Magnetic Field Dependent Tunneling of Atoms and Molecules in Non-Magnetic Disordered Solids

The low-temperature properties of disordered solids, such as glasses or crystals with certain substitutional defects are governed by atomic tunneling systems. Until recently it was believed that the dielectric properties of insulating materials devoid of magnetic impurities should not—or only very weakly—depend on external magnetic fields. In contrast, new experiments on glasses and crystalline defect systems show a pronounced magnetic field dependence of the dielectric properties of such materials at ultra-low temperatures. In particular, the low-frequency dielectric susceptibility and the amplitude of polarization echoes appear to be strongly affected by magnetic fields. These very surprising findings clearly indicate that atomic tunneling systems can couple to magnetic fields. We summarize the available data and discuss the possible origin of these intriguing phenomena.     

Quantum phenomena in glasses at low temperatures

Glasses exhibit surprising low-temperature properties caused by the tunneling motion of small atomic clusters. We report here on recent dielectric measurements on a glass with the components BaO–Al₂O₃–SiO₂. In contrast to expectation, below 100 mK the dielectric properties become sensitive to weak magnetic fields. In this temperature range dielectric constant and dielectric loss show an oscillatory behavior with increasing magnetic field. Below 6 mK a phase transition within the ensemble of tunneling systems is observed.     

Novel isotope effects observed in polarization echo experiments in glasses

In recent years unexpected magnetic field effects have been observed in dielectric measurements on insulating glasses at very low temperatures. Polarization echo experiments have indicated that atomic tunneling systems are responsible for these effects and that the nuclear properties of the tunneling particles are of importance. Subsequently, it was suggested that the magnetic field effects are caused by tunneling systems carrying a nuclear quadrupole moment. Now we have studied the isotope effect in echo experiments on fully deuterated and ordinary glycerol clearly showing the crucial role of the nuclear quadrupole moments for the magnetic field effects. In addition, we have observed a new effect in the decay of spontaneous echoes in zero magnetic field for the deuterated samples which can be explained in terms of a quantum beating involving the quadrupole levels.     

Effect of magnetic field on tunneling systems in glasses

The effect of a magnetic field on two-level tunneling systems in dielectric glasses originates from the magnetic-field-induced rotation of nuclear spins and the ensuing rearrangement of ordered regions (clusters) in the glass structure. This process accounts for the observed variation of both the spontaneous-polarization echo amplitude and the dielectric constant in a magnetic field at low temperatures. The proposed theory is compared with experiment.     

Atomic tunneling states in low-temperature dielectric and elastic losses of diamagnetic glassy semiconductors

A mechanism of low-temperature dielectric and elastic losses in some diamagnetic amorphous structures (e.g. glassy semiconductors) is presented. The mechanism is associated with atomic tunneling states which (as earlier stated) cause also the low-temperature anomalies of thermal properties of such systems. Some related questions are also discussed.     

Pressure induced quantum critical point and non-fermi-liquid behavior in BaVS3

We report on experiments giving evidence for quantum effects of electromagnetic flux in barium alumosilicate glass. In contrast to expectation, below 100 mK the dielectric response becomes sensitive to magnetic fields. The experimental findings include both lifting of the dielectric saturation by weak magnetic fields and oscillations of the dielectric response in the low temperature resonant regime. As the origin of these effects we suggest that the magnetic induction field violates the time reversal invariance leading to a flux periodicity in the energy levels of tunneling systems. At low temperatures, this effect is strongly enhanced by the interaction between tunneling systems and thus becomes measurable.     

Direct coupling of magnetic fields to tunneling systems in glasses

We report on investigations of spontaneous polarization echoes in the nonmagnetic multicomponent glass BaO-Al2O3-SiO2 in static magnetic fields. While the echo decay is only marginally influenced, the echo amplitude depends strongly on magnetic fields. It seems that the intrinsic magnetic moment of tunneling systems causes dephasing effects which are detected in our echo experiments. In addition we find a strong increase of the echo amplitude with magnetic fields. This result shows that the coupling of the tunneling systems to magnetic fields is surprisingly strong and cannot be understood on the basis of current theories.     

Thermodynamic properties of small amorphous and crystalline Silica particles at low temperatures

We have measured the low-temperature (K) specific heat and heat release of small amorphous and crystalline SiO₂ particles embedded in Teflon and of Vycor. The temperature and time dependence of these properties have been interpreted in terms of the tunneling model. We found that the particle size influences the density of states of tunneling systems of the composite. The smaller the size of the particles the larger is the density of states of tunneling systems P₀. Quartz grains with dimensions in the micrometer range show similar glass-like properties as vitreous silica. In comparison with bulk vitreous silica, Vycor shows a much larger P₀ in agreement with the behavior we found for small SiO₂ particles. We discuss the implication of our results on the origin of the universal low-temperature properties of glasses. Received 9 April 1998     

Partial ordered homogeneous magnetic moments in the Hubbard-model

In this contribution, we study the behaviour of magnetically disordered electron systems. In a model with localized magnetic fields at the atomic sites, a CPA-like method, which has regard for the vector character of the fields, is used to examine the case where the localized fields, which correspond to atomic magnetic moments, are distributed statistically. In an example for a non-isotropic distribution of the fields, we construct a system state with partial homogeneous order of the localized fields (or moments). The ordering behaviour of the system and the comparison of the new magnetic state with pure magnetic and non-magnetic band states and with the non-magnetic state of (isotropic) stochastic distribution of the localized fields is discussed. With this paper we are able to introduce typical properties of localized models into a band model.Work supported in part by the Deutsche Forschungsgemeinschaft     

Optical properties of some magnetic materials for coherent magnetic vibrations

In this communication we report experimental results concerning optical properties of magnetic crystals such as rare-earth iron garnets and orthoferrites doped with semimetals such as bismuth and tin. The components of the dielectric tensor of the doped magnetic material exhibit favourable magnetic properties. Such kinds of materials are employed for generating coherent magnetic vibrations having amplitudes sufficiently large for detection.     

Effect of nuclear quadrupole interactions on the dynamics of two-level systems in glasses

The standard tunneling model describes quite satisfactorily the properties of amorphous solids at temperatures T < 1K in terms of an ensemble of two-level systems including the logarithmic temperature dependence of the dielectric constant. Yet, experiments have shown that at ultralow temperatures T< 5 mK such a temperature behavior breaks down and the dielectric constant becomes temperature independent (plateau effect). In this Letter we suggest an explanation of this behavior exploiting the effect of the nuclear quadrupole interaction on tunneling. We also predict that the application of a sufficiently large magnetic field B> 10T should restore the logarithmic dependence because of the suppression of the nuclear quadrupole interaction.     

Dark-state imaging for two-dimensional mapping of a magnetic field

By imaging the dark states that are due to coherent population trapping in Na atomic vapor, we have succeeded in depicting a spatially inhomogeneous magnetic field. Highly resolved dark lines represent cross sections of the surfaces of constant magnetic fields, and they agree well with the predicted hyperfine Zeeman splitting and the two-photon selection rules of the Na atom. Mapping was made two dimensionally in real time, but extension to three dimensions is straightforward. Furthermore, unlike the previous techniques based on optical pumping, this method can be used for any magnetic-field directions.     

Electromagnetically induced magnetochiral anisotropy in a resonant medium

Chirality has been extensively studied for well over a century, and its potential applications range from optics to chemistry, medicine, and biology. Ingenious experiments have been designed to measure this naturally small effect. Here we discuss the possibility of producing a medium having a large chiral effect by using the ideas of coherent control. The coherent fields resonant with appropriate transitions in atomic or molecular systems can be used to manipulate the optical properties of a medium. We demonstrate experimentally very large magnetochiral anisotropy by using electromagnetic fields in atomic Rb vapors.     

Three level atom optics in dipole traps and waveguides

An analogy is explored between a setup of three atomic traps coupled via tunneling and an internal atomic three-level system interacting with two laser fields. Within this scenario we describe a STIRAP like process which allows to move an atom between the ground states of two trapping potentials and analyze its robustness. This analogy is extended to other robust and coherent transport schemes and to systems of more than a single atom. Finally it is applied to manipulate external degrees of freedom of atomic wave packets propagating in waveguides.     

Quantum oscillation of the tunneling conductance in fully epitaxial double barrier magnetic tunnel junctions

We investigated spin-dependent tunneling conductance properties in fully epitaxial double MgO barrier magnetic tunnel junctions with layered nanoscale Fe islands as a middle layer. Clear oscillations of the tunneling conductance were observed as a function of the bias voltage. The oscillation, which depends on the middle layer thickness and the magnetization configuration, is interpreted by the modulation of tunneling conductance due to the spin-polarized quantum well states created in the middle Fe layer. This first observation of the quantum size effect in the fully epitaxial double barrier magnetic tunnel junction indicates great potential for the development of the spin-dependent resonant tunneling effect in coherent tunneling regime.     

The filamentary structure of the sunspot magnetic fields

The filamentary structure of the magnetic fields as well as the coherent radiations that emanate from a sunspot are explained considering solar burst as a non-equilibrium process. Methods of irreversible statistical mechanics have been applied to the problem of an electron gas in a constant magnetic field to explain the above features. We have obtained the non-equilibrium distribution function in the self-consistent field approximation. The dielectric function, we obtained, is a function of time, besides being a function of frequency and wavevector. We have thus taken the non-linearity of the system as well. This theory explains many features of stria bursts, chain bursts as well as the type III bursts. This also accounts for the bunching of the magnetic field lines as a consequence of quantisation of flux in the Landau sense.     

Finite-temperature density functional theory of atoms in strong magnetic fields

The density functional theory of atomic electrons in strong magnetic fields is generalized to finite-temperature systems. General integral formulations are developed in the format of Mermin-Kohn-Sham finite-temperature density functional theory. The lowest order of the general theory leads to a temperature-dependent extended Thomas-Fermi (TETF)-like functional, which is simple enough to be analyzed. The general theory provides a new way of calculating the equilibrium properties of many-electron systems in strong magnetic fields.     

Two-channel Kondo effect in glasslike ThAsSe

We present low-temperature heat and charge transport as well as caloric properties of a ThAsSe single crystal. An extra -AT(1/2) term in the electrical resistivity, independent of magnetic fields as high as 14 T, provides evidence for an unusual scattering of conduction electrons. Additionally, both the thermal conductivity and the specific heat show a glass-type temperature dependence which signifies the presence of tunneling states. These observations apparently point to an experimental realization of a two-channel Kondo effect derived from structural two-level systems.     

Mössbauer studies of iron-rich metallic glasses

Iron-based metallic glasses have recently become an important class of ferromagnetic materials exhibiting excellent soft magnetic properties coupled with good mechanical properties. These glasses are usually prepared by rapid quenching techniques and are produced in thin long ribbon form with widths ranging from a few mm to 150 mm or more.⁵⁷Fe Mössbauer spectroscopy has been extensively used to study hyperfine interaction parameters in these metallic glasses to understand ferromagnetism in amorphous structure. In particular, Mössbauer spectra have been carefully analyzed to reveal information about the distribution of hyperfine fields resulting from the randomness of the atomic arrangement and to understand the temperature dependence of hyperfine fields, spin-wave excitations, magnetic structure, thermal stability and crystallization, the quenched-in magnetization axis, the Curie temperature and its dependence on compositions, the effect of stress and pressure on the magnetic properties, corrosion behaviour, local order and atomic arrangement, phase transformation, etc. This paper reviews the application of⁵⁷Fe Mössbauer spectroscopy to magnetic studies on metallic glasses mainly based on the iron-boron alloy system, and some of the significant results obtained which are characteristic of the glassy/amorphous state.     

Effects of electron inelastic transport through the potential relief of a spin dimer in a magnetic field

The influence of inelastic effects on electron quantum transport through the potential relief of a dimer system was studied by exact solution of the Schrodinger equation. The nature of this problem is due to the coherent superposition of the different potential profiles through which the spin-polarized electron tunnels. It was found that the low magnetic field initiates new peaks of resonant tunneling. In high magnetic fields, the transport of electrons with opposite spin polarization is qualitatively different.