Tilted Magnetic Field Research Articles

Vortex cores and vortex motion in superconductors with anisotropic Fermi surfaces

Explaning static and dynamic properties of the vortex lattice in anisotropic superconductors requires a careful characterization of vortex cores. The vortex core contains Andreev bound states whose spatial extension depends on the anisotropy of the electronic band-structure and superconducting gap. This might have an impact on the anisotropy of the superconducting properties and on vortex dynamics. Here we briefly summarize basic concepts to understand anisotropic vortex cores and review vortex core imaging experiments. We further discuss moving vortex lattices and the influence of vortex core shape in vortex motion. We find vortex motion in highly tilted magnetic fields. We associate vortex motion to the vortex entry barrier and the screening currents at the surface. We find preferential vortex motion along the main axis of the vortex lattice. After travelling integers of the intervortex distance, we find that vortices move more slowly due to the washboard potential of the vortex lattice.

The nature of the light variability of magnetic Of?p star HD 191612

A small fraction of hot OBA stars host global magnetic fields with field strengths of the order of 0.1-10 kG. This leads to the creation of persistent surface structures (spots) in stars with sufficiently weak winds as a result of the radiative diffusion. These spots become evident in spectroscopic and photometric variability. This type of variability is not expected in stars with strong winds, where the wind inhibits the radiative diffusion. Therefore, a weak photometric variability of the magnetic Of?p star HD 191612 is attributed to the light absorption in the circumstellar clouds. We study the nature of the photometric variability of HD 191612. We assume that the variability results from variable wind blanketing induced by surface variations of the magnetic field tilt and modulated by stellar rotation. We used our global kinetic equilibrium (NLTE) wind models with radiative force determined from the radiative transfer equation in the comoving frame (CMF) to predict the stellar emergent flux. Our models describe the stellar atmosphere in a unified manner and account for the influence of the wind on the atmosphere. The models are calculated for different wind mass-loss rates to mimic the effect of magnetic field tilt on the emergent fluxes. We integrate the emergent fluxes over the visible stellar surface for individual rotational phases, and calculate the rotationally modulated light curve of HD 191612. The wind blanketing that varies across surface is able to explain a part of the observed light variability in this star. The mechanism is able to operate even at relatively low mass-loss rates. The remaining variability is most likely caused by the flux absorption in circumstellar clouds. The variable wind blanketing is an additional source of the light variability in massive stars. The presence of the rotational light variability may serve as a proxy for the magnetic field.

Resistance oscillations of two-dimensional electrons in crossed electric and tilted magnetic fields

Effect of dc electric field on transport of highly mobile 2D electrons is studied in wide GaAs single quantum wells placed in titled magnetic fields. The study shows that in perpendicular magnetic field resistance oscillates due to electric field induced Landau-Zener transitions between quantum levels that corresponds to geometric resonances between cyclotron orbits and periodic modulation of electron density of states. Magnetic field tilt inverts these oscillations. Surprisingly the strongest inverted oscillations are observed at a tilt corresponding to nearly absent modulation of the electron density of states in regime of magnetic breakdown of semiclassical electron orbits. This phenomenon establishes an example of quantum resistance oscillations due to Landau quantization, which occur in electron systems with a constant density of states.

Hybridization and Field Driven Phase Transitions in Hexagonally Warped Topological Insulators

In this paper, we discuss the role of material parameters and external field effects on a thin film topological insulator(TI) in the context of quantum phase transition (QPT). First, we consider an in-plane tilted magnetic field and determine the band structure of the surface states as a function of the tilt angle. We show that the presence of either a hybridization term or hexagonal warping or a combination of both leads to a semi-metal to insulator phase transition which is facilitated by their [Formula: see text] symmetry breaking character. We then note that while the introduction of an electric field does not allow for this QPT since it does not break [Formula: see text] symmetry, it can be used in conjunction with a tunneling element to reach a phase transition efficiently. The corresponding critical point is then nontrivially dependent on the electric field, which is pointed out here. Then, we demonstrate that including a hexagonal warping term leads to an immediate [Formula: see text] symmetry violating QPT.

Fluctuations of Electrical Conductivity: A New Source for Astrophysical Magnetic Fields.

We consider the generation of a magnetic field by the flow of a fluid for which the electrical conductivity is nonuniform. A new amplification mechanism is found which leads to dynamo action for flows much simpler than those considered so far. In particular, the fluctuations of the electrical conductivity provide a way to bypass antidynamo theorems. For astrophysical objects, we show through three-dimensional global numerical simulations that the temperature-driven fluctuations of the electrical conductivity can amplify an otherwise decaying large scale equatorial dipolar field. This effect could play a role for the generation of the unusually tilted magnetic field of the iced giants Neptune and Uranus.

Emergence of helical edge conduction in graphene at theν=0quantum Hall state

The conductance of graphene subject to a strong, tilted magnetic field exhibits a dramatic change from insulating to conducting behavior with tilt-angle, regarded as evidence for the transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) \nu=0 quantum Hall state. We develop a theory for the electric transport in this system based on the spin-charge connection, whereby the evolution in the nature of collective spin excitations is reflected in the charge-carrying modes. To this end, we derive an effective field theoretical description of the low-energy excitations, associated with quantum fluctuations of the spin-valley domain wall ground-state configuration which characterizes the two-dimensional (2D) system with an edge. This analysis yields a model describing a one-dimensional charged edge mode coupled to charge-neutral spin-wave excitations in the 2D bulk. Focusing particularly on the FM phase, naively expected to exhibit perfect conductance, we study a mechanism whereby the coupling to these bulk excitations assists in generating back-scattering. Our theory yields the conductance as a function of temperature and the Zeeman energy - the parameter that tunes the transition between the FM and CAF phases - with behavior in qualitative agreement with experiment.

Collective bulk and edge modes through the quantum phase transition in graphene atν=0

Undoped graphene in a strong, tilted magnetic field exhibits a radical change in conduction upon changing the tilt-angle, which can be attributed to a quantum phase transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) bulk state at filling factor $\nu=0$. This behavior signifies a change in the nature of the collective ground state and excitations across the transition. Using the time-dependent Hartree-Fock approximation, we study the collective neutral (particle-hole) excitations in the two phases, both in the bulk and on the edge of the system. The CAF has gapless neutral modes in the bulk, whereas the FM state supports only gapped modes in its bulk. At the edge, however, only the FM state supports gapless charge-carrying states. Linear response functions are computed to elucidate their sensitivity to the various modes. The response functions demonstrate that the two phases can be distinguished by the evolution of a local charge pulse at the edge.

Disorder effects on helical edge transport in graphene under a strong tilted magnetic field

In a recent experiment, Young et al. [Nature {\bf 505}, 528 (2014)] observed a metal to insulator transition as well as transport through helical edge states in monolayer graphene under a strong, tilted magnetic field. Under such conditions, the bulk is a magnetic insulator which can exhibit metallic conduction through helical edges. It was found that two-terminal conductance of the helical channels deviates from the expected quantized value at low-temperatures ($=e^2/h$ per edge, at zero temperature). Motivated by this observation, we study the effect of disorder on the conduction through the edge channels. We show that, unlike the situation in semiconducting quantum wells, a disorder Rashba spin-orbit coupling does not lead to backscattering, at least to leading order. Instead we find the lack of perfect anti-alignment of the electron spins in the helical channels to be the most likely source for backscattering arising from scalar (i.e. spin-independent) impurities. The intrinsic spin-orbit coupling and other time-reversal symmetry breaking and/or sublattice-parity breaking potentials also lead to (sub-leading) corrections to the channel conductance.

Non-Fermi-Liquid Crossovers in a Quasi-One-Dimensional Conductor in a Tilted Magnetic Field.

We consider a theoretical problem of electron-electron scattering time in a quasi-one-dimensional (Q1D) conductor in a magnetic field, perpendicular to its conducting axis. We show that inverse electron-electron scattering time becomes of the order of characteristic electron energy, 1/τ~ε~T, in a high magnetic field, directed far from the main crystallographic axes, which indicates breakdown of the Fermi-liquid theory. In a magnetic field, directed close to one of the main crystallographic axis, inverse electron-electron scattering time becomes much smaller than characteristic electron energy and, thus, applicability of Fermi-liquid theory restores. We suggest that there exist crossovers (or phase transitions) between Fermi-liquid and some non-Fermi-liquid states in a strong enough tilted magnetic field. Application of our results to the Q1D conductor (Per)_{2}Au(mnt)_{2} shows that it has to be possible to observe the above-mentioned phenomenon in feasibly high magnetic fields of the order of H≥H^{*}≃25 T.

Magnetotransport properties of corrugated stanene in the presence of electric modulation and tilted magnetic field

We investigated the effects of external electric and magnetic fields on the electronic and transport properties of corrugated stanene. We match density functional calculations with the effective Dirac model and applied external fields in the Dirac model. We calculate both the Fermi velocity and the strength of spin–orbit interaction in stanene within the framework of density functional theory. In the presence of intrinsic spin–orbit interaction accompanying with external electric field splits spin‐up and down states. We also study the spin‐dependent Landau levels and transport for stanene in the presence of periodically modulated electric and tilted magnetic fields with corrugation. Additionally, the Weiss and spin‐polarization oscillations with inverse of magnetic field in stanene are reported.

Canted antiferromagnetic to ferromagnetic phase transition in bilayer graphene

We performed electrical transport measurements in strong tilted magnetic fields (up to 30 T) on a high quality bilayer graphene sample. The possibility of tuning the system into a topological state at zero filling factor, associated to a ferromagnetic phase, is demonstrated on the basis of an insulator-to-metal transition and simultaneous arising of strong non-local response. Based on recent theoretical predictions, relevant aspects of the phase transition are quantitatively extracted. New experimental developments on the topics are proposed in the final section.

The Effect of Tilted Magnetic Field in Graphene Cone

We investigate the combined effect of both homogeneous and tilted magnetic fields on the energy spectrum of gapped graphene cones together with long range Coulomb impurity in the framework of perturbation theory. The results illustrate that the tilted magnetic field can significantly affect the spectrum of single particle low-energy effective Hamiltonian, besides lifting the degeneracy of angular momentum channels in the presence of topological defects. The breaking of the electron-hole symmetry by magnetic fields is explained as a direct consequence of sub band Landau-level coupling.

Asymmetric Landau bands due to spin–orbit coupling

We show that the Landau bands obtained in a two-dimensional lateral semiconductor superlattice with spin–orbit coupling (SOC) of the Rashba/Dresselhaus type, linear in the electron momentum, placed in a tilted magnetic field, do not follow the symmetry of the spatial modulation. Moreover, this phenomenology is found to depend on the relative tilt of magnetic field and on the SOC type: (a) when only Rashba SOC exists and the magnetic field is tilted in the direction of the superlattice (b) Dresselhaus SOC exists and the magnetic field is tilted in the direction perpendicular to the superlattice. Consequently, measurable properties of the modulated system become anisotropic in a tilted magnetic field when the field is conically rotated around the z axis, at a fixed polar angle, as we demonstrate by calculating the resistivity and the magnetization.

Confined Dirac particles in a constant and tilted magnetic field

We study the confinement of charged Dirac particles in a plane embedded in 3 + 1 space–time due to the presence of a constant magnetic field that is tilted on the given plane. We focus on the nature of the solutions of the Dirac equation and on how they depend on the choice of vector potential that gives rise to the magnetic field. In particular, we select a "Landau gauge" such that the momentum is conserved along the direction of the vector potential yielding spinor wave functions, which are localized in the plane containing the magnetic field and normal to the vector potential. These wave functions are expressed in terms of the Hermite polynomials. We point out the relevance of these findings to the relativistic quantum Hall effect and compare with the results obtained for a constant magnetic field normal to the plane in 2 + 1 dimensions.

Effect of interminiband tunneling on the generation of current in a semiconducting superlattice

The effect of the bandgap width between the first and second energy minibands on the charge transport in a semiconducting superlattice to which electric and tilted magnetic fields are applied is studied theoretically. The time dependences of the current passing through the superlattice are calculated, and the dependences of the amplitude and frequency of electric current oscillations on the applied voltage are constructed. It is found that the interminiband electron tunneling facilitates a decrease in the amplitude of current oscillations, but simultaneously increases their frequency.

Transient magnetized plasma as an optical element for high power laser pulses

Underdense plasma produced in gas jets by low intensity laser prepulses in the presence of a static magnetic field, $B\ensuremath{\sim}0.3\text{ }\text{ }\mathrm{T}$, is shown experimentally to become an optical element allowing steering of tightly focused high power femtosecond laser pulses within several degrees along with essential enhancement of pulse's focusability. Strong laser prepulses form a density ramp perpendicularly to magnetic field direction and, owing to the light refraction, main laser pulses propagate along the magnetic field even if it is tilted from the laser axis. Electrons generated in the laser pulse wake are well collimated and follow in the direction of the magnetic field; their characteristics are measured to be not sensitive to the tilt of magnetic field up to angles $\ifmmode\pm\else\textpm\fi{}5\ifmmode^\circ\else\textdegree\fi{}$.

Effects of a tilted magnetic field in a Dirac double layer

We calculate the energy spectrum of a Dirac double layer, where each layer has the Dirac electronic dispersion, in the presence of a tilted magnetic field and small interlayer tunneling. We show that the energy splitting between the Landau levels has an oscillatory dependence on the in-plane magnetic field and vanishes at a series of special tilt angles of the magnetic field. Using a semiclassical analysis, we show that these special tilt angles are determined by the Berry phase of the Dirac Hamiltonian. The interlayer tunneling conductance also exhibits an oscillatory dependence on the magnetic field tilt angle, known as the angular magnetoresistance oscillations (AMRO). Our results are applicable to graphene double layers and thin films of topological insulators.

Collective edge modes near the onset of a graphene quantum spin Hall state

Graphene subject to a strong, tilted magnetic field exhibits an insulator-metal transition tunable by tilt-angle, attributed to the transition from a canted antiferromagnetic (CAF) to a ferromagnetic (FM) bulk state at filling factor zero. We develop a theoretical description for the spin and valley edge textures in the two phases, and the implied evolution in the nature of edge modes through the transition. In particular, we show that the CAF has gapless neutral modes in the bulk, but supports gapped charged edge modes. At the transition to the FM state the charged edge modes become gapless and are smoothly connected to the helical edge modes of the FM state. Possible experimental consequences are discussed.

Surface Acoustic Waves Probe of the Spin Phase Transition at v=2/3 in n-GaAs/AlGaAs structure

High frequency (ac) conductivity in the single quantum well AlGaAs/GaAs/AlGaAs with high mobility was investigated by contactless acoustic methods in the fractional quantum Hall effect regime in perpendicular and tilted magnetic fields. We studied the dependence of ac conductivity σac=σ1-iσ2 on both the temperature and magnetic field tilt angle. Tilting the magnetic field relative to the sample surface enabled us to change the position of the conductivity oscillation minimum at v= 2/3. We measured the temperature dependence of ac conductivity for each tilt angle and for the 2/3 state we calculated the activation energy ΔE which was derived by constructing the Arrhenius plot ln σ1 against 1/T. Analyzing behavior of the activation energy in total magnetic field for the filling factor 2/3 we observed a distinct minimum which can be interpreted as the spin unpolarized-polarized phase transition.

Two-dimensional Fermi surfaces in Kondo insulator SmB₆.

In the Kondo insulator samarium hexaboride (SmB6), strong correlation and band hybridization lead to an insulating gap and a diverging resistance at low temperature. The resistance divergence ends at about 3 kelvin, a behavior that may arise from surface conductance. We used torque magnetometry to resolve the Fermi surface topology in this material. The observed oscillation patterns reveal two Fermi surfaces on the (100) surface plane and one Fermi surface on the (101) surface plane. The measured Fermi surface cross sections scale as the inverse cosine function of the magnetic field tilt angles, which demonstrates the two-dimensional nature of the conducting electronic states of SmB6.