Evidence for magnetic field induced changes of the phase of tunneling states: spontaneous echoes in (KBr)(1-x)(KCN)(x) in magnetic fields

Recently it was discovered that, in contrast to expectations, the low-temperature dielectric properties of some multicomponent glasses depend strongly on magnetic fields. The low-temperature dielectric response of these materials is governed by atomic tunneling systems. We now have investigated the influence of magnetic fields on the coherent properties of atomic tunneling states in a crystalline host in two-pulse echo experiments. As in glasses, we observe a very strong magnetic field dependence of the echo amplitude. Moreover, for the first time we have direct evidence that the magnetic fields change the phase of coherent tunneling systems.     

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.     

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.     

Oscillations of echo amplitude in glasses in a magnetic field induced by nuclear dipole-dipole interaction

The influence of a magnetic field on the dipole echo amplitude in glasses (at temperatures of about 10 mK) induced by the dipole-dipole interaction of nuclear spins has been theoretically studied. It has been shown that a change in the mutual position of nuclear spins at tunneling and the Zeeman energy E _H of their interaction with the external magnetic field lead to a nonmonotonic magnetic-field dependence of the dipole echo amplitude. The approximation that the nuclear dipole-dipole interaction energy E _d is much smaller than the Zeeman energy has been found to be valid in the experimentally important cases. It has been shown that the dipole echo amplitude in this approximation may be described by a simple universal analytic function independent of the microscopic structure of the two-level systems. An excellent agreement of the theory with the experimental data has been obtained without fitting parameters (except for the unknown echo amplitude).     

Zero-field splittings of NQR spectra for bismuth(III) oxy compounds revealed by quadrupole spin echo envelopes

Local magnetic fields up to 250 G were earlier found by measuring the NQI parameters in bismuth(III) oxy compounds conventionally considered as diamagnets, a strong increase in the ²⁰⁹Bi line intensities being observed in external magnetic fields. An approach based on registration of the quadrupole spin-echo envelopes enabled to reveal small (within an inhomogeneous line broadening) splittings in some other compounds of this type. The modeling of time dependence of the quadrupole spin echo amplitude indicated that modulations of the spin echo envelope in BaBiO₂Cl and Bi₃B₅O₁₂ resulted from weak (≤5 G) local magnetic fields. By using this approach, it was found that an increase in the ²⁰⁹Bi resonance intensity in external magnetic fields is related to an influence of the fields on the nuclear spin-spin relaxation rate for the appropriate compounds.     

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 a magnetic field on the dipole echo in glasses caused by nuclear quadrupole moments

The effect of a magnetic field on the amplitude of the dipole echo in glasses at a temperature of about 10 mK caused by the presence of nonspherical nuclei with electric quadrupole moments in the glass has been considered theoretically. It has been shown that in this case, the two-level systems (TLSs) that determine the properties of glasses at low temperatures are transformed into more complicated multilevel systems. These systems have new properties as compared to usual TLSs and, in particular, exhibit oscillations of the electric dipole echo amplitude in the magnetic field. A general formula that describes the echo amplitude in an arbitrarily split TLS has been derived in perturbation theory. Detailed analytic and numerical analysis of the formula has been performed. The theory agrees qualitatively and quantitatively well with the experimental data.     

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.     

Dissipative control in thermal ensembles using tunneling ionization

It has been observed that non-resonant tunneling ionization of diatomic molecules by strong laser fields can lead to readily observable coherent vibrations in the molecular ground state of both hydrogen and iodine. Moreover, we have shown that this process, called “Lochfrass” or “R-selective ionization,” produces larger amplitude coherent motion in hotter systems than cold. In contrast, reversible interactions, like bond-softening, become less effective as the temperature increases. In this paper, we present a density matrix analysis to demonstrate this unusual temperature dependence and suggest that dissipative interactions, like tunneling ionization, can provide strong-field control in hot systems.     

Effect of a magnetic field on thermally stimulated ionization of impurity centers in semiconductors by submillimeter radiation

The probability of electron tunneling from a bound state into a free state in crossed ac electric and dc magnetic fields is calculated in the quasiclassical approximation. It is shown that a magnetic field decreases the electron tunneling probability. This decreases the probability of thermally activated ionization of deep impurity centers by submillimeter radiation. The logarithm of the ionization probability is a linear function of the squared amplitude of the electric field and increases rapidly with the frequency of the electric field.     

Photon echo in gases: From the non-Faraday rotation to unpolarized echo

A retrospective review of both theoretical and experimental works on photon echo in gases in the presence of longitudinal magnetic fields is presented from the viewpoint of new possibilities opened by this research for polarization echo spectroscopy of gases. The main attention is given to the physical scenario of the magnetic field’s effect on the properties of the photon echo. New results on the photon echo and stimulated photon echo in ytterbium vapor at the 1 ? 0 transition in the presence of the longitudinal magnetic field whose strength ranges from weak to strong are presented. Possible applications of the magnetic field effects for optical data storage and processing are analyzed.     

Energy gap of a charge density wave in NbSe3 induced by a high magnetic field above the Peierls transition temperature

The effect of a magnetic field on the energy gap of the charge density wave (CDW) in NbSe₃ near the temperature T _p2 of the lower Peierls transition has been investigated using interlayer tunneling spectroscopy. It has been shown that the magnetic field increases the energy gap and can even induce it at temperatures higher than T _p2 by 15–20 K. As the field strength increases, the peak amplitude of the gap singularity of the tunneling spectrum first increases, reaches its maximum at 20–30 T, and then decreases. The increase in the gap peak amplitude is attributed to the field-induced improvement of the condition of the CDW nesting, while the decrease in the amplitude in high fields, to the breakdown of the ground state caused by its Zeeman splitting.     

Resonant tunneling transport through GaAs/AlGaAs superlattices in strong tilted magnetic field

An analysis is presented of the transverse resonant tunneling transport through GaAs/AlGaAs superlattices due to tunneling between Landau levels in quantum wells in a strong tilted magnetic field. A high tunneling rate is demonstrated between Landau levels with Δn ≠ 0 in a magnetic field with a nonzero in-plane component. This leads to substantial broadening and shift of the tunneling resonance and significant changes in the current-voltage characteristics of superlattices. The predicted behavior of the current-voltage characteristics of superlattices in tilted magnetic fields is demonstrated experimentally.     

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.     

Formation of the pulsed electron-electron double resonance signal in the case of a finite amplitude of microwave fields

A theoretical study of the effect of microwave (MW) fields of finite amplitude on the process of pulsed electron-electron double resonance (PELDOR) signal formation is carried out. It is shown that the behavior of the experimentally observed values can be described by four vectors of partial magnetizations whose motion is reduced to precession in effective magnetic fields. In the case of strong spin-spin interaction, the PELDOR effect can be observed when a sufficiently powerful MW field is applied at pumping frequency to affect both components of the Pake doublet. The possibility of a “two-frequency” spin echo to appear under the action of two pulses with different carrier frequencies in the system where the inhomogeneous broadening of the electron spin resonance line contour is mainly determined by the dipole-dipole interaction is demonstrated.     

Observation of Spin and Valley Splitting of Landau Levels under Magnetic Tunneling in Graphene/Boron Nitride/Graphene Structures

Resonance magnetic tunneling in heterostructures formed by graphene single sheets separated by a hexagonal boron nitride barrier and bounded by two gates has been investigated in a strong magnetic field, which has allowed observing transitions between spin- and valley-split Landau levels with various indices belonging to different graphene sheets. An unexpected increase with the temperature in the interlayer tunneling conductance owing to transitions between the Landau levels in strong magnetic fields cannot be explained by existing theories.     

Josephson effect in ferromagnetic superconductors

The DC Josephson effect in ferromagnetic superconductors is theoretically studied in the presence of external magnetic fields. In addition to the usual term for the Josephson tunneling current, a new term expressing the pair tunneling current induced by the magnetization appears. The amplitude and period of the Fraunhofer pattern of the maximum Josephson current drastically diminish near the phase transition temperature for the magnetization bound around the junction.     

Quantum Hall effect in intentionally disordered two‐dimensional electron systems

In this work intentionally disordered two‐dimensional electron systems in modulation doped GaAs/GaAlAs heterostructures are studied by magnetotransport experiments. The disorder is provided by a δ‐doped layer of negatively charged beryllium acceptors. In low magnetic fields a strong negative magnetoresistance is observed that can be ascribed to magnetic‐field‐induced delocalization. At increased magnetic fields the quantum Hall effect exhibits broad Hall plateaus whose centers are shifted to higher magnetic fields, i.e. lower filling factors. This shift can be explained by an asymmetric density of states. Consistently, the transition into the insulating state of quantum Hall droplets in high magnetic fields occurs at critical filling factors around ν_c=0.4, i.e. well below the value 1/2 that is expected for symmetric disorder potentials. The insulator transition is characterized by the divergence of both the longitudinal resistance as well as the Hall resistance. This is contrary to other experiments which observe a finite Hall resistance in the insulating regime and has not been observed previously. According to recent theoretical studies the divergence of the Hall resistance points to quantum coherent transport via tunneling between quantum Hall droplets. The magnetotransport experiments are supplemented by simulations of potential landscapes for random and correlated distributions of repulsive scatterers, which enable the determination of percolation thresholds, densities of states, and oscillator strengths for far‐infrared excitations. These simulations reveal that the strong shift of the Hall plateaus and the observed critical filling factor for the insulator transition in high magnetic fields require an asymmetric density of states that can only be generated by a strongly correlated beryllium distribution. Cyclotron resonance on the same samples also indicates the possibility of correlations between the beryllium acceptors.     

Ehrenfest ‘phase-space’ trajectories and quantum chaos in He atom under strong,oscillating magnetic fields: an application of time-dependent quantum fluid density functional theory (TDQFDFT)

Using a dynamical signature proposed earlier from our laboratory, quantum chaos in He atom interacting with strong, oscillating magnetic fields has been studied through a comparison between the nonlinear divergence of two neighbouring Ehrenfest ‘phase-space’ (EPS) trajectories differing slightly in initial conditions and the Loschmidt echo. The dynamical EPS signature can detect quantum chaos independently of the Loschmidt echo and in agreement with the latter, even for low-lying states, in the same spirit as that of classical chaos. This time-dependent signature extends the concept of quantum chaos to systems which have no classical counterparts and brings the concept of quantum chaos closer to that of classical chaos.     

Strong coupling superconductivity and prominent superconducting fluctuations in the new superconductor Bi4O4S3

Electric transport and scanning tunneling spectrum(STS)have been investigated on polycrystalline samples of the new superconductor Bi4O4S3.A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic field.Interestingly,a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely suppressed.This kink associated with the upper critical field as well as the wide range of excess conductance at low fields and high temperatures is explained as the possible evidence of strong superconducting fluctuation.From the tunneling spectra,a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2Δ/kB TC~16.6.This value is much larger than the one predicted by the BCS theory in the weak coupling regime(2Δ/kB TC~3.53),which suggests the strong coupling superconductivity in the present system.Furthermore,the gapped feature persists on the spectra until 14 K in the STS measurement,which suggests a prominent fluctuation region of superconductivity.Such a superconducting fluctuation can survive at very high magnetic fields,which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements.