands calculated from Quantum Monte Carlo and NRG calculations. Our conclusion is that the half-filled symmetric PAM contains a {\em metal-semimetal transition}, not a metal-insulator transition as has been suggested previously.Comment: ReVteX, 10 pages, 2 EPS figures. Minor corrections made in the text and in the figure captions from the first version. More references added. Accepted for publication in Physical Review

Phase diagrams of spin ladders with ferromagnetic legs

The low-temperature properties of the spin S=1/2 ladder with anisotropic ferromagnetic legs are studied using the continuum limit bosonization approach. The weak-coupling ground state phase diagram of the model is obtained for a wide range of coupling constants and several unconventional gapless ''spin-liquid'' phases are shown to exist for ferromagnetic coupling. The behavior of the ladder system in the vicinity of the ferromagnetic instability point is discussed in detail.Comment: 11 pages, 4 figure

Origin of Low-Energy Excitations in Charge-Ordered Manganites

The low-energy excitations in the charge-ordered phase of polycrystalline La0.25Ca0.75MnO3 are explored by frequency-domain terahertz spectroscopy. In the frequency range from 4 cm^-1 to 700 cm^-1 (energies 0.4 meV to 90 meV) and at temperatures down to 5 K, we do not detect any feature that can be associated with the collective response of the spatially modulated charge continuum. In the antiferromagnetically ordered phase, broad absorption bands appear in the conductivity and permittivity spectra around 30 cm^-1 and 100 cm^-1 which are assigned to former acoustic phonons optically activated due to a fourfold superstructure in the crystal lattice. Our results indicate that characteristic energies of collective excitations of the charge-ordered phase in La0:25Ca0:75MnO3, if any, lie below 1 meV. At our lowest frequencies of only few wavenumbers a strong relaxation is observed above 100 K connected to the formation of the charge-ordered state.Comment: 5 pages, 3 figure

Strong rejuvenation in a chiral-glass superconductor

The glassy paramagnetic Meissner phase of a Bi$_2$Sr$_2$CaCu$_2$O$_x$ superconductor ($x$ = 8.18) is investigated by squid magnetometry, using ``dc-memory'' experiments employed earlier to study spin glasses. The temperature dependence of the zero-field-cooled and thermo-remanent magnetization is recorded on re-heating after specific cooling protocols, in which single or multiple halts are performed at constant temperatures. The 'spin' states equilibrated during the halts are retrieved on re-heating. The observed memory and rejuvenation effects are similar to those observed in Heisenberg-like spin glasses.Comment: REVTeX 4 style; 5 pages, 5 figure

Specific heat of an S=1/2 Heisenberg ladder compound Cu2_2(C5_5H12_{12}N2_2)2_2Cl4_4 under magnetic fields

Specific heat measurements down to 0.5 K have been performed on a single crystal sample of a spin-ladder like compound Cu$_{2}$(C$_{5}$H$_{12}$N$_{2}$)$_{2}$Cl$_{4}$ under magnetic fields up to 12 T. The temperature dependence of the observed data in a magnetic field below 6 T is well reproduced by numerical results calculated for the S=1/2 two-leg ladder with $J_{\rm{rung}}$/$J_{\rm{leg}}$=5. In the gapless region above 7 T ($H_{\rm{c1}}$), the agreement between experiment and calculation is good above about 2 K and a sharp and a round peak were observed below 2 K in a magnetic field around 10 T, but the numerical data show only a round peak, the magnitude of which is smaller than that of the observed one. The origin of the sharp peak and the difference between the experimental and numerical round peak are discussed.Comment: 14 pages, 11 figures, Submitted to PR

Antiferromagnetically coupled alternating spin chains

The effect of antiferromagnetic interchain coupling in alternating spin (1,1/2) chains is studied by mean of a spin wave theory and density matrix renormalization group (DMRG). In particular, two limiting cases are investigated, the two-leg ladder and its two dimensional (2D) generalization. Results of the ground state properties like energy, spin gap, magnetizations, and correlation functions are reported for the whole range of the interchain coupling $J_{\perp}$. For the 2D case the spin wave results predict a smooth dimensional crossover from 1D to 2D keeping the ground state always ordered. For the ladder system, the DMRG results show that any $J_{\perp}>0$ drives the system to a gapped ground state. Furthermore the behaviour of the correlation functions closely resemble the uniform spin-1/2 ladder. For $J_{\perp}$ lower than 0.3, however, the gap behaves quadratically as $\Delta\sim0.6 J^2_{\perp}$. Finally, it is argued that the behaviour of the spin gap for an arbitrary number of mixed coupled spin chains is analogous to that of the uniform spin-1/2 chains.Comment: 5 pages, 7 ps-figure

Electronic correlation in the infrared optical properties of the quasi two dimensional κ\kappa-type BEDT-TTF dimer system

The polarized optical reflectance spectra of the quasi two dimensional organic correlated electron system $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]$Y$, $Y =$ Br and Cl are measured in the infrared region. The former shows the superconductivity at $T_{\rm c} \simeq$ 11.6 K and the latter does the antiferromagnetic insulator transition at $T_{\rm N} \simeq$ 28 K. Both the specific molecular vibration mode $\nu_{3}(a_{g})$ of the BEDT-TTF molecule and the optical conductivity hump in the mid-infrared region change correlatively at $T^{*} \simeq$ 38 K of $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br, although no indication of $T^{*}$ but the insulating behaviour below $T_{\rm ins} \simeq$ 50-60 K are found in $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl. The results suggest that the electron-molecular vibration coupling on the $\nu_{3}(a_{g})$ mode becomes weak due to the enhancement of the itinerant nature of the carriers on the dimer of the BEDT-TTF molecules below $T^{*}$, while it does strong below $T_{\rm ins}$ because of the localized carriers on the dimer. These changes are in agreement with the reduction and the enhancement of the mid-infrared conductivity hump below $T^{*}$ and $T_{\rm ins}$, respectively, which originates from the transitions between the upper and lower Mott-Hubbard bands. The present observations demonstrate that two different metallic states of $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Br are regarded as {\it a correlated good metal} below $T^{*}$ including the superconducting state and {\it a half filling bad metal} above $T^{*}$. In contrast the insulating state of $\kappa$-(BEDT-TTF)$_{2}$Cu[N(CN)$_{2}$]Cl below $T_{\rm ins}$ is the Mott insulator.Comment: 8 pages, 7 figure

Phase Transitions Between Topologically Distinct Gapped Phases in Isotropic Spin Ladders

We consider various two-leg ladder models exhibiting gapped phases. All of these phases have short-ranged valence bond ground states, and they all exhibit string order. However, we show that short-ranged valence bond ground states divide into two topologically distinct classes, and as a consequence, there exist two topologically distinct types of string order. Therefore, not all gapped phases belong to the same universality class. We show that phase transitions occur when we interpolate between models belonging to different topological classes, and we study the nature of these transitions.Comment: 11 pages, 16 postscript figure

Paramagnetic effect in YBaCuO grain boundary junctions

A detailed investigation of the magnetic response of YBaCuO grain boundary Josephson junctions has been carried out using both radio-frequency measurements and Scanning SQUID Microscopy. In a nominally zero-field-cooled regime we observed a paramagnetic response at low external fields for 45 degree asymmetric grain boundaries. We argue that the observed phenomenology results from the d-wave order parameter symmetry and depends on Andreev bound states.Comment: To be published in Phys. Rev.