arXiv subjects

G. B. Martins

Publications and source records attributed to G. B. Martins.

42 records · Page 3Linked to original sources

arXiv2000

Magnetic properties of the frustrated AFM spinel ZnCr_2O_4 and the spin-glass Zn_{1-x}Cd_xCr_2O_4 (x=0.05,0.10)

The $T$-dependence (2- 400 K) of the electron paramagnetic resonance (EPR), magnetic susceptibility, $χ(T)$, and specific heat, $C_{v}(T)$, of the $normal$ antiferromagnetic (AFM) spinel ZnCr$_{2}$O$_{4}$ and the spin-glass (SG) Zn$_{1-x}$Cd$_{x}$Cr$_{2}$O$_{4}$ ($x=0.05,0.10$) is reported. These systems behave as a strongly frustrated AFM and SG with $% T_{N}$ $ \approx T_{G}\approx 12$ K and -400 K $\gtrsim Θ_{CW}\gtrsim -500$ K. At high-$T$ the EPR intensity follows the $χ(T)$ and the $g$-value is $T$-independent. The linewidth broadens as the temperature is lowered, suggesting the existence of short range AFM correlations in the paramagnetic phase. For ZnCr$_{2}$O$_{4}$ the EPR intensity and $χ(T)$ decreases below 90 K and 50 K, respectively. These results are discussed in terms of nearest-neighbor Cr$^{3+}$ (S $=3/2$%) spin-coupled pairs with an exchange coupling of $| J/k| \approx $ 50 K. The appearance of small resonance modes for $T\lesssim 17$ K, the observation of a sharp drop in $χ(T)$ and a strong peak in $C_{v}(T)$ at $T_{N}=12$ K confirms, as previously reported, the existence of long range AFM correlations in the low-$T$ phase. A comparison with recent neutron diffraction experiments that found a near dispersionless excitation at 4.5 meV for $T\lesssim T_{N}$ and a continuous gapless spectrum for $T\gtrsim T_{N}$, is also given.

cond-mat.str-el↗

arXiv2000

Indications of Spin-Charge Separation at Short Distance and Stripe Formation in the Extended t-J Model on Ladders and Planes

The recently discussed tendency of holes to generate nontrivial spin environments in the extended two-dimensional t-J model (G. Martins, R. Eder, and E. Dagotto, Phys. Rev. B{\bf 60}, R3716 (1999)) is here investigated using computational techniques applied to ladders with several number of legs. This tendency is studied also with the help of analytic spin-polaron approaches directly in two dimensions. Our main result is that the presence of robust antiferromagnetic correlations between spins located at both sides of a hole either along the x or y axis, observed before numerically on square clusters, is also found using ladders, as well as applying techniques based on a string-basis expansion. This so-called "across-the-hole" nontrivial structure exists even in the two-leg spin-gapped ladder system, and leads to an effective reduction in dimensionality and spin-charge separation at short-distances, with a concomitant drastic reduction in the quasiparticle (QP) weight Z. In general, it appears that holes tend to induce one-dimensional-like spin arrangements to improve their mobility. Using ladders it is also shown that the very small J/t$\sim$0.1 regime of the standard t-J model may be more realistic than anticipated in previous investigations, since such regime shares several properties with those found in the extended model at realistic couplings. Another goal of the present article is to provide additional information on the recently discussed tendencies to stripe formation and spin incommensurability reported for the extended t-J model.

cond-mat.str-el↗

arXiv2000

Stripes in the Ising Limit of Models for the Cuprates

The hole-doped standard and extended t-J models on ladders with anisotropic Heisenberg interactions are studied computationally in the interval $0.0 \leq λ\leq 1.0$ ($λ=0$, Ising; $λ=1$, Heisenberg). It is shown that the approximately half-doped stripes recently discussed at $λ=1$ survive in the anisotropic case ($λ$$<$1.0), particularly in the "extended" model. Due to the absence of spin fluctuations in the Ising limit and working in the rung basis, a simple picture emerges in which the stripe structure can be mostly constructed from the solution of the t-J model on chains. A comparison of results in the range $0.0 \leq λ\leq 1.0$ suggests that this picture is valid up to the Heisenberg limit.

cond-mat.str-el↗

arXiv2000

Doped Stripes in Models for the Cuprates Emerging from the One-hole Properties of the Insulator

The extended and standard t-J models are computationally studied on ladders and planes, with emphasis on the small J/t region. At couplings compatible with photoemission results for undoped cuprates, half-doped stripes separating $π$-shifted antiferromagnetic (AF) domains are found, as in Tranquada's interpretation of neutron experiments. Our main result is that the elementary stripe `"building-block" resembles the properties of $one$ hole at small J/t, with robust AF correlations across-the-hole induced by the local tendency of the charge to separate from the spin (G. Martins {\it et al.}, Phys. Rev. B{\bf 60}, R3716 (1999)). This suggests that the seed of half-doped stripes already exists in the unusual properties of the insulating parent compound.

cond-mat.str-el↗

arXiv1998

Diagonalization in Reduced Hilbert Spaces using a Systematically Improved Basis: Application to Spin Dynamics in Lightly Doped Ladders

A method is proposed to improve the accuracy of approximate techniques for strongly correlated electrons that use reduced Hilbert spaces. As a first step, the method involves a change of basis that incorporates exactly part of the short distance interactions. The Hamiltonian is rewritten in new variables that better represent the physics of the problem under study. A Hilbert space expansion performed in the new basis follows. The method is successfully tested using both the Heisenberg model and the $t-J$ model with holes on 2-leg ladders and chains, including estimations for ground state energies, static correlations, and spectra of excited states. An important feature of this technique is its ability to calculate dynamical responses on clusters larger than those that can be studied using Exact Diagonalization. The method is applied to the analysis of the dynamical spin structure factor $S(q,ω)$ on clusters with $2 \times 16$ sites and 0 and 2 holes. Our results confirm previous studies (M. Troyer, H. Tsunetsugu, and T. M. Rice, Phys. Rev. $ B 53$, 251 (1996)) which suggested that the state of the lowest energy in the spin-1 2-holes subspace corresponds to the bound state of a hole pair and a spin-triplet. Implications of this result for neutron scattering experiments both on ladders and planes are discussed.

cond-mat.stat-mech↗

arXiv1997

High-field Electron Spin Resonance of Cu_{1-x}Zn_{x}GeO_{3}

High-Field Electron Spin Resonance measurements were made on powder samples of Cu_{1-x}Zn_{x}GeO_{3} (x=0.00, 0.01, 0.02, 0.03 and 0.05) at different frequencies (95, 110, 190, 220, 330 and 440 GHz) at low temperatures. The spectra of the doped samples show resonances whose positions are dependent on Zn concentration, frequency and temperature. The analysis of intensity variation of these lines with temperature allows us to identify them as originating in transitions within states situated inside the Spin Peierls gap. A qualitative explanation of the details of the spectra is possible if we assume that these states in the gap are associated with "loose" spins created near the Zn impurities, as recently theoreticaly predicted. A new phenomenon of quenching of the ESR signal across the Dimerized to Incommensurate phase-boundary is observed.

cond-mat.str-el↗