Large External Field Research Articles

Anisotropic magnetodielectric coupling behavior of Ca3Co1.4Rh0.6O6 due to geometrically frustrated magnetism

We have investigated the magnetic, dielectric, and magnetodielectric (MDE) behavior of a geometrically frustrated spin-chain system, Ca3Co1.4Rh0.6O6 (related to Ca3CoRhO6), in the single crystalline form for different orientations. The results bring out that the magnetic behavior of this compound is by itself interesting in the sense that this compound exhibits an anisotropic glassy-like magnetic behavior with a huge frequency (ν) dependence of ac susceptibility (χ) peak for an orientation along the spin-chain in the range of 30–60 K; this behavior is robust to applications of large external magnetic fields (H) unlike in canonical spin-glasses. The temperature dependence of dielectric constant also shows strong ν-dependence with similar robustness to H. The isothermal H-dependent dielectric results at low temperatures establish anisotropic MDE coupling. It is intriguing to note that there is a “step” roughly at one-third of saturation values as in the case of isothermal magnetization curves for same temperatures (for orientation along spin-chain), a correlation hitherto unrealized for geometrically frustrated systems.

Band engineering and absorption spectra in compressively strained Ge0.92Sn0.08/Ge (001) double quantum well for infrared photodetection

AbstractWe report a theoretical investigation of strained Ge1−xSnx/Ge (001)‐oriented quantum wells as a bulding block in active region of infrared photodetector. The electronic band parameters, gaps, discontinuities and effective masses for heterointerfaces between compressively strained Ge1−xSnx and relaxed Ge have beencomputed at room temperature. From this preliminary and mendatory work, we conclude that pseudomorphic Ge1−xSnx alloys become direct band gap semiconductors at a Sn‐fraction of 15.3%, e.g. a lattice mismatch as high as 2.3%. Due to achievable critical layer thickness and mainly solid solubility limit, a type‐I compressively strained Ge/Ge0.92Sn0.08/Ge (double) quantum well is studied by solving Schrödinger equation without and applied bias voltage. A strong absorption coefficient (> 1×104 cm−1) and a Stark shift of the direct transition between 2.01 μm and 2.25 μm at large external fields (40kV/cm) are attractive characteristics for the design of infrared photodetectors (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Ferroelectric domain evolution in 0.9625Bi0.5Na0.5TiO3−0.0375BiZn0.5Ti0.5O3 piezoceramic

The domain evolution of 0.9625Bi0.5Na0.5TiO3−0.0375BiZn0.5Ti0.5O3 ceramic is studied by piezoelectric force microscopy. Nanodomains are the most common in the as-prepared ceramic, and they can transform to monodomain in a micrometer-scale grain under a large external electric field, which corresponds to a maximum piezoelectric d33 coefficient. After the field is removed, the mondomain commonly decomposes gradually, and occasionally it transforms to periodic stripe-like domains. This experiment exhibits the domain evolution during poling and depoling processes of a normal ferroelectric ceramic, which is different from ergodic relaxors.

Molecular dynamics simulation of electrokinetic flow of an aqueous electrolyte solution in nanochannels.

Electrokinetic flows of an aqueous NaCl solution in nanochannels with negatively charged surfaces are studied using molecular dynamics simulations. The four transport coefficients that characterize the response to weak electric and pressure fields, namely, the coefficients for the electrical current in response to the electric field (M(jj)) and the pressure field (M(jm)), and those for the mass flow in response to the same fields (M(mj) and M(mm)), are obtained in the linear regime using a Green-Kubo approach. Nonequilibrium simulations with explicit external fields are also carried out, and the current and mass flows are directly obtained. The two methods exhibit good agreement even for large external field strengths, and Onsager's reciprocal relation (M(jm) = M(mj)) is numerically confirmed in both approaches. The influence of the surface charge density on the flow is also considered. The values of the transport coefficients are found to be smaller for larger surface charge density, because the counter-ions strongly bound near the channel surface interfere with the charge and mass flows. A reversal of the streaming current and of the reciprocal electro-osmotic flow, with a change of sign of M(mj) due to the excess co-ions, takes places for very high surface charge density.

Dynamical quark mass generation in a strong external magnetic field

We investigate the effect of a strong magnetic field on dynamical chiral symmetry breaking in quenched and unquenched QCD. To this end we apply the Ritus formalism to the coupled set of (truncated) Dyson-Schwinger equations for the quark and gluon propagator under the presence of an external constant Abelian magnetic field. We work with an approximation that is trustworthy for large fields eH > \Lambda_{QCD}^2 but is not restricted to the lowest Landau level. We confirm the linear rise of the quark condensate with large external field previously found in other studies and observe the transition to the asymptotic power law at extremely large fields. We furthermore quantify the validity of the lowest Landau level approximation and find substantial quantitative differences to the full calculation even at very large fields. We discuss unquenching effects in the strong field propagators, condensate and the magnetic polarization of the vacuum. We find a significant weakening of magnetic catalysis caused by the back reaction of quarks on the Yang-Mills sector. Our results support explanations of the inverse magnetic catalysis found in recent lattice studies due to unquenching effects.

Linear and nonlinear density response functions for a simple atomic fluid

We use molecular dynamics simulations to investigate the linear and nonlinear density response functions for simple fluids under the influence of spatially periodic external fields. Using a direct Fourier space decomposition of the instantaneous microscopic density for the perturbed fluid we can clearly identify the distinct order of response. Using a single component sinusoidal longitudinal force for a set of wavelengths and amplitudes we show that in the linear response regime the proportionality between the external field amplitude and the density perturbation can be used to determine the linear density response function, and hence the pair correlation function, static liquid structure factor, and lowest order direct correlation function. We show also that for large external field amplitudes a single component external field can be used to determine the form for lowest order and second lowest order nonlinear response functions for restricted regions of the total response function spaces.

The mitigating effect of magnetic fields on Rayleigh-Taylor unstable inertial confinement fusion plasmas

Rayleigh-Taylor (RT) instabilities at interfaces of disparate mass densities have long been known to generate magnetic fields during inertial confinement fusion implosions. An externally applied magnetic field can also be efficiently amplified by RT instabilities. The focus here is on magnetic field generation and amplification at the gas-ice interface which is RT unstable during the deceleration phase of the implosion. RT instabilities lead to undesirable mix of hot and cold plasmas which enhances thermal energy loss and tends to produce a more massive warm-spot instead of a hot-spot. Two mechanisms are shown here to mitigate the thermal energy loss from the hot-spot. The first mechanism is the reduction of electron thermal conductivity with interface-aligned magnetic fields. This can occur through self-generated magnetic fields via the Biermann battery effect as well as through externally applied magnetic fields that undergo an exponential growth via the stretch-and-fold magnetohydrodynamic dynamo. Self-generated magnetic fields during RT evolution can result in a factor of 2−10 decrease in the electron thermal conductivity at the gas-ice interface, while externally applied magnetic fields that are compressed to 6–1000 T at the onset of deceleration (corresponding to pre-implosion external fields of 0.06–10 T) could result in a factor of 2–500 reduction in electron thermal conductivity at the gas-ice interface. The second mechanism to mitigate thermal energy loss from the hot-spot is to decrease the interface mixing area between the hot and cold plasmas. This is achieved through large external magnetic fields of 1000 T at the onset of deceleration which damp short-wavelength RT modes and long-wavelength Kelvin-Helmholtz modes thus significantly slowing the RT growth and reducing mix.

Tuning the composition of Zn–Fe–O nanotube arrays: from zinc ferrite ZnFe2O4 to hematite α-Fe2O3

Large-scale vertically aligned nanotube arrays with composition of zinc ferrite (ZnFe2O4), hematite (α-Fe2O3) and their composite (ZnFe2O4/α-Fe2O3) are fabricated by a simple sacrificial ZnO-nanowire-array templating method. Hydrolysis of Fe3+ ions on the surface of ZnO nanowires in the arrays leads to deposition of Fe(OH)3 shell at the expense of ZnO. The controllable ratio between the newly deposited Fe(OH)3 and the remaining ZnO determines the chemical composition of the final products after solid state reaction between the two at elevated temperatures. We have shown that the nanotube morphology can always be obtained while its chemical composition can be tuned from stoichiometric ZnFe2O4 to ZnFe2O4/α-Fe2O3 composite, and eventually to α-Fe2O3. By using ZnO nanowires as both sacrificing and reacting template, the present work suggests a feasible methodology for ternary nanotube array growth. The optical and magnetic properties of obtained nanotube arrays are studied. Both ZnFe2O4 and α-Fe2O3 nanotubes show strong visible light absorption. ZnFe2O4 nanotubes exhibit relatively large ferromagnetism at both 3 K and 300 K, while α-Fe2O3 nanotubes demonstrate a Morin transition around 235 K under small external field and a spin-flop transition under large external field.

Research on the Magnetic Force of a Blood Pump Driven by Large Gap External Magnetic Field

The calculation model of the single turn rectangle current carrying coil was established. The theoretic formula for calculating the magnetic field intensity of any point in space was derived. For a pair of radial magnetizing permanent magnets, the formula for calculating the magnetic force of permanent magnet in the magnetic field was deduced based on the equivalent current theory of permanent magnet. According to the formula, the influencing factors and the changing rules for the magnetic force of permanent magnet can be seen directly: the current, the coil turns are proportional to its magnetic force, while the coupling distance is inversely proportional to its magnetic force.

Calculation of Lorentz Forces on Coils for Transcranial Magnetic Stimulation During Magnetic Resonance Imaging

New applications for transcranial magnetic stimulation (TMS) are rapidly being developed for diagnostic and therapeutic purposes but are restricted by a number of technical limitations. The ability to reliably perform TMS concurrently with functional magnetic resonance imaging (fMRI) would be a valuable tool for researchers. However the forces experienced by TMS coils during normal operation due to large transient magnetic fields which are produced, are problematic. When operated within a large external field, such as in an fMRI scanner these forces can result in mechanical failure of the coils. This paper presents calculations of these forces to form a basis from which coil failure can be prevented.

Theory of the SrTiO3surface state two-dimensional electron gas

We present a theory of the quasi two-dimensional electron gas (2DEG) systems which appear near the surface of SrTiO$_3$ when a large external electric field attracts carriers to the surface. We find that non-linear and non-local screening by the strongly polarizable SrTiO$_3$ lattice plays an essential role in determining 2DEG properties. The electronic structure always includes weakly bound bulk-like bands that extend over many SrTiO$_3$ layers. At 2D carrier-densities exceeding $\sim 10^{14} {\rm cm}^{-2}$ tightly bound bands emerge that are confined within a few layers of the surface.

Topological Nematic States and Non-Abelian Lattice Dislocations

An exciting new prospect in condensed matter physics is the possibility of realizing fractional quantum Hall (FQH) states in simple lattice models without a large external magnetic field. A fundamental question is whether qualitatively new states can be realized on the lattice as compared with ordinary fractional quantum Hall states. Here we propose new symmetry-enriched topological states, topological nematic states, which are a dramatic consequence of the interplay between the lattice translation symmetry and topological properties of these fractional Chern insulators. When a partially filled flat band has a Chern number N, it can be mapped to an N-layer quantum Hall system. We find that lattice dislocations can act as wormholes connecting the different layers and effectively change the topology of the space. Lattice dislocations become defects with non-trivial quantum dimension, even when the FQH state being realized is by itself Abelian. Our proposal leads to the possibility of realizing the physics of topologically ordered states on high genus surfaces in the lab even though the sample has only the disk geometry.

Spin decoherence in graphene quantum dots due to hyperfine interaction

Carbon based systems are prominent candidates for a solid-state spin-qubit due to weak spin-orbit and hyperfine interactions in combination with a low natural abundance of spin carrying isotopes. We consider the effect of the hyperfine interaction on the coherence of an electron-spin localized in a graphene quantum dot. It is known, that the hyperfine interaction in these systems is anisotropic promising interesting physics. We calculate the dynamics of an electron spin surrounded by a bath of nuclear spins in a non-Markovian approach using a generalized master equation. Considering a general form of the hyperfine interaction, we are able to extend the range of validity of our results to other systems beyond graphene. For large external magnetic fields, we find within Born approximation that the electron spin state is conserved up to small corrections, which oscillate with a frequency determined by the hyperfine interaction. The amplitude of these oscillations decays with a power law, where its initial value depends on the specific form of the anisotropy. Analyzing this in more detail, we identify two distinct classes of anisotropy, which can be both found in graphene depending on the orientation of the external magnetic field with respect to the carbon layer.

Can slip walls improve field-flow fractionation or hydrodynamic chromatography?

One way to potentially modify the performance of field-flow fractionation (FFF) would be to move the position of the maximum flow velocity away from the mid-point of the channel, for example by using walls with non-zero slip lengths. In this short communication, we extend the ideal theory of FFF to include the effects of two slip walls. Our calculations demonstrate that while the hydrodynamic chromatography limit of FFF (weak fields) is not improved by engineering devices with slip-walls, the performance of Normal-Mode FFF can be enhanced by having slip at the depletion wall in moderate fields. We also introduce a new regime, which we call Slip-Mode FFF, where a large external field (typical of Normal-Mode FFF) and a large slip at the accumulation wall lead to sharp separations characterized by an elution order that is similar to that of hydrodynamic chromatography.

An analytic solution for the magnetization of two-dimensional ferrofluids based on the mean spherical approximation

An analytic formula is derived for the magnetization of a two-dimensional dipolar hard disk fluid using a variational functional series expansion of the free energy as a function of the orientational distribution function. The excess term expressing the effect of the intermolecular forces is calculated on the basis of the mean spherical approximation. Comparison with our own Monte Carlo simulation data shows excellent agreement for large external fields and for the zero-field susceptibility. At intermediate field strengths, the agreement is satisfactory for moderate dipole moments and densities.

Spin-Controlled Vertical-Cavity Surface-Emitting Lasers

We discuss the concept of spin-controlled vertical-cavity surface-emitting lasers (VCSELs) and analyze it with respect to potential room-temperature applications in spin-optoelectronic devices. Spin-optoelectronics is based on the optical selection rules as they provide a direct connection between the spin polarization of the recombining carriers and the circular polarization of the emitted photons. By means of optical excitation and numerical simulations we show that spin-controlled VCSELs promise to have superior properties to conventional devices such as threshold reduction, spin control of the emission, or even much faster dynamics. Possible concepts for room-temperature electrical spin injection without large external magnetic fields are summarized, and the progress on the field of purely electrically pumped spin-VCSELs is reviewed.

Effect of the polarization drift in a strongly magnetized plasma

We consider a strongly magnetized plasma described by a Vlasov-Poisson system with a large external magnetic field. The finite Larmor radius scaling allows to describe its behaviour at very fine scales. We give a new interpretation of the asymptotic equations obtained by Frenod and Sonnendrucker [SIAM J. Math. Anal. 32 (2001) 1227–1247] when the intensity of the magnetic field goes to infinity. We introduce the so-called polarization drift and show that its contribution is not negligible in the limit, contrary to what is usually said. This is due to the non linear coupling between the Vlasov and Poisson equations.

Reversible, opto-mechanically induced spin-switching in a nanoribbon-spiropyran hybrid material

It has recently been shown that electronic transport in zigzag graphene nanoribbons becomes spin-polarized upon application of an electric field across the nanoribbon width. However, the electric fields required to experimentally induce this magnetic state are typically large and difficult to apply in practice. Here, using both first-principles density functional theory (DFT) and time-dependent DFT, we show that a new spiropyran-based, mechanochromic polymer noncovalently deposited on a nanoribbon can collectively function as a dual opto-mechanical switch for modulating its own spin-polarization. These calculations demonstrate that upon mechanical stress or photoabsorption, the spiropyran chromophore isomerizes from a closed-configuration ground-state to a zwitterionic excited-state, resulting in a large change in dipole moment that alters the electrostatic environment of the nanoribbon. We show that the electronic spin-distribution in the nanoribbon-spiropyran hybrid material can be reversibly modulated via noninvasive optical and mechanical stimuli without the need for large external electric fields. Our results suggest that the reversible spintronic properties inherent to the nanoribbon-spiropyran material allow the possibility of using this hybrid structure as a resettable, molecular-logic quantum sensor where opto-mechanical stimuli are used as inputs and the spin-polarized current induced in the nanoribbon substrate is the measured output.

Applications of symmetries in superconductivity

Superconductivity was discovered by Kamerlingh Onnes in 1911. It is characterized by two major properties: i) the electrical resistance exactly vanishes and ii) the magnetic field lines are expelled outside the superconducting body (Meissner effect). Both properties are potentially useful for many applications, in particular for storage and dissipationless transport of electrical energy. However, though superconductivity has been observed in many metals, it has been limited to low temperatures so far. Moreover, it cannot sustain too large external magnetic fields or current densities. Applications have thus been limited to quite specific, though significant areas (superconducting electromagnets used for Magnetic Resonance Imaging, for instance). On the fundamental research side, superconductivity has fascinated many physicists, in particular because it is a direct manifestation of quantum coherence on the macroscopic scale. The theories that have been invented to explain this phenomenon have then irrigated other areas including condensed matter, nuclear, and particle physics. The aim of these lecture notes is to highlight that the concept of symmetry has been essential in making these advances. Section 2 will present the phenomenological description of superconductivity. It will allow ascribing it to a spontaneous gauge symmetry breaking in the frame of the Landau theory of phase transitions. In Section 3, the microscopic theory of superconductivity will be introduced. Unconventional superconductors where an additional symmetry breaking takes place at the phase transition will be contrasted with conventional superconductors where only the gauge symmetry breaking takes place. Physical signatures that allow distinguishing conventional and unconventional superconductors will be discussed. Section 4 will conclude by presenting active fields of research in superconductivity where the concepts of symmetry are still being used. For the clarity of the presentation, strong simplifications will be made in the analysis and many aspects of the theory will be put aside. The reader may consult the textbooks [1–3] for further reading and references. A detailed presentation of the aspects related with symmetries can be found in Refs. [4, 5].

Charge state readout and hyperfine interaction in a few-electron InGaAs double quantum dot

A laterally defined InGaAs double quantum dot with an integrated charge readout sensor is realized in an InGaAs/InP heterostructure. The charge states of the double quantum dot are measured with the use of the charge readout sensor in the few-electron regime in which the current is too weak to be observable by direct measurements of electron transport through the double dot. We also measure the leakage current of the double quantum dot in the Pauli spin-blockade few-electron regime and study the singlet-triplet state mixing by the hyperfine coupling to the nuclear spins. The measurements of the leakage current in the weak external-magnetic-field region and for weak interdot couplings allow us to extract an effective nuclear magnetic field in the double-dot system. We also study spin relaxation and transport processes in the Pauli spin-blockade region at large external magnetic fields and observe transport through the excited triplet state in the few-electron double quantum dot.