Theory Of Magnetotransport Research Articles

Understanding magnetic field effects in organic light-emitting devices

The discovery of magnetic field effects (MFEs) in organic light-emitting devices poses a new challenge for the theoretical studies. Based on the spin-boson theory of magnetotransport that we proposed recently, this article considers various contributions to the MFE. The role of triplet exciton, as an irreducible gradient of the environment in magnetotransport, is discussed extensively. Possible effect of the current caused by secondary charge on the MFE is taken into account to discuss the relation between magnetoresistance and magnetoelectroluminescence. Further experimental correspondence is also discussed.

Electronic Transport in Itinerant Metamagnets with Strong Static Disorder

We report on electronic transport in nearly magnetic conductors with strong structural disorder. The initial motivation for this work was a large positive magnetoresistance (MR) found in magnetically ordered ground state of (Y1-xGdx)Co2 alloys. This was a surprising result since a large positive MR is not expected in a system with strong static magnetic or structural disorder. Contemporary theory of magnetotransport and common sense agree that an external magnetic field should suppress magnetic fluctuations, resulting in a negative MR. On the contrary; a positive MR suggests that an external magnetic field enhances static magnetic disorder. It was shown that unusual MR of (Y1-xGdx)Co2 alloys is related to a combination of structural disorder and metamagnetic instability of itinerant Co-3d electrons. The new mechanism of MR is common of a broad class of materials featuring a static magnetic disorder and itinerant metamagnetism. Such systems display a number of unusual properties, among them strong pressure and magnetic field dependencies of resistivity and thermopower, Non-Fermi-Liquid (NFL) behavior of resistivity and, possibly, of thermopower. We review the relevant experimental data, mostly the properties of RCo2-based alloys, and discuss the theoretical model developed for the interpretation of the experimental results. This model includes new mechanism of magnetoresistivity in structurally disordered itinerant magnetic alloys.

Graphene magnetoresistance in a parallel magnetic field: Spin polarization effect

We develop a theory for graphene magnetotransport in the presence of carrier spin polarization as induced, for example, by the application of an in-plane magnetic field $(B)$ parallel to the two-dimensional graphene layer. We predict a negative magnetoresistance $\ensuremath{\sigma}\ensuremath{\propto}{B}^{2}$ for intrinsic graphene, but for extrinsic graphene we find a nonmonotonic magnetoresistance which is positive at lower magnetic fields (below the full spin polarization) and negative at very high fields (above the full spin polarization). The conductivity of the minority spin band $(\ensuremath{-})$ electrons does not vanish as the minority carrier density $({n}_{\ensuremath{-}})$ goes to zero. The residual conductivity of $(\ensuremath{-})$ electrons at ${n}_{\ensuremath{-}}=0$ is unique to graphene. We discuss experimental implications of our theory.

Classical and quantum magnetoresistance in a two-subband electron system

We observe a large positive magnetoresistance in a bilayer electron system (double quantum well) as the latter is driven by the external gate from double to single layer configuration. Both classical and quantum contributions to magnetotransport are found to be important for explanation of this effect. We demonstrate that these contributions can be separated experimentally by studying the magnetic-field dependence of the resistance at different gate voltages. The experimental results are analyzed and described by using the theory of low-field magnetotransport in the systems with two occupied subbands.

Electronic transport in mesoscopic superconductor/2D electron gas junctions in strong magnetic field

The hybrid superconductor/2D electron gas (S/2DEG) structures based on InGaAs-InP hetero-junctions with a high-mobility 2D electron gas and superconducting NbN electrodes have been investigated. The electronic transport and current-voltage characteristics of S/2DEG/normal metal (S/2DEG/N) structures in strong perpendicular magnetic fields have been studied. Oscillations in the magnetoresistance of S/2DEG/N structures have been found in strong magnetic fields. It is shown that at bias voltages lower than the superconducting gap the amplitude of oscillations in S/2DEG/N structures significantly exceeds the oscillation amplitude in the reference N/2DEG/N samples. The experimental results can be explained within the quasiclassical theory of magnetotransport in S/2DEG structures developed by N.M. Chtchelkatchev and I.S. Burmistrov (Phys. Rev. B, 2007, vol. 75, 214 510).

Scaling Theory of Magnetoresistance in Disordered Local Moment Ferromagnets

We present a scaling theory of magneto-transport in Anderson-localized disordered ferromagnets. Within our framework a pronounced magnetic-field-sensitive resistance peak emerges naturally for temperatures near the magnetic phase transition. We find that the resistance anomaly is a direct consequence of the change in localization length caused by the magnetic transition. For increasing values of the external magnetic field, the resistance peak is gradually depleted and pushed towards higher temperatures. Our results are in good agreement with magneto-resistance measurements on a variety of disordered magnets.

Local geometry of the Fermi surface and magnetoacoustic response of two-dimensionalelectron systems in strong magnetic fields

A semiclassical theory for magnetotransport in a quantum Hall system near filling factorν = 1/2 based on the composite fermion physical picture is used to analyse the effect of localflattening of the composite fermion Fermi surface (CF-FS) upon magnetoacousticoscillations. We report on calculations of the velocity shift and attenuation of a surfaceacoustic wave (SAW) which travels above the two-dimensional electron system, and weshow that local geometry of the CF-FS could give rise to noticeable changes inthe magnitude and phase of the oscillations. We predict these changes will berevealed in experiments, and will be used in further studies of the shape andsymmetries of the CF-FS. The main conclusions reported here could be appliedto analyse magnetotransport in quantum Hall systems at higher filling factorsν = 3/2 and 5/2 for a Fermi-liquid-like state of the system.

THEORY OF THE DC MAGNETOTRANSPORT IN LATERALLY MODULATED QUANTUM HALL SYSTEMS NEAR FILLING ν=½

A quasiclassical theory for DC magnetotransport in a modulated quantum Hall system near filling factor ν=½ is presented. A weak one-dimensional electrostatic potential acts on the two-dimensional electron gas. Closed form analytic expressions are obtained for the resistivity ρ⊥ corresponding to a current at right angles to the direction of the modulation lines as well as a smaller component ρ‖ for a current along the direction of the modulation lines. It is shown that both resistivity components are affected by the presence of the modulation. Numerical results are presented for ρ⊥ and ρ‖ and show reasonable agreement with the results of recent experiments.

Magnetotransport in a modulated two-dimensional electron gas

We propose a semiclassical theory of dc magnetotransport in a two-dimensional electron gas modulated along one direction with weak electrostatic modulations. We show that oscillations of the magnetoresistivity ρ∥ corresponding to the current driven along the modulation lines observed at moderately low magnetic fields can be explained as commensurability oscillations.

On the theory of magnetotransport in a periodically modulated two-dimensional electron gas

A semiclassical theory based on the Boltzmann transport equation for a two-dimensional electron gas modulated along one direction with weak electrostatic or magnetic modulations is proposed. It is shown that oscillations of the magnetoresistivity ρ ∥ corresponding to the current driven along the modulation lines observed at moderately low magnetic fields, can be explained as classical geometric resonances reflecting the commensurability of the period of spatial modulations and the cyclotron radius of electrons.

Gate-controlled spin-orbit quantum interference effects in lateral transport.

In situ control of spin-orbit coupling in coherent transport using a clean GaAs/AlGaAs two-dimensional electron gas is realized, leading to a gate-tunable crossover from weak localization to antilocalization. The necessary theory of 2D magnetotransport in the presence of spin-orbit coupling beyond the diffusive approximation is developed and used to analyze experimental data. With this theory the Rashba contribution and linear and cubic Dresselhaus contributions to spin-orbit coupling are separately estimated, allowing the angular dependence of spin-orbit precession to be extracted at various gate voltages.

Determination of the Fermi velocity by angle-dependent periodic orbit resonance measurements in the organic conductorα−(BEDT−TTF)2KHg(SCN)4

We report on detailed angle-dependent studies of the microwave $(\ensuremath{\nu}=50--90\mathrm{GHz})$ interlayer magnetoelectrodynamics of a single crystal sample of the organic charge-density-wave (CDW) conductor $\ensuremath{\alpha}\ensuremath{-}(\mathrm{BEDT}\ensuremath{-}\mathrm{TTF}{)}_{2}\mathrm{KHg}(\mathrm{SCN}{)}_{4}.$ Recently developed instrumentation enables both magnetic-field $(\mathbf{B})$ sweeps for a fixed sample orientation and angle sweeps at fixed $\ensuremath{\nu}/\mathbf{B}.$ We observe series' of resonant absorptions, which we attribute to periodic orbit resonances (POR)---a phenomenon closely related to cyclotron resonance. The angle dependence of the POR indicates that they are associated with the low-temperature quasi-one-dimensional (Q1D) Fermi surface (FS) of the title compound; indeed, all of the resonance peaks collapse onto a single set of $\ensuremath{\nu}/\mathbf{B}$ versus angle curves, generated using a semiclassical magnetotransport theory for a single Q1D FS. We show that Q1D POR measurements provide one of the most direct methods for determining the Fermi velocity, without any detailed assumptions concerning the band structure; our analysis yields an average value of ${v}_{F}=6.5\ifmmode\times\else\texttimes\fi{}{10}^{4}\mathrm{m}/\mathrm{s}.$ Quantitative analysis of the POR harmonic content indicates that the Q1D FS is strongly corrugated. This is consistent with the assumption that the low-temperature FS derives from a reconstruction of the high-temperature quasi-two-dimensional FS, caused by the CDW instability. Detailed analysis of the angle dependence of the POR yields parameters associated with the CDW superstructure, which are consistent with published results. Finally, we address the issue as to whether or not the interlayer electrodynamics are coherent in the title compound. We obtain a relaxation time from the POR linewidths, which is considerably longer than the interlayer hopping time, indicating that the transport in this direction is coherent.

HIGH PRESSURE TRANSPORT STUDY OF NON-FERMI LIQUID BEHAVIOUR IN U2Pt2In AND U3Ni3Sn4

The strongly correlated metals U 2 Pt 2 In and U 3 Ni 3 Sn 4 show pronounced non-Fermi liquid (NFL) phenomena at ambient pressure. Here we review single-crystal electrical resistivity measurements under pressure (p ≤ 1.8 GPa) conducted to investigate the stability of the NFL phase. For tetragonal U 2 Pt 2 In (I ‖ a) we observe a rapid recovery of the Fermi-liquid T2-term with pressure. The Fermi-liquid temperature varies as T FL ~ p-p c , where p c =0 is a critical pressure. The analysis within the magnetotransport theory of Rosch provides evidence for the location of U 2 Pt 2 In at a p=0 antiferromagnetic critical point (QCP). In the case of cubic U 3 Ni 3 Sn 4 we find T FL ~ (p-p c )1/2. The analysis provides evidence for an antiferromagnetic QCP in U 3 Ni 3 Sn 4 at a negative pressure p c = -0.04±0.04 GPa

Spin-Dependent Transport in Heavily Mn-Doped GaAs

The theory of magnetotransport in heavily Mn-doped GaAs, a p-type ferromagnetic semiconductor, is developed in the weak coupling limit by calculating the temperature and magnetic field dependences of the spin disorder scattering-limited hole mobility, The theory is based on the exchange interaction between the itinerant holes and the localized 3d spins of the Mn ions, and it is developed by using Zubarev's double-time Green's functions. The relaxation time of the charge carriers is calculated from the imaginary part of the self-energy appearing in the derived hole propagator. Adding a contribution from impurity scattering the total hole mobility is estimated. Comparison to the experimental resistivity data shows that a good agreement with the measured results is obtained only if the short-range order and the consequent T dependence of the critical scattering are taken into account in the paramagnetic region. The measured resistivity peak near Curie temperature is broader than the calculated one, which together with the estimated large exchange parameter J pd = 2.3 eV calls for an extension of the present theory to the intermediate or strong coupling cases.

High-pressure study of the non-Fermi liquid material U 2 Pt 2 In

The effect of hydrostatic pressure ( p ? 1.8 GPa) on the non-Fermi liquid state of U2Pt2In is investigated by electrical resistivity measurements in the temperature interval 0.3-300 K. The experiments were carried out on single-crystals with the current along ( I || c ) and perpendicular ( I || a ) to the tetragonal axis. The pressure effect is strongly current-direction dependent. For I || a we observe a rapid recovery of the Fermi-liquid T2-term with pressure. A comparison of the data with the magnetotransport theory of Rosch provides evidence for the location of U2Pt2In at an antiferromagnetic quantum critical point. For I || c the resistivity increases under pressure, indicating the enhancement of an additional scattering mechanism.

Surface-scattering-limited magnetotransport in thin metallic films

Electron transport in metallic films with little bulk scattering strongly depends on film roughness. The quantum theory of transport in this surface-scattering-limited regime has been investigated during the past decade using different approaches, but without considering the effect of magnetic field. We develop in this paper a theory of magnetotransport in this regime and compare the predictions with the rich experimental behavior reported for various systems.

Recovery of the Fermi-liquid state inU3Ni3Sn4by pressure

Measurements of the resistivity of single-crystalline U_3Ni_3Sn_4 under hydrostatic pressure up to 1.8 GPa are reported. We show that the temperature T_FL, below which the resistivity obeys the Fermi-liquid expression rho ~ T^2, increases with pressure as T_FL ~ (p-p_c)^1/2. The analysis of the data within the magnetotransport theory of Rosch lends strong support for an antiferromagnetic quantum critical point in U_3Ni_3Sn_4 at a negative critical pressure p_c of -0.04+-0.02 GPa. The proximity to an antiferromagnetic instability in U_3Ni_3Sn_4 is consistent with the previously reported non-Fermi liquid term in the low-temperature specific heat.

Magnetoresistance of a 2DEG in a 2D superlattice: Boltzmann theory of commensurability oscillations and their gradual suppression with increasing mobility

We present analytical and numerical results of a classical magnetotransport theory, based on Boltzmann's equation, for a two-dimensional electron gas in a two-dimensional superlattice of square symmetry. We show that commensurability oscillations of the magnetoresistance are increasingly suppressed with increasing modulation amplitude and increasing mean free path λtr, but will be observable for sufficiently weak modulation and short λtr. The calculation shows different type commensurability effects at stronger modulation.

The magnetoconductivity of a square lattice in a periodically modulated magnetic field

The static magnetoconductivity components σ xx and σ yy are calculated as a function of magnetic field for a square lattice with period a in a periodically modulated magnetic field (PMMF) with period b. We develop a magneto-transport theory for a periodic magnetic field B 1 sin(2 πx/b). Our model permits us to predict anomalies, such as the appearance of peaks, arising from the commensurability ratio β= a/ b between the period a of the crystal lattice and the period b of the PMMF, which originate from contributions to the conductivity due to the eigenstates whose energies are in the vicinity of the Fermi energy. Numerical results are obtained for the energy eigenvalue spectra as a function of the magnetic field and the energy dispersion.

Theory of magnetoacoustic transport in modulated quantum Hall systems nearν=1/2

Motivated by the experimental results of Willett et al. [Phys. Rev. Lett. $78,$ 4478 (1997)], we develop a magnetotransport theory for the response of a two-dimensional electron gas (2DEG) in the fractional quantum Hall regime near Landau-level filling factor $\ensuremath{\nu}=1/2$ to the surface acoustic wave (SAW) in the presence of an added periodic density modulation. We assume there exists a composite fermion Fermi surface (CF-FS) at $\ensuremath{\nu}=1/2,$ which is a circle, and we show that the deformation of the circular CF-FS due to the density modulation can be at the origin of the observed transport anomalies for the experimental conditions. Our analysis is carried out particularly for the nonlocal case, which corresponds to the SAW experiments. We introduce a concrete model of a deformed CF-FS. The model permits us to explain anomalous features of the response of the modulated 2DEG to the SAW near $\ensuremath{\nu}=1/2:$ namely, the nonlinear wave-vector dependence of the electron conductivity, the appearance of peaks in the SAW velocity shift and attenuation, and the anisotropy of the effect, all of which originate from contributions to the conductivity tensor due to the regions of the originally circular CF-FS, which are flattened by the applied modulation.