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The energy of magnetopolaron levels is calculated at the resonance of the first excited level with the ground-state level plus one longitudinal-optical phonon for a parabolic quantum well using an improved resonance approximation. The degeneracy between these levels is lifted by the electron-phonon interaction and the levels are resonance split. It is shown by numerical calculations that the renormalization of the levels ΔE± above and below the crossing energy depends strongly on the confinement energy of the parabolic potential. We obtain analytic results for the energy shifts ΔE± in the limits of two-dimensional (2D) and three-dimensional (3D) resonant magnetopolarons.

We study the density of states (DOS) for a periodically modulated two-dimensional (2D) electronic system in a perpendicular magnetic field in the presence of a smooth random potential. We find an exact analytical solution for the DOS which is valid under the conditions that both the period of modulation and the correlation length of random potential are larger than the magnetic length. The magnetocapacitance, calculated with the use of this DOS, fits nicely the experimental data by Weiss et al. for the case of one-dimensional periodic potential. The fit of experimental data by Ismail et al. for magnetocapacitance of the samples with two-dimensional periodic modulation is also presented.

We investigate the energy levels of the negatively and positively charged excitons (also called trions) in a 200 Angstrom wide GaAs quantum well in the presence of a perpendicular magnetic field. A comparison is made with the experimental results of Glasberg et al. (Phys. Rev. B. 59 (1999) R10 425) and of Yusa et al. (cond-mat/0103505). (C) 2002 Elsevier Science B.V. All rights reserved.

Comprehension of the origins of asymmetric behavior during the magnetization reversal is an outstanding issue in the studies of exchange bias (EB). By far, two intrinsic origins of the asymmetric magnetization reversal behavior (AMRB) have been found in different systems separately. The compatibility of the two origins has not been proved-yet. Here, we report on the AMRBs derived from cooperation between the two AMRB origins. Novel AMRBs, including the odd shape of the hysteresis loops and a positive EB, were found in the Co/FeMn bilayers grown under a perpendicular magnetic field. The striking AMRBs we observed could be explained by a model based on spin incoherent rotation and antiferromagnetic interfacial coupling. It suggests that the two AMRB origins are indispensable in the development of these unique AMRBs.

An external magnetic field, H, applied parallel to a quasi-two-dimensional carrier system modifies quantitatively and qualitatively the density of states. We examine how this affects primary thermodynamic properties, namely, the entropy, S, the internal and free energy, U and F, the magnetization, M, and the magnetic susceptibility, χ(m), using a self-consistent numerical approach. Although M is mainly in the opposite direction to H, the system is not linear. Hence, surprisingly, [Formula: see text] swings between negative and positive values, i.e. a diamagnetic to paramagnetic transition of entirely orbital origin is predicted. This phenomenon is important compared to the ideal de Haas-van Alphen effect, i.e. the corresponding phenomenon under perpendicular magnetic field. By augmenting temperature, the diamagnetic to paramagnetic transition fades away. The overall behaviour of entropy is also foreseen and consistently interpreted. While the entropy contribution to the free energy is very small at low temperatures, entropy shows a clear dependence on the external magnetic field.

We study the effect of electron-electron interaction and spin on electronic and transport properties of gated graphene nanoribbons (GNRs) in a perpendicular magnetic field in the regime of the lowest Landau level (LL). The electron-electron interaction is taken into account using the Hartree and Hubbard approximations, and the conductance of GNRs is calculated on the basis of the recursive Greens function technique within the Landauer formalism. We demonstrate that, in comparison to the one-electron picture, electron-electron interaction leads to the drastic changes in the dispersion relation and structure of propagating states in the regime of the lowest LL showing a formation of the compressible strip and opening of additional conductive channels in the middle of the ribbon. We show that the latter are very sensitive to disorder and get scattered even if the concentration of disorder is moderate. In contrast, the edge states transport is very robust and can not be suppressed even in the presence of a strong spin-flipping., 6 pages, 3 figures

We present transport measurements on a strongly coupled graphene quantum dot in a perpendicular magnetic field. The device consists of an etched single-layer graphene flake with two narrow constrictions separating a 140 nm diameter island from source and drain graphene contacts. Lateral graphene gates are used to electrostatically tune the device. Measurements of Coulomb resonances, including constriction resonances and Coulomb diamonds prove the functionality of the graphene quantum dot with a charging energy of approximately 4.5 meV. We show the evolution of Coulomb resonances as a function of perpendicular magnetic field, which provides indications of the formation of the graphene specific 0th Landau level. Finally, we demonstrate that the complex pattern superimposing the quantum dot energy spectra is due to the formation of additional localized states with increasing magnetic field., Nanoscale Research Letters, 6 (1), ISSN:1931-7573, ISSN:1556-276X

We analyze the four-dimensional Hamiltonian proposed to describe the band structure of the single-Dirac-cone family of topological insulators in the presence of a uniform perpendicular magnetic field. Surface Landau level(LL) states appear, decoupled from the bulk levels and following the quantized energy dispersion of a purely two-dimensional surface Dirac Hamiltonian. A small hybridization gap splits the degeneracy of the central n=0 LL with dependence on the film thickness and the field strength that can be obtained analytically. Explicit calculation of the spin and charge densities show that surface LL states are localized within approximately one quintuple layer from the surface termination. Some new surface-bound LLs are shown to exist at a higher Landau level index., Comment: 8 pages, 4 figures

We investigate the phenomenon of integer quantum Hall effect in a square lattice, subjected to a perpendicular magnetic field, through Landauer-B\"uttiker formalism within the tight-binding framework. The oscillating nature of longitudinal resistance and near complete suppression of momentum relaxation processes are examined by studying the flow of charge current using Landauer-Keldysh prescription. Our analysis for the lattice model corroborates the finding obtained in the continuum model and provides a simple physical understanding., Comment: 5 pages, 5 figures

Twisted graphene monolayers form a sample of two closely spaced two dimensional systems. We have performed transport measurements on such decoupled graphene layers being jointly contacted and conducting in parallel to investigate the properties of charge carriers in both layers. Varying the charge carrier concentration and applying perpendicular magnetic field, the electric field effect, Shubnikov‐de Haas oscillations and plateaus in the Hall resistance are observed. At the charge neutrality point, the resistance decreases by 15 percent with increasing temperature from 1.5 to 50 Kelvin.

Properties of the diffusive motion in a strong magnetic field are discussed. We describe two possible phase memory breaking mechanisms. We present a high magnetic field form of the discussion propagator which can be used to describe the conductance fluctuations of a two-dimensional electron gas in the regime of well developed Shubnikov-de Haas oscillations.

The dynamics of two-dimensional electrons in a circular ring in an external perpendicular magnetic field is investigated. The boundaries of the ring are approximated by infinite potential barriers. Within the quasiclassical approximation, a complete classification of the electron states generated by confinement and magnetic field is provided.

Coherent microwave-induced effects in 2D arrays of hysteretic Josephson junctions placed in a perpendicular magnetic field are theoretically investigated. Two new phenomena: fractional inverse giant Josephson effect and modification of the V-A characteristic beyond Shapiro steps are found. A theory of these effects based on the idea of a moving vortex superlattice is developed. A new model of coherent phase dynamics is proposed.

We investigate transport in locally gated graphene devices, where carriers are injected and collected along, rather than across, the gate edge. Tuning densities into the $p\mathrm{\text{\ensuremath{-}}}n$ regime significantly reduces resistance along the $p\mathrm{\text{\ensuremath{-}}}n$ interface, while resistance across the interface increases. This provides an experimental signature of snake states, which zigzag along the $p\mathrm{\text{\ensuremath{-}}}n$ interface and remain stable as applied perpendicular magnetic field approaches zero. Snake states appear as a peak in transverse resistance measured along the $p\mathrm{\text{\ensuremath{-}}}n$ interface. The generic role of snake states in disordered graphene is also discussed.

We put forward a two-terminal valley filter based on a bulk graphene sheet under the modulations of both a local perpendicular magnetic field and a substrate strain. When only one of the two modulations is present, no valley polarization can be generated. A combination of the two modulations leads to a different (but not opposite) shifts of the K and K' valleys, which could be utilized to generate a valley-polarized current. The degree of the valley polarization can be tuned by the strain strength and the inclusion of a scalar potential. The valley polarization changes its polarity as the local magnetic field switches its direction.

We investigate theoretically quantum transport through graphene nanorings in the presence of a perpendicular magnetic field. Our theoretical results demonstrate that the graphene nanorings behave like a resonant tunneling device, contrary to the Aharonov-Bohm oscillations found in conventional semiconductor rings. The resonant tunneling can be tuned by the Fermi energy, the size of the central part of the graphene nanorings and the external magnetic field.

The conductance of strongly confined one-dimensional constrictions fabricated from Si/SiGe two-dimensional electron gases is investigated. Conductance measurements reveal conductance quantization in units of G(0)=2e(2)/h rather than 2G(0)=4e(2)/h, as expected in the presence of valley and spin degeneracy. Furthermore, at temperatures below T=400 mK, small steps and peaklike features, superimposed to the conductance plateaus, become visible. The conductance in the presence of parallel and perpendicular magnetic field shows that significant valley splitting is present even at zero magnetic field. The enhanced valley splitting observed in our etched devices is related to the strong in-plane confinement.

The effects of electron interaction on the magnetoconductance of graphene nanoribbons (GNRs) are studied within the Hartree approximation. We find that a perpendicular magnetic field leads to a suppression instead of an expected improvement of the quantization. This suppression is traced back to interaction-induced modifications of the band structure leading to the formation of compressible strips in the middle of GNRs. It is also shown that the hard wall confinement combined with electron interaction generates overlaps between forward and backward propagating states, which may significantly enhance backscattering in realistic GNRs. The relation to available experiments is discussed., 4 pages, 3 figures

The intrinsic pinning properties of FeSe0.5Te0.5, which is the superconductor with Tc of about 14 K, were studied by the analysis of magnetization curves by the extended critical state model. In the magnetization measurements by SQUID magnetometer, the external magnetic fields were applied parallel and perpendicular to c-axis of the sample. The critical current density Jc's under the perpendicular field of 1 T were estimated by using the Kimishima model as about 1.6 x 10^4, 8.8 x 10^3, 4.1 x 10^3, and 1.5 x 10^3 A/cm2 at 5, 7, 9, and 11 K, respectively, and the temperature dependence of Jc could be fitted with the exponential law of Jc(0)xexp(-{\alpha}T /Tc) up to 9 K and power law of Jc(0)x(1-T / Tc)n near Tc., Comment: 19 pages, 7 figures

A 2-D electron system in the presence of a perpendicular magnetic field of arbitrary strength is expected to form a Wigner solid in certain regimes of density and filling factor. Some estimates of the phase diagram in these two parameters are presented and a few recent experimental results are reviewed.

The authors discuss the dynamics of voltage-driven Josephson junction arrays with negligible capacitances. The array dynamics shows a sensitive dependence on initial phases, leading to current-spike structures in the time-averaged I-V relations. They demonstrate this effect explicitly for the simple system of a two-junction chain, which they also use to illustrate the main features of current response to external driving voltages. They then show that a class of simplified ansatz solutions of two-dimensional voltage-driven Josephson junction arrays under a perpendicular magnetic field can be reduced to two-junction dynamics. In both cases, they find that coherent phase-slip processes can lead to subharmonic lockings with an external RF field.

We demonstrate that ballistic electrons in ultra-high mobility two-dimensional electron systems can be controlled in ways analogous to the manipulation of photons in optical systems. Among the electron-optical structures we demonstrate are emitters, detectors, absorbers, and refractors. These advances may enable the design of new types of electronic circuits.

Based on the Bean critical state model j???jc for both longitudinal and transverse geometry, the total dissipation power P along the virgin magnetization curves is calculated. The dissipation power obtained from the corresponding magnetic measurements for a disk-shaped YBa2Cu3O7-? thin film in a perpendicular magnetic field is verified. The virgin magnetization curves for the sample at different temperatures are calculated.

Two quantized charge pumps are operated in parallel. The total current generated is shown to be far more accurate than the current produced with just one pump operating at a higher frequency. With the application of a perpendicular magnetic field the accuracy of quantization is shown to be $l20\text{ }\text{ppm}$ for a current of 108.9 pA. The scheme for parallel pumping presented in this work has applications in quantum information processing, the generation of single photons in pairs and bunches, neural networking, and the development of a quantum standard for electrical current. All these applications will benefit greatly from the increase in output current without the characteristic decrease in accuracy as a result of high-frequency operation.

The authors report on modeling of transport spectroscopy in split-gate controlled quantum constrictions. A mixed momentum-coordinate representation is employed to solve a set of time-dependent Lippmann-Schwinger equations with intricate coupling between the subbands and the sidebands. Our numerical results show that the transport properties are tunable by adjusting the ac-biased split-gates and the applied perpendicular magnetic field. We illustrate time-modulated quasibound-state features involving inter-sideband transitions and the Aharonov-Bohm oscillation characteristics in the split-gated systems., RevTeX (pdf-LaTeX), 8 pages with 10 included jpg figures

Time dependent Ginzburg‐Landau equation was used to investigate the commensurate effect in superconductor having lattice holes arrays. Study was focused on the visualization of the commensurate effects on the superconductor having holes arrays and on the critical cunent density enhancement. It was found that when the superconductor having electric current density exposed on perpendicular magnetic field, the critical current density was enhanced through commensurate effect. The enhancement factors on critical current were influenced by the holes size, number of holes, and lattice holes geometry.

We investigate an effective low energy theory of HgTe quantum wells near their mass inversion thickness in a perpendicular magnetic field. By comparison of the effective band structure with a more elaborated and well-established model, the parameter regime and the validity of the effective model is scrutinized. Optical transitions in HgTe quantum wells are analyzed. We find selection rules which we functionalize to optically manipulate edge state transport. Qualitatively, our findings equally apply to optical edge current manipulation in graphene.

We study the coherent transport of heavy holes through a one-dimensional ring in the presence of spin-orbit coupling. Spin-orbit interaction of holes, cubic in the in-plane components of momentum, gives rise to an angular momentum dependent spin texture of the eigenstates and influences transport. We analyze the dependence of the resulting differential conductance of the ring on hole polarization of the leads and the signature of the textures in the Aharonov-Bohm oscillations when the ring is in a perpendicular magnetic field. We find that the polarization-resolved conductance reveals whether the dominant spin-orbit coupling is of Dresselhaus or Rashba type, and that the cubic spin-orbit coupling can be distinguished from the conventional linear coupling by observing the four-peak structure in the Aharonov-Bohm oscillations., Comment: 12 pages, 11 figures

The wave functions of the disordered two-dimensional electron gas (2DEG) placed in strong perpendicular magnetic field are calculated numerically. Even though the purpose of this work is mainly pedagogical, we shall shortly discuss our motivation which is possible application of the results to the calculations of the conductivity tensor in transition region between consecutive plateaus of the Hall conductivity in the quantum Hall effect.

The binding energy for on-centre impurities in a rectangular quantum well wire is calculated as a function of the width of the wire and perpendicular magnetic field. The results for zero-magnetic-field case are in perfect agreement with previous calculations.

We analytically calculate the energy spectrum of a circular graphene quantum dot with radius R subjected to a perpendicular magnetic field B by applying the infinite-mass boundary condition. We can retrieve well-known limits for the cases R, B going to infinity and B going to zero. Our model is capable of capturing the essential details of recent experiments. Quantitative agreement between theory and experiment is limited due to the fact that a circular dot is not close enough to the experimental geometry, that disorder plays a significant role, and that interaction effects may be relevant., 4 pages, 3 figures

We describe the effects of vacancies on the electronic properties of a graphene sheet in the presence of a perpendicular magnetic field: from a single defect to an organized vacancy lattice. An isolated vacancy is the minimal possible inner edge, showing an antidotlike behaviour, which results in an extra level between consecutive Landau levels. Two close vacancies may couple to each other, forming a vacancy molecule tuned by the magnetic field. We show that a vacancy lattice introduce an extra band in between Landau levels with localization properties that could lead to extra Hall resistance plateaus., 6 pages, 4 figures, few comments added after referees - accepted to publication in Phys. Rev. B

We investigate the electron transport through a graphene p-n junction under a perpendicular magnetic field. By using Landauar-Buttiker formalism combining with the non-equilibrium Green function method, the conductance is studied for the clean and disordered samples. For the clean p-n junction, the conductance is quite small. In the presence of disorders, it is strongly enhanced and exhibits plateau structure at suitable range of disorders. Our numerical results show that the lowest plateau can survive for a very broad range of disorder strength, but the existence of high plateaus depends on system parameters and sometimes can not be formed at all. When the disorder is slightly outside of this disorder range, some conductance plateaus can still emerge with its value lower than the ideal value. These results are in excellent agreement with the recent experiment., Comment: 5 pages, 5 figures, submit to PRL on May 20, 2008

With large thin superconducting films in a perpendicular magnetic field, the usual definition and calculation of the magnetization M via currents or as the difference of two fields fail, since the spatially averaged magnetic field in the film coincides with the uniform applied field and the demagnetization factor is unity. The definition of M as field-derivative of the free energy, however, still works in this limit. We generalize the virial theorem, previously derived for infinite bulk superconductors, to infinitely extended films of arbitrary thickness. An expression for M is obtained that indeed reproduces the M computed from the field derivative of the free energy.

We consider an ideal parabolic quantum wire in a perpendicular magnetic field. A simple Gaussian shaped scattering potential well or hill is flashed softly on and off with its maximum at $t=0$, mimicking a temporary broadening or narrowing of the wire. By an extension of the Lippmann-Schwinger formalism to time-dependent scattering potentials we investigate the effects on the continuous current that is driven through the quantum wire with a vanishingly small forward bias. The Lippmann-Schwinger approach to the scattering process enables us to investigate the interplay between geometrical effects and effects caused by the magnetic field., RevTeX (pdf-LaTeX), 11 pages with 15 included jpg figures

Strongly correlated two-dimensional electronic systems subject to a perpendicular magnetic field at lowest Landau level (LLL) filling factors: 1/2, 1/4 and 1/6 are believed to be composite fermion (CF) Fermi liquid phases. Even though a Bose Laughlin wave function cannot describe these filling factors we investigate whether such a wave function provides a lower energy bound to the true CF Fermi liquid energies. By using Monte Carlo simulations in disk geometry we compute the Bose Laughlin energies and compare them to corresponding results for the spin-polarized LLL CF Fermi liquid state and avalable data from literature.We find the unexpected result that, for filling factors 1/4 and 1/6, the Bose Laughlin ground state energy is practically identical to the true CF liquid energy while this is not the case at 1/2 where the Bose Laughlin ground state energy is sizeably lower than the energy of the CF Fermi liquid state., 7 pages, 2 figures, 2 tables

We propose and analyze a four-terminal metal-semiconductor device that uses hot-electron transport through thin ferromagnetic films to inject and detect a charge-coupled spin current transported through the conduction band of an arbitrary semiconductor. This provides the possibility of realizing a spin field-effect-transistor in Si, using electrostatic transit-time control in a perpendicular magnetic field, rather than Rashba effect with spin-orbit interaction.

Superconductivity When the thickness of a superconducting film becomes comparable to the typical size of its electron pairs, its superconductivity enters a two-dimensional (2D) regime. Thinner films usually have higher amounts of disorder, making it difficult to isolate the 2D effects. To circumvent this limitation, Saito et al. induced charge carriers on the surface of the 3D insulator ZrNCl. This approach produced a clean superconducting layer thinner than the unit cell of the crystal. The superconducting state was extremely sensitive to the application of a perpendicular magnetic field, as expected for clean systems. Science , this issue p. [409][1] [1]: /lookup/doi/10.1126/science.1259440

High-resolution nuclear magnetic resonance (NMR) spectrometry is possible in nonhomogeneous magnetic fields-such as those in portable equipment-if the static and the radio-frequency (RF) magnetic fields are perpendicular and correlated in the measurement volume. From the easy-axis rotation theorem and the Amperian currents model, it is possible to exactly match two magnetic fields in two-dimensional systems. We derive a basic probe element that fulfills these conditions. Then we present a portable NMR probe design. The static and RF magnetic fields of the probe are matched on a large volume

Magneto-optical imaging was used to observe flux dendrites with opposite polarities simultaneously penetrate superconducting, ring-shaped $\mathrm{Mg}{\mathrm{B}}_{2}$ films. By applying a perpendicular magnetic field, branching dendritic structures nucleate at the outer edge and abruptly propagate deep into the rings. When these structures reach close to the inner edge, where flux with opposite polarity has penetrated the superconductor, they occasionally trigger anti-flux-dendrites. These antidendrites do not branch, but instead trace the triggering dendrite in the backward direction. Two trigger mechanisms, a nonlocal magnetic and a local thermal mechanism, are considered as possible explanations for this unexpected behavior. Increasing the applied field further, the rings are perforated by dendrites which carry flux to the center hole. Repeated perforations lead to a reversed field profile and new features of dendrite activity when the applied field is subsequently reduced.

The nucleation of the superconductive phase in macroscopic type-I foils in a slow, continuously decreasing perpendicular magnetic field is observed to first occur at the field $H_{c3}$, and reinitiate at $H_{c2}$. The observed variation of $H_{c2}$ with temperature further yields Ginzburg-Landau parameters for the materials approximately a factor 2 below tabulations, consistent with previous determinations in small grains.

How the vortex lattice orders at long range in a layered superconductor with weak point pinning centers is studied through a duality analysis of the corresponding frustrated XY model. Vortex-glass order emerges out of the vortex liquid across a macroscopic number of weakly coupled layers in perpendicular magnetic field as the system cools down. Further, the naive magnetic-field scale determined by the Josephson coupling between adjacent layers is found to serve as an upperbound for the stability of any possible conventional vortex lattice phase at low temperature in the extreme type-II limit., 13 pgs., 1 table, published version

In a two-dimensional quantum wire in a perpendicular magnetic field with a smooth embedded repulsive scattering potential we find in the multimode conductance resonances caused by bound states with negative binding energy. These resonances are the counterexamples to well-known dips in the conductance and evanescent states caused by quasibound states of attractive scattering centers in the wire.

Small numbers N

In the last years noncommutative quantum mechanics has been investigated intensively. We consider the influence of magnetic field on decoherence of a system in the noncommutative quantum plane. Particularly, we point out a model in which the magnetic field allows {\it in situ} dynamical control of decoherence as well as, in principle, observation of noncommutativity., 12 AMSTex pages

Considered is a quantum system of N(?:_ 2) charged particles moving in the plane R2 under the influence of a perpendicular magnetic field. Each particle feels the magnetic field concenrated in the positions of the other particles. The gauge potential which gives this magnetic field is called a winding gauge potential. Properties of the momentum operators with the winding gauge potential are investigated. The momentum operators with the winding gauge potential are represented by the fibre direct integral of Arni's momentum operators [1]. Using this fibre direct integral decomposition, commutation properties of the momentum operators are investigated. A notion of local quantization of the magnetic flux is introduced to characterize the strong commutativity of the momentum operators. Aspects of the representation of the canonical commutation relations (CCR) are discussed. There is an interesting relation between the representation of the CCR with respect to this system and Arni's representation. Some applications of those results are also discussed.

It is rigorously proved that the Green's function of a uniform two-dimensional interacting electron gas in a perpendicular magnetic field is diagonal with respect to single-particle states in the Landau gauge. The implication of this theorem is briefly discussed.

We present the results of an experimental study of superconducting, disordered, thin-films of amorphous Indium Oxide. These films can be driven from the superconducting phase to a reentrant insulating state by the application of a perpendicular magnetic field ($B$). We find that the high-$B$ insulator exhibits activated transport with a characteristic temperature, $T_I$. $T_I$ has a maximum value ($T_{I}^p$) that is close to the superconducting transition temperature ($T_c$) at $B$ = 0, suggesting a possible relation between the conduction mechanisms in the superconducting and insulating phases. $T_{I}^p$ and $T_c$ display opposite dependences on the disorder strength., Comment: Tex file and 5 figures; Revised version; To appear in Phys. Rev. Lett. (2004)