arXiv subjects

Timothy Ziman

Publications and source records attributed to Timothy Ziman.

30 records · Page 2Linked to original sources

arXiv2006

Non-perturbative $J_{pd}$ model and ferromagnetism in dilute magnets

We calculate magnetic couplings in the $J_{pd}$ model for dilute magnets, in order both to identify the relevant parameters which control ferromagnetism and also to bridge the gap between first principle calculations and model approaches. The magnetic exchange interactions are calculated non-perturbatively and disorder in the configuration of impurities is treated exacly, allowing us to test the validity of effective medium theories. Results differ qualitatively from those of weak coupling. In contrast to mean field theory, increasing $J_{pd}$ may not favor high Curie temperatures: $T_C$ scales primarily with the bandwidth. High temperature ferromagnetism at small dilutions is associated with resonant structure in the p-band. Comparison to diluted magnetic semiconductors indicate that Ga(Mn)As has such a resonant structure and thus this material is already close to optimality.

cond-mat.str-el↗

arXiv2006

Comparison between ab-initio and phenomenological modeling of the exchange couplings in diluted magnetic semiconductors: the case of $Zn_{1-x}Cr_{x}Te$

Using a recently developed semi-analytical method (Self-Consistent Local RPA or SC-LRPA) we study the stability of the ferromagnetic phase in diluted magnetic systems where the exchange coupling between magnetic impurities are of RKKY form. A short discussion of the relevance of these calculations with respect to the ferromagnetism observed in diluted ferromagnetic materials is provided. Then, within a two step approach, we study ferromagnetism in $Zn_{1-x}Cr_{x}Te$. In the first step of our study, we calculate the magnetic couplings between Mn impurities within the LDA. In the second step, we diagonalize the resulting effective Heisenberg Hamiltonian using the SC-LRPA. We also compare, when available, our calculations with Monte Carlo simulations and experimental measurements.

cond-mat.dis-nn↗

arXiv2006

Compensation, interstitial defects and ferromagnetism in diluted semiconductors

We present a quantitative theory for ferromagnetism in diluted III-V ferromagnetic semi-conductors in the presence of the two types of defects commonly supposed to be responsible for compensation: As anti-sites and Mn interstitials. In each case we reduce the description to that of an effective random Heisenberg model with exchange integrals between active magnetic impurities provided by ab initio calculation. The effective magnetic Hamiltonian is then solved by a semi-analytical method (locally self-consistent RPA), where disorder is treated exactly. Measured Curie temperatures are shown to be inconsistent with the hypothesis that As anti-sites provide the dominant mechanism for compensation. In contrast, if we assume that Mn interstitials are the main source for compensation, we obtain a very good agreement between the calculated Curie temperature and the measured values, in both as-grown and annealed samples.

cond-mat.dis-nn↗

arXiv2006

Model for vacancy-induced d$^0$ ferromagnetism in oxide compounds

We propose a model with few parameters, for vacancy-induced ferromagnetism based on a correlated model for anions (oxygen orbitals) with random potentials that represent cation vacancies. There is a range of potential strength for which moments appear on oxygen sites neighboring the vacancies. By using techniques which allow us to treat the randomness exactly, we calculate the magnetic couplings between the total moment around each vacancy, the Curie temperature and dynamical correlations as a function of the effective stren gth of the vacancy potential and doping properties, the density of vacancies, as well as the correlation and band width of the host. For physically reasonable parameters this predicts Curie temperatures well above room temperature for small concentrations of vacancies and appropriate parameters. We disc uss our results in relation to questions of stability and reproducibility raised in experiments. To circumvent the difficulties of controlling intrinsic defects, we propose specific non-magnetic host doping that could be, for example, substituted for cations in HfO$_2$ or ZrO$_2$.

cond-mat.str-el↗

arXiv2005

Why RKKY exchange integrals are inappropriate to describe ferromagnetism in diluted magnetic semiconductors

We calculate Curie temperatures and study the stability of ferromagnetism in diluted magnetic materials, taking as a model for the exchange between magnetic impurities a damped Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction and a shor t range term representing the effects of superexchange. To properly include effects of spin and thermal fluctuations as well as geometric disorder, we solve the effective Heisenberg Hamiltonian by means of a recently developed semi-analytical approach. This approach, ``self-consistent local Random Phase Approximation (SC-L RPA)'', is explained. We show that previous mean-field treatments, which have been widely used in the literature, largely overestimate both the Curie temperatures and the stability of ferromagnetism as a function of carrier density. The discr epancy when compared to the current approach was that effects of frustration in RKKY oscillations had been strongly underestimated by such simple mea n-field theories. We argue that the use, as is frequent, of a weakly-disordered RKKY exchange to model ferromagnetism in diluted III-V systems is inconsistent with the observation of ferromagnetism over a wide region of itinerant carrier densities. This may be puzzling when compared to the apparent success of calculations based on {\it ab-initio} estimates of the coupling; we propose a resolution to this issue by taking RKKY-like interactions between resonant states close to the Fermi level.

cond-mat.str-el↗

arXiv2004

Modified Spin-Wave Theory for Nanomagnets : Application to the Keplerate Molecule Mo(72)Fe(30)

We adapt Takahashi's modified spin-wave theory to the context of nano-magnets, and apply it to the molecular compound based on the giant magnetic molecule Mo(72)Fe(30). This involves solving numerically the mean-field equations and then forcing the sublattice magnetizations to zero by means of local chemical potentials for the magnons. We have thus constructed a quantum state with no local magnetization at all temperatures, appropriate to a finite-size system, but with strong correlations. We compare theoretical results to specific heat and ESR measurements.

cond-mat.mtrl-sci↗

arXiv2004

Boosting the Curie temperature with correlations in diluted magnetic semiconductors

We present a quantitative theory for the effects of correlated doping on the ferromagnetism of diluted magnetic semiconductors. It predicts that room temperature ferromagnetism should be possible in homogeneous, but correlated, samples of Mn$_x$Ga$_{1-x}$As. The theory predicts lower critical temperatures for Mn$_x$Ga$_{1-x}$N.

cond-mat.dis-nn↗

arXiv2004

Calculating the Curie Temperature reliably in diluted III-V ferromagnetic semiconductors

We present a semi-analytic theory for the Curie temperature in diluted magnetic semi-conductors that treats disorder effects exactly in the effective Heisenberg Hamiltonian, and spin fluctuations within a local RPA. The exchange couplings are taken from concentration dependent {\it ab initio} estimates. The theory gives very good agreement with published data for well-annealed samples of Mn$_x$Ga$_{1-x}$As. We predict the critical temperatures for Mn$_x$Ga$_{1-x}$N lower than in doped GaAs, despite the stronger nearest-neighbour ferromagnetic coupling. We also predict the dependence on the hole concentration.

cond-mat.dis-nn↗

arXiv2004

Theory of phonon-assisted "forbidden" optical transitions in spin-gapped systems

We consider the absorption of light with emission of one S(tot)=1 magnetic excitation in systems with a spin gap induced by quantum fluctuations. We argue that an electric dipole transition is allowed on the condition that a virtual phonon instantaneously breaks the inversion symmetry. We derive an effective operator for the transition and argue that the proposed theory explains the polarized experiments in CuGeO(3) and SrCu(2)[BO(3)](2).

cond-mat.str-el↗

arXiv2000

An Electron Spin Resonance Selection Rule for Spin-Gapped Systems

The direct electron spin resonance (ESR) absorption between a singlet ground state and the triplet excited states of spin gap systems is investigated. Such an absorption, which is forbidden by the conservation of the total spin quantum number in isotropic Hamiltonians, is allowed by the Dzyaloshinskii-Moriya interaction. We show a selection rule in the presence of this interaction, using the exact numerical diagonalization of the finite cluster of the quasi-one-dimensional bond-alternating spin system. The selection rule is also modified into a suitable form in order to interpret recent experimental results on CuGeO$_3$ and NaV$_2$O$_5$.

cond-mat.str-el↗

arXiv1993

Poisson versus GOE statistics in integrable and non-integrable quantum hamiltonian

We calculate the level statistics by finding the eigenvalue spectrum for a variety of one-dimensional many-body models, namely the Heisenberg chain, the t-J model and the Hubbard model. In each case the generic behaviour is GOE, however at points corresponding to models known to be exactly integrable Poisson statistics are found, in agreement with an argument we outline.

cond-mat↗

arXiv1992

Dynamical Properties of a Single Hole in an Antiferromagnet

A finite size scaling analysis of the spectral function and of the optical conductivity of a single hole moving in an antiferromagnetic background is performed. It is shown that both the low energy quasiparticle peak and the broad higher energy structure are robust with increasing cluster size from $4\times 4$ to $\sqrt{26}\times\sqrt{26}$ sites. In the abscence of spin fluctuations, for most static or dynamical quantities saturation occurs when the size exceeds a characteristic size $N_c(J_z)$. Typically, 16 and 26 site clusters give reliable results for $J_z>0.75$ and $J_z>0.3$ respectively. The hole optical mass is shown to be very large ($>20$) in agreement with the small bandwidth. Due to the energy gap to flip a spin in the vicinity of a hole, a small gap $\propto J_z$ separates the low energy delta-function from the rest of the spectrum in the dynamical correlation functions. On the other hand, with $J_\perp$ this gap seems to disappear with increasing system size as one would expect since the spin waves are gapless in the thermodynamic limit. The large momentum dependence of the quasiparticle weight in the isotropic case is inconsistent with a string picture but agrees well with the self-consistent Born approximation. An accurate estimation of the higher energy part of the spectral functions of the t--J model can be made for momenta close to $(0,0)$ or $(π,π)$;

cond-mat↗