Spin freezing in geometrically frustrated β-Mn probed by muon spin relaxation

Muon spin relaxation was measured down to 0.05 K on a geometrically frustrated metallic system, β-Mn. The observed relaxation rate increases with decreasing temperature and unexpectedly makes a peak at 0.5 K indicating a spin freezing below the temperature. Actually the asymmetry is almost recovered by applying a longitudinal magnetic field of 0.1 T. But since the tail amplitude is more than half and the dynamical relaxation is still appreciable at the lowest temperature, it is likely that the frozen moments are associated with imperfection as oxygen impurity and the intrinsic ground state is fluctuating spin liquid. This revised version was published online in August 2006 with corrections to the Cover Date.     

Fano Interference versus Kondo Effect in Strongly Correlated T-Shaped Quantum Dots Embedded in an Aharonov-Bohm Ring

We theoretically investigate the properties of the ground state of the strongly correlated T-shaped double quantum dots embedded in an Aharonov-Bohm ring in the Kondo regime by means of the one-impurity Anderson Hamiltonian. It is found that in this system, the persistent current depends sensitively on the parity and size of the ring. With the increase of interdot coupling, the persistent current is suppressed due to the enhancing Fano interference weakening the Kondo effect. Moreover, when the spin of quantum dot embedded in the Aharonov- Bohm ring is screened, the persistent current peak is not affected by interdot coupling. Thus this model may be a new candidate for detecting Kondo screening cloud.     

Current and Shot Noise in a Quantum Dot Coupled to Ferromagnetic Leads in the Kondo Regime

Using the Keldysh nonequilibrium Green function technique, we study the current and shot noise spectroscopy of an interacting quantum dot coupled to two ferromagnetic leads with different polarizations in the Kondo regime. General formulas of current and shot noise are obtained, which can be applied in both the parallel （P） and antiparallel （AP） alignment cases. For large polarization values, it is revealed that the behaviour of differential conductance and shot noise are completely different for spin up and spin down configurations in the P alignment case. However, the differential conductance and shot noise have similar properties for different spin configurations in the P alignment case with the small polarization value and in the AP alignment case with any polarization value.     

Tunable Kondo Effect of a Three-Terminal Transport Quantum Dot Embedded in an Aharonov-Bohm Ring

We theoretically investigate the Kondo effect of a three-terminal transport quantum dot （QD） embedded in an Aharonov-Bohm ring in the Kondo regime by means of the one-impurity Anderson Hamiltonian. The Hamiltonian is solved by means of the slave-boson mean-field theory. We find that in this system, the Kondo effect depends sensitively oil the parity and size of the ring; the Kondo screening cloud can be tuned by tuning the coupling strength of the reservoir-dot. Thus this model might be a candidate for future device applications.     

Kondo Resonance versus Fano Interference in Double Quantum Dots Coupled to a Two-Lead One-Ring System

We analyse the transport properties of a coupled double quantum dot （DQD） device with one of the dots （QD1） coupled to metallic leads and the other （QD2） embedded in an Aharonov-Bhom （A-B） ring by means of the slaveboson mean-field theory. It is found that in this system, the Kondo resonance and the Fano interference exist simultaneously, the enhancing Kondo effect and the increasing hopping of the QD2-Ring destroy the localized electron state in the QD2 for the QD1-leads, and accordingly, the Fano interference between the DQD-leads and the QD1-leads are suppressed. Under some conditions, the Fano interference can be quenched fully and the single Kondo resonance of the QD1-leads comes into being. Moreover, when the magnetic flux of the A-B ring is zero, the influence of the parity of the A-B ring on the transport properties is very weak, but this influence becomes more obvious with non-zero magnetic flux. Thus this model may be a candidate for future device applications.     

Photon recycling transport in bulk GaAs probed by μSR with laser irradiation

A newly developed μSR method combined with a tunable, high power and sharply pulsed laser was applied to GaAs. Anomalous transport phenomena of the photo excited effects into bulk crystal were observed, given that the stopping range of implanted muons is about 200 times larger than photo-absorption depth. In addition, the measurement of photon energy dependence of muon spin depolarization under laser irradiation suggests the impurity level associated with Mu _BC ⁰ is located within the band gap of GaAs. This revised version was published online in August 2006 with corrections to the Cover Date.     

A Decoherence-Reduction Scheme by Waveguides in Quantum Information Processing

Based on the result of cavity quantum electrodynamics, we suggest a method, in which the Fabry-Perot cavity or the confocal cavity is replaced by a waveguide with the size comparable to the wavelength of the photon, to reduce decoherence caused by spontaneous emission in quantum information processing, especially in the realization of quantum computation. Since a waveguide has a lowest cutoff frequency while a Fabry-Perot cavity or a confocal cavity has none, the spontaneous emission of excited atoms will be forbidden in an ideal waveguide with an appropriate size. To avoid the influence of the non-ideal conducting walls on the atom in a realistic waveguide, which will lead to decoherence, we suggest that the waveguide should be coated by a thin film of transparent insulating medium. In our method, the quantum information is represented by a multi-level atom or molecule; any two of its levels can be used to represent a qubit in principle. Our method greatly extends the choice of the material to be used in the realization of quantum computation, and it can be used in most schemes to reduce the decoherence caused by spontaneous emission.     

Observation of Quantum Beat in Rb by Parametric Four-Wave Mixing

Two coupled paralnetric four-wave-mixing processes in Rb atoms are studied using perturbation theory, which reveals clear evidence of tile appearance of quantum beat at 608cm^-1, corresponding to the energy difference of the 7s - 5d states of Rb atoms, in the parametric four-wave-mixing signals. A pump-probe technique is utilized to observe the quantum beat. Tinle-varying characteristics of the quantum beat are investigated using time-dependellt Fourier transform. The results show that the time-varying characteistics of the quantum beat not oldy offers a sensitive detecting method for observing the decay of atomic wave packets, but also provides a potential tool for monitoriHg the dissociation of molecules.     

Quantum critical properties in the topological Ginzburg–Landau theory of self-dual Josephson junction arrays

This paper examines the multicritical behavior of a generalized U(_N1)×U(_N2)$U\left(N_1\right)\times U\left(N_2\right)$ Ginzburg–Landau theory containing two multicomponent complex fields which couple differently to two gauge fields described by two Maxwell terms and one mixed-Chern–Simons term. This model is relevant to the dynamics of Cooper pairs and vortices in a self-dual Josephson junction array system near its superconductor–insulator transition. We develop a renormalization group flow at fixed dimension and obtain the beta functions at one loop when both disorder fields are critical. Two sets of infrared-stable charged fixed points solutions are found for N>_Nc$N>N_c$: partially charged solutions with respect to the gauge fields exist with _Nc=35.6$N_c=35.6$, and fully charged solutions exist with _Nc=12.16$N_c=12.16$. We show that fine tuning the ratio of the two energy scales in the model has the effect of reducing the critical number _Nc$N_c$ and thus enlarges the region where the quantum phase transition is continuous. It is also found that the decoupled fixed point which is stable in the neutral case is no longer attainable in the presence of fluctuating gauge fields. We probe the conductivity at the critical point and show that it has a universal character determined by the renormalization group infrared-stable fixed-point values of the gauge couplings.     

Effect of substitution of Ce by La in the CeNixPt1 − x dense Kondo ferromagnets

The effects of substitution of Ce by La in the orthorhombic CeNi_{1 − x}Pt_x dense Kondo ferromagnets are studied by means of magnetization and electrical resistivity measurements. A decrease of the exchange RKKY interactions leads to a decrease of the Curie temperature T_c as a function of the La content and hence to an enhancement of the Kondo character in the thermal dependence of the resisitivity. However, the Ce moment is almost independent of the La amount. The Kondo temperature being also independent, this surprising result seems in contradiction with the available Kondo lattice models.     

Global Estimates of Errors in Quantum Computation by the Feynman–Vernon Formalism

The operation of a quantum computer is considered as a general quantum operation on a mixed state on many qubits followed by a measurement. The general quantum operation is further represented as a Feynman–Vernon double path integral over the histories of the qubits and of an environment, and afterward tracing out the environment. The qubit histories are taken to be paths on the two-sphere \(S^2\) as in Klauder’s coherent-state path integral of spin, and the environment is assumed to consist of harmonic oscillators initially in thermal equilibrium, and linearly coupled to to qubit operators \(\hat{S}_z\). The environment can then be integrated out to give a Feynman–Vernon influence action coupling the forward and backward histories of the qubits. This representation allows to derive in a simple way estimates that the total error of operation of a quantum computer without error correction scales linearly with the number of qubits and the time of operation. It also allows to discuss Kitaev’s toric code interacting with an environment in the same manner.     

Quantum Cryptography in Spin Networks

In this paper we propose a new scheme of long-distance quantum cryptography based on spin networks with qubits stored in electron spins of quantum dots. By＂ conditional Faraday- rotation, single photon polarization measurement, and quantum state transfer, maximal-entangled Bell states for quantum cryptography between two long-distance parties are created. Meanwhile, efficient quantum state transfer over arbitrary＂ distances is obtained in a spin chain by＂ a proper choice of coupling strengths and using spin memory- technique improved. We also analyse the security＂ of the scheme against the cloning-based attack which can be also implemented in spin network and discover that this spin network cloning coincides with the optimal fidelity- achieved by＂ an eavesdropper for entanglement-based cryptography.     

Insights into copper coordination in the EcoRI–DNA complex by ESR spectroscopy

The EcoRI restriction endonuclease requires one divalent metal ion in each of two symmetrical and identical catalytic sites to catalyse double-strand DNA cleavage. Recently, we showed that Cu²⁺ binds outside the catalytic sites to a pair of new sites at H114 in each sub-unit, and inhibits Mg²⁺-catalysed DNA cleavage. In order to provide more detailed structural information on this new metal ion binding site, we performed W-band (～94 GHz) and X-band (～9.5 GHz) electron spin resonance spectroscopic measurements on the EcoRI–DNA–(Cu²⁺)₂ complex. Cu²⁺ binding results in two distinct components with different g_zz and A_zz values. X-band electron spin echo envelope modulation results indicate that both components arise from a Cu²⁺ coordinated to histidine. This observation is further confirmed by the hyperfine sub-level correlation results. W-band electron nuclear double resonance spectra provide evidence for equatorial coordination of water molecules to the Cu²⁺ ions.     

Femtosecond coherence in poly(p-Phenylene- vinylene) – polarization beatings and phase-relaxation probed by wavepacket fluorescence interferometry

Coherent, optical excitation transients have been measured in the bottom-states of the dynamically disordered multi-chromophore poly(p-phenylenevinylene) at low phonon temperatures. Interferometric femtosecond (fs) excitation/probing experiments using freely propagating 70 fs pulses have been employed to generate spatially overlapping electronic and nuclear wavepackets. Narrow-bandwidth detection tuned to two vibronic 01 S₁S₀ fluorescence transitions with discrete arrival-states , , respectively, projects-out two distinct site-subensembles from the pulsed target-area of site excitations. By monitoring the interferences of the fluorescence correlations for - and -photons on the detector, the coherent superposition can be probed as a typical polarization beating. The fluorescence interferograms show strong damping on an overall time-scale of 200 fs with typically, half-to-single cycle beating waves. The relaxation of coherence is indicative of a predominantly homogeneous site-dephasing and hence, gives rise to the observation of the structural equilibration of the initial Franck–Condon (FC)-excitation, i.e. the formation of the dressed state. PACS 78.47.+p; 42.50.Md; 42.25.Kb     

Quantum coherence in λ-three-level system driven by bichromatic coupling field

We present a theoretical study of quantum coherent effects in a λ-three-level system with a strong bichromatic coupling field and a weak probe field. When one component of the strong bichromatic coupling field is resonant with a corresponding transition and the other is detuning with an integer fraction of the Rabi frequency of the resonant field, the absorption spectrum exhibits a series of symmetrical doublets. While two frequencies of the strong bichromatic coupling field are symmetrically detuned from the transition, the position and the relative intensity of the absorption peak are both affected by the coupling field intensity and detuning. An explanation of the spectrum is given in term of the dressed-state formalism.     

Dynamical Localization in an Asymmetric Two-Electron Quantum Dot Molecule by an Alternating-Current Electric Field

We investigate the dynamics of two interacting electrons in an asymmetric double coupled quantum dot under an ac electric field. The numerical results demonstrate that dynamical localization and Rabi oscillation still exist in such a system under the stronger electron correlation. The two electrons can be regarded as a quasiparticle, which move together between two dots similarly to a boson. The dynamics of two electrons in such a quantum system are mainly confined in a Q subspace, which is constructed by two double-occupied states.     

Quantum critical behaviour in Ce3Pd20Si6?

The cage compound Ce₃Pd₂₀Si₆ has recently been shown to undergo two successive low-temperature phase transitions which are strongly affected by an applied magnetic field. Here we show that, as the lower, probably antiferromagnetic transition is suppressed to zero in a field slightly above 1 T, the electrical resistivity shows a non-Fermi-liquid-like linear-in-T   temperature dependence while it follows the usual Fermi liquid T²$T^2$ temperature dependence both at smaller and larger fields. This suggests that a field-induced quantum critical point exists in Ce₃Pd₂₀Si₆.     

Hyperfine fields in Fe crystal detected by β-emitting12N

Hyperfine magnetic fields in a single crystal Fe were measured by use of asymmetric decay and NMR detection of spin polarized -emitting¹²N.