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E. M. Chudnovsky

Publications and source records attributed to E. M. Chudnovsky.

88 records · Page 5Linked to original sources
arXiv2001

Spin tunneling via dislocations in Mn-12 acetate crystals

We show that dislocations should be the main source of spin tunneling in Mn-12 crystals. Long-range strains caused by dislocations produce broad distribution of relaxation times that has been seen in many experiments. When the external magnetic field is applied along the c-axis of the crystal, local rotations of the magnetic anisotropy axis due to dislocations result in the effective local transverse magnetic field that unfreezes odd tunneling resonances. Scaling law is derived that provides universal description of spin tunneling for all resonances.

cond-mat.stat-mech↗
arXiv2001

Dipolar ordering in Fe8?

We show that the low-temperature physics of molecular nanomagnets, contrary to the prevailing one-molecule picture, must be determined by the long-range magnetic ordering due to many-body dipolar interactions. The calculations here performed, using Ewald's summation, suggest a ferromagnetic ground state with a Curie temperature of about 130 mK. The energy of this state is quite close to those of an antiferromagnetic state and to a glass of frozen spin chains. The latter may be realized at finite temperature due to its high entropy.

cond-mat.mes-hall↗
arXiv2000

Magnetic qubits as hardware for quantum computers

We propose two potential realisations for quantum bits based on nanometre scale magnetic particles of large spin S and high anisotropy molecular clusters. In case (1) the bit-value basis states |0> and |1> are the ground and first excited spin states Sz = S and S-1, separated by an energy gap given by the ferromagnetic resonance (FMR) frequency. In case (2), when there is significant tunnelling through the anisotropy barrier, the qubit states correspond to the symmetric, |0>, and antisymmetric, |1>, combinations of the two-fold degenerate ground state Sz = +- S. In each case the temperature of operation must be low compared to the energy gap, Δ, between the states |0> and |1>. The gap Δin case (2) can be controlled with an external magnetic field perpendicular to the easy axis of the molecular cluster. The states of different molecular clusters and magnetic particles may be entangled by connecting them by superconducting lines with Josephson switches, leading to the potential for quantum computing hardware.

cond-mat.mes-hall↗
arXiv2000

Non-Kramers Freezing and Unfreezing of Tunneling in the Biaxial Spin Model

The ground state tunnel splitting for the biaxial spin model in the magnetic field, H = -D S_{x}^2 + E S_{z}^2 - g μ_B S_z H_z, has been investigated using an instanton approach. We find a new type of spin instanton and a new quantum interference phenomenon associated with it: at a certain field, H_2 = 2SE^{1/2}(D+E)^{1/2}/(g μ_B), the dependence of the tunneling splitting on the field switches from oscillations to a monotonic growth. The predictions of the theory can be tested in Fe_8 molecular nanomagnets.

cond-mat.soft↗
arXiv2000

Ferromagnetic Film on a Superconducting Substrate

We study the equilibrium domain structure and magnetic flux around a ferromagnetic (FM) film with perpendicular magnetization M_0 on a superconducting (SC) substrate. At 4πM_0 > 1. Here λ_L is the London penetration length. For 4πM_0 > H_{c1} and l_{N} in excess of about 35 λ_{L}, the domains are connected by SC vortices. We argue that pinning of vortices by magnetic domains in FM/SC multilayers can provide high critical currents.

cond-mat.supr-con↗
arXiv2000

Escape-Rate Crossover between Quantum and Classical Regimes in Molecular Magnets: A Diagonalization Approach

We have studied numerically the quantum-classical crossover in the escape-rate for an uniaxial spin system with an arbitrarily directed field. Using the simple quantum transition-state theory, we have obtained the boundary separating the first- and the second-order crossover and the escape-rate in the presence of the transverse and longitudinal field. The results apply to the molecular nanomagnet, ${\rm Mn_{12}}$.

cond-mat.mes-hall↗
arXiv2000

Quantum statistical metastability for a finite spin

We study quantum-classical escape-rate transitions for uniaxial and biaxial models with finite spins S=10 (such as Mn_12Ac and Fe_8) and S=100 by a direct numerical approach. At second-order transitions the level making a dominant contribution into thermally assisted tunneling changes gradually with temperature whereas at first-order transitions a group of levels is skipped. For finite spins, the quasiclassical boundaries between first- and second-order transitions are shifted, favoring a second-order transition: For Fe_8 in zero field the transition should be first order according to a theory with S \to \infty, but we show that there are no skipped levels at the transition. Applying a field along the hard axis in Fe_8 makes transition the strongest first order. For the same model with S=100 we confirmed the existence of a region where a second-order transition is followed by a first-order transition [X. Martines Hidalgo and E. M. Chudnovsky, J. Phys.: Condensed Matter (in press)].

cond-mat.stat-mech↗
arXiv2000

Spin Tunneling in Mn-12 Cannot be Assisted by Phonons at Millikelvin Temperatures [Comment on PRL 83, 416 (1999) by Bellessa et al]

Recently, Bellessa et al in a Physical Review Letter entitled "Phonon-Assisted Tunneling in High-Spin Molecules: Experimental Evidence", reported observation of a peak in the imaginary part of the ac-susceptibility of Mn-12 molecular nanomagnet, that decreases strongly below 0.1 K down to 0.02 K. They explained this effect by a "phonon-induced tunneling process". The purpose of this Comment is to show that the explanation of Bellessa et al is logically inconsistent and physically impossible. All formulas in the Letter supporting that explanation are incorrect.

cond-mat.stat-mech↗
arXiv2000

Crystal Field -AS_z^2 Does Not Produce One-Phonon Transitions With Delta S_z=+-2 [Comment on EPL 46, 692 (1999) by Leuenberger and Loss]

Recently Leuenbeger and Loss suggested a theory of phonon-assisted relaxation in a molecular nanomagnet Mn-12 that "contrary to previous results is in reasonably good agreement ... with all experimental parameter values known so far". The purpose of this Comment is to show that the model of Leuenberger and Loss and its comparison with experiment are premised upon their incorrect use of the linear formula for the strain tensor. The spin-phonon coupling introduced by Leuenberger and Loss disappears if the nonlinear term in the strain tensor is taken into account.

cond-mat.stat-mech↗
arXiv1999

Tunneling of a large spin via hyperfine interactions

We consider a large spin \bf S in the magnetic field parallel to the uniaxial crystal field, interacting with N >> 1 nuclear spins \bf I_i via Hamiltonian \cal H = -DS_z^2 - H_zS_z+ A{\bf S}\cdot \sum_{i=1}^N {\bf I}_i with A << D, at temperature T. Tunneling splittings and the selection rules for the resonant values of H_z are obtained perturbatively. The quantum coherence exists at T << ASI while at T >= ASI the coherence is destroyed and the relaxation of \bf S is described by a stretched dependence which can be close to log t under certain conditions. Relevance to Mn-12 acetate is discussed.

cond-mat.stat-mech↗
arXiv1999

Quantum-Classical Escape-Rate Transition of a Biaxial Spin System with a Longitudinal Field: a Perturbative Approach

The quantum-classical transition of the escape rate of the spin model \cal H= -DS_z^2 - H_zS_z + B S_x^2 is investigated by a perturbative approach with respect to B [D. A. Garanin, J. Phys A {\bf 24}, L61 (1991)]. The transition is first order for B<B_c(H_z), the boundary line going to zero as B_c/D ~ 1 - H_z/(2SD) in the strongly biased limit. The range of the first-order transition is thus larger than for the model \cal H = -DS_z^2 - H_zS_z - H_x S_x studied earlier, where in the strongly biased case H_{xc}/(2SD) ~ [1-H_z/(2SD)]^{3/2}. The temperature of the quantum-classical transition, T_0, behaves linearly in the strongly biased case for both models: T_0 ~ 2SD - H_z.

cond-mat.stat-mech↗
arXiv1998

Quantum Coherence in Fe8 Molecular Nanomagnets

We report observation of coherent quantum oscilations in spin-10 Fe8 molecular clusters. The powder of magnetically oriented Fe8 crystallites was placed inside a resonator, in a dc magnetic field perpendicular to the magnetization axis. The field dependence of the ac-susceptibility was measured up to 5 T, at 680 MHz, down to 25 mK. Two peaks in the imaginary part of the susceptibility have been detected, whose positions coincide, without any fitting parameters, with the predicted two peaks corresponding to the quantum splitting of the ground state in the magnetic field parallel and perpendicular to the hard magnetization axis.

cond-mat↗
arXiv1998

Quantum-Classical Transition of the Escape Rate of a Uniaxial Spin System in an Arbitrarily Directed Field

The escape rate Γof the large-spin model described by the Hamiltonian H = -DS_z^2 - H_zS_z - H_xS_x is investigated with the help of the mapping onto a particle moving in a double-well potential U(x). The transition-state method yields $Γ$ in the moderate-damping case as a Boltzmann average of the quantum transition probabilities. We have shown that the transition from the classical to quantum regimes with lowering temperature is of the first order (dΓ/dT discontinuous at the transition temperature T_0) for h_x below the phase boundary line h_x=h_{xc}(h_z), where h_{x,z}\equiv H_{x,z}/(2SD), and of the second order above this line. In the unbiased case (H_z=0) the result is h_{xc}(0)=1/4, i.e., one fourth of the metastability boundary h_{xm}=1, at which the barrier disappears. In the strongly biased limit δ\equiv 1-h_z << 1, one has h_{xc} \cong (2/3)^{3/4}(\sqrt{3}-\sqrt{2})δ^{3/2}\cong 0.2345 δ^{3/2}, which is about one half of the boundary value h_{xm} \cong (2δ/3)^{3/2} \cong 0.5443 δ^{3/2}.The latter case is relevant for experiments on small magnetic particles, where the barrier should be lowered to achieve measurable quantum escape rates.

cond-mat.stat-mech↗
arXiv1998

Thermally Activated Resonant Magnetization Tunneling in Molecular Magnets: Mn_12Ac and others

The dynamical theory of thermally activated resonant magnetization tunneling in uniaxially anisotropic magnetic molecules such as Mn_12Ac (S=10) is developed.The observed slow dynamics of the system is described by master equations for the populations of spin levels.The latter are obtained by the adiabatic elimination of fast degrees of freedom from the density matrix equation with the help of the perturbation theory developed earlier for the tunneling level splitting [D. A. Garanin, J. Phys. A, 24, L61 (1991)]. There exists a temperature range (thermally activated tunneling) where the escape rate follows the Arrhenius law, but has a nonmonotonic dependence on the bias field due to tunneling at the top of the barrier. At lower temperatures this regime crosses over to the non-Arrhenius law (thermally assisted tunneling). The transition between the two regimes can be first or second order, depending on the transverse field, which can be tested in experiments. In both regimes the resonant maxima of the rate occur when spin levels in the two potential wells match at certain field values. In the thermally activated regime at low dissipation each resonance has a multitower self-similar structure with progressively narrowing peaks mounting on top of each other.

cond-mat.stat-mech↗
arXiv1998

First- and Second-Order Transitions between Quantum and Classical Regimes for the Escape Rate of a Spin System

We have found a novel feature of the bistable large-spin model described by the Hamiltonian H = -DS_z^2 - H_xS_x.The crossover from thermal to quantum regime for the escape rate can be either first (H_x<SD/2) or second (SD/2<H_x<2SD) order, that is, sharp or smooth, depending on the strength of the transverse field. This prediction can be tested experimentally in molecular magnets like Mn_12Ac.

cond-mat.stat-mech↗