共查询到20条相似文献,搜索用时 15 毫秒
1.
Rong Lü Hui Hu Jia-Lin Zhu Xiao-Bing Wang Lee Chang Bing-Lin Gu 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,14(2):349-361
Resonant quantum tunneling of the Néel vector between nonequivalent magnetic wells is investigated theoretically for a nanometer-scale
single-domain antiferromagnet with biaxial crystal symmetry in the presence of an external magnetic field applied along the
easy anisotropy axis, based on the two-sublattice model. Both the Wentzel-Kramers-Brillouin exponent and the preexponential
factors are evaluated in the instanton contribution to the tunneling rate for finite and zero magnetic fields by applying
the instanton technique in the spin-coherent-state path-integral representation, respectively. The quantum interference or
spin-parity effects induced by the topological phase term in the Euclidean action are discussed in the rate of quantum tunneling
of the Néel vector. In the absence of an external applied magnetic field, the effect of destructive phase interference or
topological quenching on resonant quantum tunneling of the Néel vector is evident for the half-integer excess spin antiferromagnetic
nanoparticle. In the weak field limit, the tunneling rates are found to oscillate with the external applied magnetic field
for both integer and half-integer excess spins. We discuss the experimental condition on the applied magnetic field which
may allow one to observe the topological quenching effect for nanometer-scale single-domain antiferromagnets with half-integer
excess spins. Tunneling behavior in resonant quantum tunneling of the magnetization vector between nonequivalent magnetic
wells is also studied for a nanometer-scale single-domain ferromagnet by applying the similar technique, but in the large
noncompensation limit.
Received 4 June 1999 相似文献
2.
E. Rastelli A. Tassi 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(3):341-347
Ferromagnetic spin chains of a hexagonal lattice coupled by a weak antiferromagnetic interaction J1 develop a helix arrangement if the intrachain antiferromagnetic NNN exchange J2 is sufficiently large. We show that the classical minimum energy spin configuration is an umbrella when an external magnetic field is applied. The scenario is dramatically changed by quantum fluctuations. Indeed we find
that the zero point motion forces the spins in a plane containing the magnetic field so that classical expectation is deceptive
for our model. Our result is obtained by controlled expansion in the low field-long wavelength modulation limit.
Received: 9 September 1997 / Revised: 15 October 1997 / Accepted: 17 November 1997 相似文献
3.
Magnetic with anisotropic antiferromagnetic exchange interactions and special topology of coupling in the square lattice with spins pairs ordering is studied by quantum Monte Carlo method. The antiferromagnetic order is found to be more stable as compared to spin liquid state. Exchange interactions and wave vector of structure modulation for Cu3B2O6 is estimated. Neel temperature versus strength of exchange in spin pair is calculated. Plateau and modulation of magnetic structure in field magnetization dependence is revealed. 相似文献
4.
The macroscopic quantum coherence in a biaxial antiferromagnetic molecular magnet in the presence of magnetic field acting parallel to its hard anisotropy axis is studied within the two-sublattice model.On the basis of instanton technique in the spin-coherent-state path-integral representation,both the rigorous Wentzel-Kramers-Brillouin exponent and pre-exponential factor for the ground-state tunnel splitting are obtained.We find that the quantum fluctuations around the classical paths can not only induce a new quantum phase previously reported by Chiolero and Loss (Phys.Rev.Lett.80(1998)169),but also have great influence on the intensity of the ground-state tunnel splitting.Those features clearly have no analogue in the ferromagnetic molecular magnets.We suggest that they may be the universal behaviors in all antiferromagnetic molecular magnets.The analytical results are complemented by exact diagonalization calculation. 相似文献
5.
Magnetization measurements of Mn12 molecular nanomagnets with spin ground states of S=10 and S=19/2 show resonance tunneling at avoided energy level crossings. The observed oscillations of the tunnel probability as a function of the magnetic field applied along the hard anisotropy axis are due to topological quantum phase interference of two tunnel paths of opposite windings. Spin-parity dependent tunneling is established by comparing the quantum phase interference of integer and half-integer spin systems. 相似文献
6.
A charge-current switch manipulated by the macroscopic quantum coherence of a single-molecule magnet
B. Chang J.-Q. Liang 《The European Physical Journal B - Condensed Matter and Complex Systems》2009,69(4):515-522
We report a theoretical analysis of electron transport
through a quantum dot with an embedded biaxial single-molecule
magnet, which is coupled to ferromagnetic electrodes of parallel and
antiparallel magnet-configurations. For the antiparallel
configuration of complete polarization it is shown that the
originally prohibited electron transport can be opened up by the
macroscopic quantum coherence of the molecular magnet, which
provides a spin-flipping mechanism. The charge-current and
differential conductance are controllable by variation of the
magnitude and orientation of an external magnetic field, which in
turn manipulates the macroscopic quantum coherence of the molecular
magnet. Moreover, the transport can be switched off at particular
values of the magnetic field, where the tunnel splitting is quenched
by the quantum phase interference of tunnel paths.The transport
current and differential conductance as functions of the
electrode-polarization and magnetic field are extensively studied,
which may be useful in practical applications. A new transport
channel is found in the completely polarized parallel-configuration
induced by the tunnel splitting of molecular magnet and
resonance-peak splits of the conductance are observed in
non-completely polarized configurations. 75.50.Xx Molecular magnets 相似文献
7.
8.
S. Wessel S. Haas 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,16(3):393-396
Three-dimensional magnetic ordering transitions are studied theoretically in strongly anisotropic quantum magnets. An external
magnetic field can drive quasi-one-dimensional subsystems with a spin gap into a gapless regime, thus inducing long-range
three-dimensional magnetic ordering due to weak residual magnetic coupling between the subsystems. Compounds with higher spin
degrees of freedom, such as N-leg spin-1/2 ladders, are shown to have cascades of ordering transitions. At high magnetic fields, zero-point fluctuations
within the quasi-1D subsystems are suppressed, causing quantum corrections to the ordering temperature to be reduced.
Received 24 March 2000 相似文献
9.
Entanglement in One-Dimensional Random XY Spin Chain with Dzyaloshinskii--Moriya Interaction 下载免费PDF全文
The impurities of exchange couplings, external magnetic fields and Dzyaloshinskii-Moriya (DM) interaction considered as Gaussian distribution, and the entanglement in one-dimensional random XY spin systems is investigated by the method of solving the different spin-spin correlation functions and the average magnetization per spin. The entanglement dynamics at central locations of ferromagnetic and antiferromagnetic chains have been studied by varying the three impurities and the strength of DM interaction. (i) For the ferromagnetic spin chain, the weak DM interaction can improve the amount of entanglement to a large value, and the impurities have the opposite effect on the entanglement below and above critical DM interaction. (ii) For the antiferromagnetic spin chain, DM interaction can enhance the entanglement to a steady value. Our results imply that DM interaction strength, the impurity and exchange couplings (or magnetic field) play competing roles in enhancing quantum entanglement. 相似文献
10.
Wu H George RE Wesenberg JH Mølmer K Schuster DI Schoelkopf RJ Itoh KM Ardavan A Morton JJ Briggs GA 《Physical review letters》2010,105(14):140503
Strong coupling between a microwave photon and electron spins, which could enable a long-lived quantum memory element for superconducting qubits, is possible using a large ensemble of spins. This represents an inefficient use of resources unless multiple photons, or qubits, can be orthogonally stored and retrieved. Here we employ holographic techniques to realize a coherent memory using a pulsed magnetic field gradient and demonstrate the storage and retrieval of up to 100 weak 10?GHz coherent excitations in collective states of an electron spin ensemble. We further show that such collective excitations in the electron spin can then be stored in nuclear spin states, which offer coherence times in excess of seconds. 相似文献
11.
We demonstrate that in a single molecule magnet strongly coupled to electrodes the Kondo effect involves all magnetic excitations. This Kondo effect is induced by the quantum tunneling of the magnetic moment. Importantly, the Kondo temperature TK can be much larger than the magnetic splittings. We find a strong modulation of the Kondo effect as a function of the transverse anisotropy parameter or a longitudinal magnetic field. Both for integer and half-integer spin this can be used for an accurate transport spectroscopy of the magnetic states in low magnetic fields on the order of the easy-axis anisotropy parameter. We set up a relationship between the Kondo effects for successive integer and half-integer spins. 相似文献
12.
We study theoretically the spin dynamics of antiferromagnetic molecular rings, such as the ferric wheel Fe10. For a single nuclear or impurity spin coupled to one of the electron spins of the ring, we calculate nuclear and electronic spin correlation functions and show that nuclear magnetic resonance (NMR) and electron spin resonance (ESR) techniques can be used to detect coherent tunneling of the Néel vector in these rings. The location of the NMR/ESR resonances gives the tunnel splitting and its linewidth an upper bound on the decoherence rate of the electron spin dynamics. We illustrate the experimental feasibility of our proposal with estimates for Fe10 molecules. 相似文献
13.
By the method of finite difference, the anisotropic spin splitting of the AlxGa1-xAs/GaAs/Aly Ga1-yAs/AlxGal-xAs step quantum wells (QWs) are theoretically investigated considering the interplay of the bulk inversion asymmetry and structure inversion asymmetry induced by step quantum well structure and external electric field. We demonstrate that the anisotropy of the total spin splitting can be controlled by the shape of the QWs and the external electric field. The interface related Rashba effect plays an important effect on the anisotropic spin splitting by influencing the magnitude of the spin splitting and the direction of electron spin. The Rashba spin splitting presents in the step quantum wells due to the interface related Rashba effect even without external electric field or magnetic field. 相似文献
14.
Based on a parameterized electronic many-body theory we calculate the spectrum of optical second-harmonic generation (SHG)
on an antiferromagnetic (AF) surface: NiO (001). The occurrence of AF spectral lines is further exploited for the calculation
of ultra-fast spin dynamics. This spin dynamics is observable in SHG. It is characterized by both a sharp drop of the AF signal
within a few femtoseconds and a long persistence (up to nanoseconds) of the spin coherence. These two features constitute
an ideal system for possible applications in both magnetic recording and quantum computing.
Received: 16 October 2001 / Revised version: 21 March 2002 / Published online: 6 June 2002 相似文献
15.
Xiao-Feng Shi 《Frontiers of Physics》2021,16(5):52501
Alkaline-earth-like (AEL) atoms with two valence electrons and a nonzero nuclear spin can be excited to Rydberg state for quantum computing. Typical AEL ground states possess no hyperfine splitting, but unfortunately a GHz-scale splitting seems necessary for Rydberg excitation. Though strong magnetic fields can induce a GHz-scale splitting, weak fields are desirable to avoid noise in experiments. Here, we provide two solutions to this outstanding challenge with realistic data of well-studied AEL isotopes. In the first theory, the two nuclear spin qubit states |0〉 and |1〉 are excited to Rydberg states |r〉 with detuning Δ and 0, respectively, where a MHz-scale detuning Δ arises from a weak magnetic field on the order of 1 G. With a proper ratio between Δ and Ω, the qubit state |1〉 can be fully excited to the Rydberg state while |0〉 remains there. In the second theory, we show that by choosing appropriate intermediate states a two-photon Rydberg excitation can proceed with only one nuclear spin qubit state. The second theory is applicable whatever the magnitude of the magnetic field is. These theories bring a versatile means for quantum computation by combining the broad applicability of Rydberg blockade and the incomparable advantages of nuclear-spin quantum memory in two-electron neutral atoms. 相似文献
16.
A. F. Popkov G. D. Demin N. E. Kulagin N. S. Mazurkin 《Bulletin of the Russian Academy of Sciences: Physics》2013,77(1):72-77
The phenomena of spin tunneling and spin torque transfer between magnetic layers of a tunnel spin-valve setup under weak and strong field emissions of spin-polarized electrons are considered. Bifurcational features of changes in the macrospin states under the impact of a tunnel current are discussed for varying directions of the spin-polarization vector. 相似文献
17.
A. Cuccoli A. Fort A. Rettori E. Adam J. Villain 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(1):39-46
We study the magnetic relaxation of a system of localized spins interacting through weak dipole interactions, at a temperature
large with respect to the ordering temperature but low with respect to the crystal field level splitting. The relaxation results
from quantum spin tunneling but is only allowed on sites where the dipole field is very small. At low times, the magnetization
decrease is proportional to as predicted by Prokofiev and Stamp, and at long times the relaxation can be described as an extension of a relaxed zone.
The results can be directly compared with very recent experimental data on Fe8 molecular clusters.
Received 9 February 1999 相似文献
18.
We propose a mean field approach to the transport properties of carbon nanotube quantum dots. Quantum interaction between spin and orbital pseudo-spin degrees of freedom results in an SU(4) Kondo effect at low temperatures. By calculating the chemical potentials and the tunnelling strengths, and hence the spectral functions for different coupling constants and applied magnetic fields, we find that this exotic Kondo effect manifests as a four-peak splitting in the non-linear conductance when an axial magnetic field is applied. 相似文献
19.
A family of highly sensitive devices based on a graphene nanobridge and superconducting electrodes has been developed, manufactured,
and examined. These devices can be used to create a graphene-based integral receiver. A cold-electron bolometer prototype
with superconductor-insulator-normal metal tunnel junctions has been studied. Its response to a change in the temperature
and external microwave radiation has been measured. A superconducting quantum interferometer with a graphene strip as a weak
coupling between superconducting electrodes has been examined. The corresponding modulation of the voltage by a magnetic field
at a given current has been measured. The effect of the gate voltage on the resistance of graphene has been analyzed for these
samples. To confirm that graphene is single-layer, measurements with the reference samples were performed in high magnetic
fields, displaying the half-integer quantum Hall effect. 相似文献
20.
The level splitting formulae of excited states as well as ground state for a biaxial spin particle in the presence of an applied magnetic field are obtained in a simple way from Schrödinger theory. Considering the boundary condition of the wave function, we obtain the tunneling splitting of the energy levels for half-integral spins as well as for the integral spins. The results obtained are compared with those previously derived by complicated pseudoparticle methods and numerical calculation values.Received: 12 May 2004, Published online: 3 August 2004PACS:
75.45. + j Macroscopic quantum phenomena in magnetic systems - 75.50.Xx Molecular magnets - 73.40.Gk Tunneling 相似文献