Low Magnetic Field Regime Research Articles

Antiferromagnetic Spin Seebeck Effect.

We report on the observation of the spin Seebeck effect in antiferromagnetic MnF_{2}. A device scale on-chip heater is deposited on a bilayer of MnF_{2} (110) (30 nm)/Pt (4nm) grown by molecular beam epitaxy on a MgF_{2} (110) substrate. Using Pt as a spin detector layer, it is possible to measure the thermally generated spin current from MnF_{2} through the inverse spin Hall effect. The low temperature (2-80K) and high magnetic field (up to 140kOe) regime is explored. A clear spin-flop transition corresponding to the sudden rotation of antiferromagnetic spins out of the easy axis is observed in the spin Seebeck signal when large magnetic fields (>9 T) are applied parallel to the easy axis of the MnF_{2} thin film. When the magnetic field is applied perpendicular to the easy axis, the spin-flop transition is absent, as expected.

Magnetoresistive polyaniline-silicon carbide metacomposites: plasma frequency determination and high magnetic field sensitivity.

The Drude model modified by Debye relaxation time was introduced to determine the plasma frequency (ωp) in the surface initiated polymerization (SIP) synthesized β-silicon carbide (β-SiC)/polyaniline (PANI) metacomposites. The calculated plasma frequency for these metacomposites with different loadings of β-SiC nanoparticles was ranging from 6.11 × 10(4) to 1.53 × 10(5) rad s(-1). The relationship between the negative permittivity and plasma frequency indicates the existence of switching frequency, at which the permittivity was changed from negative to positive. More interestingly, the synthesized non-magnetic metacomposites, observed to follow the 3-dimensional (3-D) Mott variable range hopping (VRH) electrical conduction mechanism, demonstrated high positive magnetoresistance (MR) values of up to 57.48% and high MR sensitivity at low magnetic field regimes.

Emergence of high-mobility minority holes in the electrical transport of theBa(Fe1−xMnxAs)2iron pnictides

In Fe pnictide (Pn) superconducting materials, neither Mn- nor Cr- doping to the Fe site induces superconductivity, even though hole carriers are generated. This is in strong contrast with the superconductivity appearing when holes are introduced by alkali metal substitution on the insulating blocking layers. We investigate in detail the effects of Mn doping on magneto-transport properties in Ba(Fe$_{1-x}$Mn$_x$As)$_2$ for elucidating the intrinsic reason. The negative Hall coefficient for $x$ = 0 estimated in the low magnetic field ($B$) regime gradually increases as $x$ increases, and its sign changes to a positive one at $x$ = 0.020. Hall resistivities as well as simultaneous interpretation using the magnetoconductivity tensor including both longitudinal and transverse transport components clarify that minority holes with high mobility are generated by the Mn doping via spin density wave (SDW) transition at low temperatures, while original majority electrons and holes residing in the parabolic-like Fermi surfaces (FSs) of the semimetallic Ba(FeAs)$_2$ are negligibly affected. Present results indicate that the mechanism of hole doping in Ba(Fe$_{1-x}$Mn$_x$As)$_2$ is greatly different from that of the other superconducting FePns family.

Critical current and flux pinning properties of the superconducting Ti–V alloys

We present a study on the critical current density and the flux-line pinning properties of a series of annealed and as cast samples of Ti100−xVx (x=20, 30, 40 and 60) alloys, performed with the help of magnetization measurements. We show that the grain boundaries are the dominant flux pinning centers in the Ti–V alloys in the low and intermediate magnetic field regime. In the high magnetic field regime, the dislocation networks and the point pinning mechanism provided by the ω phase present in these alloys are more effective than the grain boundaries. While in the low magnetic fields we find the signature of surface effects enhancing the pinning force density, a peak effect in the critical current is observed in these alloys in the high field regime near the upper critical field HC2. Using the existing theories we provide an analysis of the pinning force density over a large range of magnetic fields, and obtain some new insight on the influence of various metallurgical phases, grain boundaries and dislocation networks on the flux-line pinning properties of these alloys.

Non-monotonic magnetoresistance in an AlGaN/GaN high-electron-mobility transistor structure in the ballistic region

In this report, we will discuss the nonmonotonic magnetoresistance (MR) in an AlGaN/GaN high-electron-mobility transistor (HEMT) in a perpendicular magnetic field B in the ballistic region (kBTτ/ħ < 1) and in the weakly-disordered limit (kFl = 159 ≫ 1), where kB, T, τ, ħ, k F, and l represent the Boltzmann constant, temperature, elastic scattering time, reduced Planck constant, Fermi wave vector and mean free path, respectively. The MR shows a local maximum between the weak localization (WL) and the Shubnikov-de Haas regions. In the low magnetic field regime, the quantum correction to the conductivity is proportional to T −3/2, which is consistent with a recent theory [T. A. Sedrakyan, and M. E. Raikh, Phys. Rev. Lett. 100, 106806 (2008)]. According to our results, as the temperature is increased, the position of the MR maximum in B increases. These results cannot be explained by present theories. Moreover, in the high-magnetic-field regime, neither the magnetic and nor the temperature dependences of the observed MR is consistent with present theories. We, therefore, suggest that while some features of the observed nonmonotonic MR can be successfully explained, further experimental and theoretical studies are necessary to obtain a thorough understanding of the MR effects.

Magnetoresistive conductive polymer-tungsten trioxide nanocomposites with ultrahigh sensitivity at low magnetic field

The effect of conductive polymer matrix including polyaniline (PANI) and polypyrrole (PPy) on the magnetoresistance (MR) behaviors in the variable range hopping (VRH) regime has been investigated in the disordered polymer nanocomposites containing tungsten trioxide (WO3) nanoparticles. These nanocomposites have demonstrated ultrahigh MR sensitivity at low magnetic field regime. The observed positive MR has been well explained by the wave-function shrinkage model. The conductive polymer matrix has shown different effects on the MR behaviors of the nanocomposites. The WO3/PANI nanocomposites have a lower localization length (a0) and density of states at the Fermi level (N(EF)), and higher average hopping distance (Rhop) and average hopping energy (W) compared with those of the WO3/PPy nanocomposites.

Anisotropic low field magnetoimpedance in (001) oriented La0.7Sr0.3MnO3 thin films

We report the evidence for the anisotropic magnetoimpedance behavior in (001) oriented La0.7Sr0.3MnO3 (LSMO) thin films, in low frequency–low magnetic field regime. (001) oriented LSMO thin films were deposited using pulsed laser deposition and characterized with X-ray diffraction and temperature dependent magnetization studies. In the in-plain configuration, an ac magnetoresistance (MRac) of ∼−0.5% was observed at 1000 Oe, at 100 Hz frequency in these films. The MRac was found to decrease with increase in frequency. We observe increases in MRac at low frequency, indicating major contribution for change of permeability from domain wall motion. At higher frequencies, it decreases due to decrease in transverse permeability, resulting from dampening of domain wall motion. Out-of-plane configuration showed MRac ∼ 5.5% at 1000 Oe, at 100 Hz frequency. The MRac turned from positive to negative with increase in frequency in out-of-plane configuration. These changes are attributed to the change in permeability of the film with the frequency and applied magnetic field.

Coherent Control of an NV- center and a nearby 13C nuclear spin

ABSTRACTIn this work, we numerically investigated the achievable fidelities when controlling an effective three-qubit system consisting of a NV- color center in diamond with a nearby strongly coupled 13C nuclear spin by means of microwave- and radio-frequency pulses in the experimentally attractive low magnetic field regime. We find that gates with straightforward square driving pulses do not achieve the fidelity currently required for the fault-tolerant quantum computing models.

Testing the transverse field Ising model in LiHoF4using capacitive dilatometry

Measurements of thermal expansion and magnetostriction on a single-crystal sample of the Ising ferromagnetic material, LiHoF${}_{4}$, are presented. The strain is measured along the crystallographic $c$ axis (the Ising direction) of the material and the magnetic field is applied transverse to this, in the $ab$ plane. The data are used to extract critical fields and critical temperatures for the ferromagnetic transition, resulting in a phase diagram for this material. The measurements are strongly focused on the very low magnetic field regime, where previous magnetic susceptibility data show a discrepancy with theory based on the transverse field Ising model, although the previous experimental data density is very sparse. The results of this study generate a high-resolution magnetic phase line that is consistent with previous measurements and therefore confirm the discrepancy. These detailed measurements can be used to significantly constrain any future theoretical work which aims to explain the observed behavior.

Overlapping-Gate Architecture for Silicon Hall Bar MOSFET Devices in the Low Electron Density and High Magnetic Field Regime

A common issue in low temperature measurements of enhancement-mode metal-oxide-semiconductor (MOS) field-effect transistors (FETs) in the low electron density regime is the high contact resistance dominating the device impedance. In that case a voltage bias applied across the source and drain contact of a Hall bar MOSFET will mostly fall across the contacts (and not across the channel) and therefore magneto-transport measurements become challenging. However, from a physical point of view, the study of MOSFET nanostructures in the low electron density regime is very interesting (impurity limited mobility [1], carrier interactions [2,3] and spin-dependent transport [4]) and it is therefore important to come up with solutions [5,6] that work around the problem of a high contact resistance in such devices (c.f. Fig. 1 (a)).

Electronic Transport in Graphitic Nanoribbon Films

We fabricate, pattern, and analyze thin films composed of multilayer graphene nanoribbons. These films are conductive at room temperature but depict noticeable insulating behavior at low temperatures (<20 K) due to their disordered structure. We study the transport in this strong localization regime by analyzing the dependence of resistivity on temperature and electric and magnetic fields in the framework of the variable range hopping theory. Resistivity dependence on the magnetic field confirms the insulating behavior of the films and can be fitted effectively by forward interference scattering and wave function shrinkage models at low and high magnetic field regimes, respectively. We extract large localization lengths in the range of ∼45-90 nm from both the magnetic and electric field dependence of resistivity and relate these values to the high conductance in the nanoribbons and/or good contact between them. By revealing the fundamental structural and transport properties of graphitic nanoribbon films, our results help devise methods to further improve these films for electronic and photonic device applications.

Investigation of absorption mechanisms in helicon discharges in conducting waveguides

This paper investigates the mechanisms by which the helicon and associated Trivelpiece–Gould waves are absorbed in helicon discharges produced in conducting chamber; the experiments were based on a recent theory of damping and absorption of helicon modes in conducting waveguides (Ganguli et al 2007 Phys. Plasmas 14 113503). In particular, it was also planned to investigate the manner in which the absorbed energy is utilized for the production of warm electrons that are needed for ionization because helicon discharges are high density, low Te discharges and the tail of the bulk electron population may not have sufficient high-energy electrons. To this end, two separate regimes were considered. The first was a low pressure (≈0.2–0.3 mTorr), low magnetic field (≈16–20 G) regime where both wave absorption and warm electron production are shown to proceed through Landau damping. The second was a moderate pressure (≈10 mTorr), moderate magnetic field (≈60–65 G) regime, where both power absorption (which is collisional) and warm electron production proceed via high-energy electrons produced by acceleration of bulk electrons (from neighboring regions) across large potential gradients.

Crossover from negative to positive magnetoresistance in the double quantum well systemwith different starting disorder

Magnetotransport measurements were performed in two widely separated double quantumwell systems with different starting disorders. In the weak magnetic field regime, acrossover from negative to positive magnetoresistance in the longitudinal resistivity wasobserved in the system with weak disorder when the electron densities in the neighboringwells were significantly unbalanced. The crossover was found to be the resultof the exchange-energy-assisted interactions between the electrons occupyingthe lowest subbands in the neighboring wells. In the case of the system withstrong disorder short range scattering dominated the scattering process and nosuch transition in longitudinal resistivity in the low magnetic field regime wasobserved. However, at high magnetic fields, sharp peaks were observed in the Hallresistance due to the interaction between the edge states in the quantum Hall regime.

Measurement of diffusion thermopower in the quantum Hall systems

We have measured diffusion thermopower in a two-dimensional electron gas at low temperature ( T = 40 mK ) in the field range 0 < B < 3.4 T , by employing the current heating technique. A Hall bar device is designed for this purpose, which contains two crossing Hall bars, one for the measurement and the other used as a heater, and is equipped with a metallic front gate to control the resistivity of the areas to be heated. In the low magnetic field regime ( B ≤ 1 T ), we obtain the transverse thermopower S yx that quantitatively agrees with the S yx calculated from resistivities using the generalized Mott formula. In the quantum Hall regime ( B ≥ 1 T ), we find that S yx signal appears only when both the measured and the heater area are in the resistive (inter-quantum Hall transition) region. Anomalous gate-voltage dependence is observed above ∼ 1.8 T , where spin-splitting in the measured area becomes apparent.

Fundamental relation between longitudinal and transverse conductivities in the quantumHall system

We investigate the relation between the diagonal (σxx) andoff-diagonal (σxy) components of the conductivity tensor in the quantum Hall system. We calculate theconductivity components for a short-range impurity potential using the linear responsetheory, employing an approximation that simply replaces the self-energy by a constantvalue with τ the scattering time. The approximation is equivalent to assuming that the broadening of aLandau level due to disorder is represented by a Lorentzian with the width . Analytic formulae are obtained for bothσxx andσxy withinthe framework of this simple approximation at low temperatures. By examining the leading terms inσxx andσxy, we find a proportionalrelation between dσxy/dB and Bσxx2. The relation, after slight modification to account for the long-range nature of the impuritypotential, is shown to be in quantitative agreement with experimental results obtained inthe GaAs/AlGaAs two-dimensional electron system at the low magnetic field regime wherespin splitting is negligibly small.

Magnetotransport properties of individual InAs nanowires

We probe the magnetotransport properties of individual InAs nanowires in a field-effect transistor geometry. In the low magnetic field regime we observe magnetoresistance that is well described by the weak localization description in diffusive conductors. The weak localization correction is modified to weak antilocalization as the gate voltage is increased. We show that the gate voltage can be used to tune the phase coherence length $({l}_{\ensuremath{\phi}})$ and spin-orbit length $({l}_{\text{so}})$ by a factor of 2. In the high field and low-temperature regime we observe that the mobility of devices can be modified significantly as a function of magnetic field. We argue that the role of skipping orbits and the nature of surface scattering is essential in understanding high-field magnetotransport in nanowires.

Phase effects in HgTe quantum structures

AbstractHgTe quantum well structures with high electron mobilities have been used to fabricate quantum interference devices. Aharonov‐Bohm oscillations have been studied in the low and high magnetic field regime. In the latter case a decrease of the effective ring radius is observed. Additionally, as a consequence of the strong Rashba spin‐orbit coupling within this material, it was possible to observe conductance oscillations which are due to the so‐called Aharonov‐Casher effect. These quantum interference effects are effectively controlled by the applied magnetic and electric field. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Magnetotransport in a doubly connected two-dimensional quantum Hall system in the low magnetic field regime

In a preceding paper [Phys. Rev. B 72, 035334 (2005)], experimental results for the so-called ``anti-Hall bar within a Hall bar'' configuration have been compared with numerical simulations for the high magnetic field regime. The sample structure is a doubly connected, double-boundary electronic system that has been experimentally investigated by Mani. The application of a network model for magnetotransport allows us to evaluate the longitudinal and Hall voltages in this geometry. Thus, within our network model, we rebuild the experimental configuration, including the sample geometry and the two independent floating current sources. In this paper we extend our calculations to the low magnetic field regime, where the quantum Hall effect is not yet established. Like in the high magnetic field regime, we realize the Hall voltages and longitudinal voltages at both the inner and outer boundaries to behave in excellent agreement with Mani's experiments. We find that the Hall voltages at the inner (anti-Hall bar) and outer (Hall bar) boundaries depend just on the individual current injected via the corresponding boundary, while the longitudinal voltage depends exactly on the sum of both injected currents.

Flux-flow voltage noise and normal-state resistance fluctuations in epitaxial (Dy, Y)Ba 2Cu 3O 7 thin films

We have studied low-frequency noise in the normal and mixed states of high- T c thin films. In the normal state, resistance fluctuations with a 1/ f power spectrum are observed from 100 K up to 300 K. The normal state noise is largely attributable to hopping or reordering of oxygen vacancies which in turn lead to fluctuations in local carrier density in the copper oxygen planes. Analysis shows a substantial density of oxygen defect states with activation energies below 0.5 eV, followed by a rapid increase at higher energies. In the mixed state, we have measured the current-induced, vortex-motion voltage noise. When the current–voltage characteristics are nonlinear there exist two distinct regimes. The first is a low magnetic field regime where measurements of the variations of voltage noise with magnetic field reveal reproducible, microstructure-dependent noise magneto-fingerprints. Rapid variations of the noise as a function of magnetic field and ac current drive experiments indicated that the noise is associated with metastability of pinned vortex configurations. Above magnetic fields which roughly match vortex separation with the grain size in the films, the magnitude and fine structure of the noise magneto-fingerprints are suppressed, suggesting a crossover to a more weakly pinned vortex state. The second regime is a low bias voltage regime where the observed noise scales linearly with bias voltage and has a 1/ f-like spectrum. The linear voltage scaling is reminiscent of vortex shot noise although no characteristic transit time could be determined. At higher voltage biases, the noise power drops off rapidly. We attribute the sharp drop off to a loss of pinning at higher temperatures and fields which results in the absence of metastability in the vortex pinned states. The temperature and magnetic field dependence of the noise are discussed in the context of various models including vortex shot noise, channel, critical current fluctuation, and vortex-glass models.

Low-field diffusion magnetothermopower of a high-mobility two-dimensional electron gas

The low magnetic field diffusion thermopower of a high mobility GaAs-heterostructure has been measured directly on an electrostatically defined micron-scale Hall-bar structure at low temperature (T = 1.6 K) in the low magnetic field regime (B < 1.2 T) where delocalized quantum Hall states do not influence the measurements. The sample design allowed the determination of the field dependence of the thermopower both parallel and perpendicular to the temperature gradient, denoted respectively by Sxx (longitudinal thermopower) and Syx (Nernst-Ettinghausen coefficient). The experimental data show clear oscillations in Sxx and Syx due to the formation of Landau levels for 0.3 T < B < 1.2 T and reveal that Syx is approximately 120 times larger than Sxx at a magnetic field of 1 T, which agrees well with the theoretical prediction.