Evidence of anisotropic Landau level splitting in topological semimetal ZrSiS under high magnetic fields

Magneto-transport study has been performed in topological semimetal ZrSiS single crystals under high pulsed magnetic fields. Obvious dependence of Landau level splitting on temperature and angular was investigated. The strong three-dimensional anisotropic nature of Landau level splitting under high pulsed magnetic fields was revealed by the angular dependent measurements, in which the orbital contribution is more dominant than Zeeman splitting. Our studies provide more insights into the physical properties of topological semimetals ZrSiS and shed light on future spintronic applications of ZrSiS.     

Enhancement of valley splitting in (100) Si MOSFETs at high magnetic fields

We report the density and magnetic field dependence of the valley splitting of two-dimensional electrons in (100) Si metal–oxide–semiconductor field-effect transistors, as determined via activation measurements in the quantum Hall regime. We find that the valley activation gap can be greatly enhanced at high magnetic fields as compared to the bare valley splitting. The observation of strong dependence of the valley activation gap on orbital Landau level occupancy and similar behavior of nearby spin gaps suggest that electron–electron interactions play a large role in the observed enhancement.     

Coulomb impurity problem of graphene in magnetic fields

Analytical solutions to the Coulomb impurity problem of graphene in the absence of a magnetic field show that when the dimensionless strength of the Coulomb potential g$g$ reaches a critical value the solutions become supercritical with imaginary eigenenergies. Application of a magnetic field is a singular perturbation, and no analytical solutions are known except at a denumerably infinite set of magnetic fields. We find solutions to this problem by numerical diagonalization of the large Hamiltonian matrices. Solutions are qualitatively different from those of zero magnetic field. All energies are discrete and no complex energies are allowed. We have computed the finite-size scaling function of the probability density containing an s$s$-wave component of the Dirac wavefunctions. This function depends on the coupling constant, regularization parameter, and the gap. In the limit of vanishing regularization parameter our findings are consistent with the expected values of the exponent ν$\nu$ which determines the asymptotic behavior of the wavefunction near r=0$r=0$.     

纳米量级超导Al粒子在磁场中的Zeeman分裂

用随机矩阵理论对BCS理论中的自洽方程进行修正.由此得到的新自洽方程能合理地描述纳米量级Al粒子的超导电性.更进一步论证在外磁场作用下,s>0态由于Zeeman效应得出了实验中已观测到的超导增强效应. 关键词： 纳米粒子 超导电性 Zeeman分裂     

超强磁场下电子朗道能级稳定性及对电子费米能的影响

磁星是一类由磁场供能、强磁化的中子星,其内部磁场远高于电子的量子临界磁场.本文通过引入电子朗道能级的稳定性系数g_n,讨论了在磁星环境下电子的朗道能级的稳定性及其对电子费米能E_F(e)的影响;研究发现,磁场越强,电子的朗道能级越不稳定,最大的朗道能级级数n_(max)越小;朗道能级数n越大,能级稳定性系数g_n越小.根据朗道能级的稳定性系数g_n随磁场的增加而减小的要求,电子费米能表达式的磁场指数β必须是正数.通过引入Dirac-δ函数,推导出超强磁场下的简并的、相对论电子费米能的一般表达式,修正了E_F(e)的特解.新的特解给出磁场指数β=1/6;特解的适用范围ρ≥10~7g·cm~(-3),Bcr≤B≤10~(17)G.本文结果将有助于中子星内部弱相互作用过程(包括修正的URCA反应和电子俘获)和磁星磁热演化机理的研究.     

Graphene in high magnetic fields

Carbon-based nano-materials, such as graphene and carbon nanotubes, represent a fascinating research area aiming at exploring their remarkable physical and electronic properties. These materials not only constitute a playground for physicists, they are also very promising for practical applications and are envisioned as elementary bricks of the future of the nano-electronics. As for graphene, its potential already lies in the domain of opto-electronics where its unique electronic and optical properties can be fully exploited. Indeed, recent technological advances have demonstrated its effectiveness in the fabrication of solar cells and ultra-fast lasers, as well as touch-screens and sensitive photo-detectors. Although the photo-voltaic technology is now dominated by silicon-based devices, the use of graphene could very well provide higher efficiency. However, before the applied research to take place, one must first demonstrates the operativeness of carbon-based nano-materials, and this is where the fundamental research comes into play. In this context, the use of magnetic field has been proven extremely useful for addressing their fundamental properties as it provides an external and adjustable parameter which drastically modifies their electronic band structure. In order to induce some significant changes, very high magnetic fields are required and can be provided using both DC and pulsed technology, depending of the experimental constraints. In this article, we review some of the challenging experiments on single nano-objects performed in high magnetic and low temperature. We shall mainly focus on the high-field magneto-optical and magneto-transport experiments which provided comprehensive understanding of the peculiar Landau level quantization of the Dirac-type charge carriers in graphene and thin graphite.     

Magnetic hyperfine splitting in superparamagnetic particles in external magnetic fields

The magnetic hyperfine splitting in Mössbauer spectra of superparamagnetic particles, induced by an external magnetic field, has been calculated. Numerical results have been obtained both for isolated particles with a finite value of the magnetic anisotropy energy constant and for strongly interacting particles. Moreover, analytical approximations are derived. The theoretical results are compared with results of experimental studies of supported α-Fe particles and magnetic particles in ferrofluids.     

Semiconductor artificial graphene: Effects in weak magnetic fields

Two-dimensional quantum transport through the stripe of the hexagonal lattice of antidots built in the multimode channel in the GaAs/AlGaAs structure has been studied numerically. It has been found that the low perpendicular magnetic fields (～3 mT) suppress the bulk currents and cause the appearance of the edge Landau states and high positive magnetic resistance on both sides of the Dirac point. Tamm edge states are present in some energy intervals; as a result, the 4e ²/h-amplitude oscillations caused by the quantization of these states on the lattice length are added to the steps of the conductance quantization G _n = (2|n| + 1)2e ²/h.     

Layered superconductors in high magnetic fields

We investigate the possible occurrance of partially depaired states in superconducting intercalated layered systems. Those states are discussed as a possible explanation of the high critical fields found in some of these materials. It is shown that the Chandrasekhar-Clogston limit does not apply to those states mentioned above and that the maximum field compatible with superconductivity is a sensitive function of the shape of the Fermi surface. Mean free path and spin-orbit effects on the partially depaired state are investigated. An experiment is proposed to decide between the partially depaired state and a large spin-orbit scattering rate as possible explanations for the large critical fields.     

Single-dot spectroscopy in high magnetic fields

We report on photoluminescence measurements from a single InAs/GaAs quantum dot in magnetic fields up to 28 T. Mesa-patterned structure has been used to limit the number of investigated dots. Three pairs of Zeeman-split emission lines with the same effective g*-factor and diamagnetic shift have been observed. The attribution of the lines to recombination of a neutral exciton, a biexciton, and a charged exciton is discussed.     

Heavy fermions in high magnetic fields

Heavy fermion materials are prototypical strongly correlated electron systems, where the strong electron–electron interactions lead to a wide range of novel phenomena and emergent phases of matter. Due to the low energy scales, the relative strengths of the Ruderman–Kittel–Kasuya–Yosida(RKKY) and Kondo interactions can often be readily tuned by non-thermal control parameters such as pressure, doping, or applied magnetic fields, which can give rise to quantum criticality and unconventional superconductivity. Here we provide a brief overview of research into heavy fermion materials in high magnetic fields, focussing on three main areas. Firstly we review the use of magnetic fields as a tuning parameter,and in particular the ability to realize different varieties of quantum critical behaviors. We then discuss the properties of heavy fermion superconductors in magnetic fields, where experiments in applied fields can reveal the nature of the order parameter, and induce new novel phenomena. Finally we report recent studies of topological Kondo systems, including topological Kondo insulators and Kondo–Weyl semimetals. Here experiments in magnetic fields can be used to probe the topologically non-trivial Fermi surface, as well as related field-induced phenomena such as the chiral anomaly and topological Hall effect.     

Iron-based superconductors in high magnetic fields

Here we review measurements of the normal and superconducting state properties of iron-based superconductors using high magnetic fields. We discuss the various physical mechanisms that limit superconductivity in high fields, and the information on the superconducting state that can be extracted from the upper critical field, but also how thermal fluctuations affect its determination by resistivity and specific heat measurements. We also discuss measurements of the normal state electronic structure focusing on measurement of quantum oscillations, particularly the de Haas–van Alphen effect. These results have determined very accurately, the topology of the Fermi surface and the quasi-particle masses in a number of different iron-based superconductors, from the 1111, 122 and 111 families.     

Heavy fermions in high magnetic fields

A qualitative picture of the metamagnetic transition in the Anderson lattice model of heavy fermion Ce compounds is described and a strong coupling spin fluctuation theory of the high field state is presented. The field dependence of the minority spin quasiparticle mass is calculated and the onset of the metamagnetic transition with decreasing field is discussed. The theory of the high field state is extended to include Landau levels and the oscillatory behaviour of the spin self-energy as a function of the inverse applied field is investigated. For the heavy fermion model considered such oscillations of the self-energy lead to significant modifications in the standard theory of the de Haas - van Alphen effect. The possible relevance to anomalous experimental results on CeRu₂Si₂ is discussed.     

Landau levels in deformed bilayer graphene at low magnetic fields

We review the effect of uniaxial strain on the low-energy electronic dispersion and Landau level structure of bilayer graphene. Based on the tight-binding approach, we derive a strain-induced term in the low-energy Hamiltonian and show how strain affects the low-energy electronic band structure. Depending on the magnitude and direction of applied strain, we identify three regimes of qualitatively different electronic dispersions. We also show that in a weak magnetic field, sufficient strain results in the filling factor ν=±4 being the most stable in the quantum Hall effect measurement, instead of ν=±8 in unperturbed bilayer at a weak magnetic field. To mention, in one of the strain regimes, the activation gap at ν=±4 is, down to very low fields, weakly dependent on the strength of the magnetic field.