Dielectric and Magnetic Anomalies in α-MnS Single Crystals at Applied Magnetic Fields

Journal of Superconductivity and Novel Magnetism - Temperature and magnetic field dependences of the magnetic susceptibility and dielectric permittivity of the α-MnS single crystal with NaCl...     

Neutron Scattering at Highest Magnetic Fields at the Helmholtz Centre Berlin

The Helmholtz Centre Berlin (HZB), formerly Hahn-Meitner Institute is a user facility for the study of structure and dynamics with neutrons and synchrotron radiation with special emphasis on experiments under extreme conditions. Neutron scattering is uniquely suited to study magnetic properties on a microscopic length scale, because neutrons have comparable wavelengths and, due to their magnetic moment, they interact with the atomic magnetic moments. At HZB a dedicated instrument for neutron scattering at extreme fields is under construction, the Extreme Environment Diffractometer ExED. It is projected according to the “time-of-flight” principle for elastic and inelastic neutron scattering and for the special geometric constraints of analysing samples in a high field magnet. The new magnet will not only allow for novel experiments, it will be at the forefront of development in magnet technology itself. The design of the magnet will follow the Series Connected Hybrid System Technology (SCH) developed at the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida. To compromise between the needs of the magnet design for highest fields and the concept of the neutron instrument, the magnetic field will be generated by means of a coned solenoid with horizontal field orientation. By using resistive insert coils, which are mounted in the room temperature bore of a superconducting cable-in-conduit (CIC) magnet, fields above 30 Tesla can be obtained in a geometry optimised for the demands of neutron scattering.     

Magneto-Absorption in the α Phase of Solid Oxygen at Megagauss Magnetic Fields

Optical absorption spectra of solid oxygen α phase and liquid oxygen were measured in high magnetic fields over 100 T using a single turn coil method. The absorption originates from the bimolecular transition was observed. The spectra strongly reflect the local magnetic interaction between two oxygen molecules. The shape of the absorption spectrum of the α oxygen changed dramatically with increasing magnetic fields, even though the absorption intensity of liquid oxygen simply declined. This difference cannot be explained by only the local magnetic interaction, and we speculate that the field induced lattice distortion plays an important role in the solid oxygen α phase.     

Sound Velocity Measurements in the Low and the High Field Phases of the Nuclear-Ordered bcc Solid 3He in Magnetic Fields

We have measured the temperature and magnetic-field dependences of the sound velocity for one longitudinal and two transverse waves in the low field phase (LFP) and the high field phase (HFP) of nuclear spin ordered bcc solid ³He crystals with a single magnetic domain along the melting curve. From sound velocity measurements for various crystal orientations as a function of the sound propagation direction, we determined the elastic stiffness constants, c _ij (T,B). In the LFP with tetragonal symmetry for the nuclear spin structure, we extracted six nuclear spin elastic stiffness constants Δc _ij ^ℓ (T,0.06 T) from the temperature dependence of the sound velocity at 0.06 T and Δc _ij ^ℓ (0.5 mK,B) from the magnetic-field dependence of sound velocity at 0.5 mK. In the HFP with cubic symmetry for the nuclear spin structure, we extracted three Δc _ij ^h (T,0.50 T) at 0.50 T and Δc _ij ^h (0.5 mK,B) at 0.5 mK. At the first-order magnetic phase transition from the LFP to the HFP at the lower critical field B _c1, large jumps in sound velocities were observed for various crystal directions and we extracted three . Using the thermodynamic relation between Δc _ij and the change in the internal energy for the exchange interaction in this system, ΔU _ex(T,B), Δc _ij are related to the generalized second-order Grüneisen constants Γ _ij ^X ≡∂ ²ln X/∂ ε _i ∂ ε _j as Δc _ij (T,B)=Γ _ij ^X ΔU _ex(T,B), where X represents some physical quantity which depends on the molar volume and ε _j is the j-th component of a strain tensor. In the LFP, the Δc _ij ^ℓ (T,0.06 T) were proportional to T ⁴, and Δc _ij ^ℓ (0.5 mK,B) were proportional to B ². We extracted for the spin wave velocity in the LFP, s ^ℓ , from Δc _ij ^ℓ (T,0.06 T) and for the inverse susceptibility, 1/χ ^ℓ from Δc _ij ^ℓ (0.5 mK,B). In the HFP, Δc _ij ^h (T,0.50 T) were proportional to T ⁴ and Δc _ij ^h (0.5 mK,ΔB) were proportional to ΔB(≡B−B _c1). We obtained for the spin wave velocity in the HFP, s ^h , from Δc _ij ^h (T,0.50 T) and for B _c1 from Δc _ij ^h (0.5 mK,ΔB). The values obtained for and were compared with the Multiple Spin Exchange model (MSE) with three parameters by using analytic expressions for s ^ℓ and χ ^ℓ . The three-parameter MSE does not agree with the observed Δc _ij in the LFP.      

The Influence of Many-Body Interactions on the Electron States in Quantum Point Contacs: Persistence of Exchange-Driven Splitting at High External Magnetic Fields

Spontaneous spin polarization of a quasi-1D electron gas in quantum point contacts (QPCs) is an important concept when analyzing conductance anomalies in the quantum limit. As suggested by recent measurements (Koop et al., J. Supercond. Nov. Magn. 20:433, 2007) there is a splitting of the subband levels in QPCs related to 0.7 conductance anomaly, both for zero- and finite in-plane magnetic fields. In the present paper we present theoretical results for spin polarization occurring in a QPC in a magnetic field as obtained from the local spin-density approximation (LSDA). Our numerical simulations are consistent with the findings of Koop et al. and support the idea that spin polarization underpins the conductance anomaly.     

Static and Dynamic Properties of the Insulator–Metal Transition in Magnetic Semiconductors, Including the Perovskites

All magnetic semiconductors experience an insulator–metal transition with decreasing temperature near and below the magnetic ordering temperature. T_c, as long as the carrier concentration, whether electrons or holes, is low enough. For somewhat higher carrier concentrations, a resistivity anomaly ordinarily occurs near T_c, which cannot be explained in terms of scattering from the magnetic fluctuations alone. The thrust of this paper is to review the magnetotransport properties of various magnetic semiconductors and to present arguments that magnetic polarons, a many body state involving charge carriers and the localized magnetic spins in their immediate neighborhoods, are involved in the physical processes leading to the insulator–metal transition. Magnetic polarons have been observed directly by a variety of physical techniques including, for example, magnetic measurements, neutron diffraction, scanning tunneling microscopy, and, most recently, by noise measurements. This article will review these physical manifestations of the existence of magnetic polarons and relate them to the transition.     

Dynamic Alignment of Single-Walled Carbon Nanotubes in Pulsed Magnetic Fields

We have used linear dichroism spectroscopy to measure the dynamic alignment of single-walled carbon nanotubes (SWNT) in pulsed magnetic fields up to 55 T. We make use of the fact that SWNTs absorb light only when the electric-field vector is oriented parallel to the tube axis. SWNTs thus produce a polarization dependent change of the optical transmission, that permits precise measurements of their orientation. In order to distinguish the influence of different mechanisms governing the alignment such as the external magnetic field, Brownian motion or the tube length, we have systematically varied parameters such as the viscosity of the aqueous solution and the sample temperature.     

Quantum Transport and Cyclotron Resonance Study of Ge/SiGe Quantum Wells in High Magnetic Fields

Shubnikov-de Haas oscillation and cyclotron resonance were studied for high mobility p-type Ge channels in strained Ge/Si_1?x Ge _x quantum wells, using pulsed high magnetic fields up to 50 T. Fine quantum oscillations were observed in ρ _xx . Reflecting the complex Landau level structure in the nearly degenerate valence bands, the Fourier transform of the oscillatory spectra consists of several peaks. Cyclotron resonance was measured at photon energies between 10 and 17 meV. Two well-defined resonance peaks were observed in two samples with different x, resulting in different strains. A large non-parabolicity and large strain dependence of the effective masses were observed.     

Development of the High-Field Imaging System in Pulsed-High Magnetic Fields and Its Application to Manganites

We developed a high-speed polarizing microscope imaging system combined with a 37 T pulse magnet. With utilizing this system, we can visualize some kinds of structural transitions induced by high magnetic fields. The efficiency of this system was tested through observation of the field-induced collapse of charge-orbital ordering (COO) in mixed-valent manganites. In perovskite-type Nd_1/2Sr_1/2MnO₃, quantitative analyses of the obtained polarizing microscope images clearly show the discontinuous change in birefringence accompanied with hysteresis, which are characteristic of the first order transitions. This observation of field-induced melting of the COO was also successful in a layered manganite La_1/2Sr_3/2MnO₄ in which changes in magnetization and magnetoresistance at the transition are less clear. The present success in observation of the melting of the COO demonstrates the potential applicability of this system to visualize other kind of structural transitions even in a tiny piece of crystal.     

Change in the Rheological Properties of High‐Paraffin Petroleums under the Action of Vibrojet Magnetic Activation

The influence of the method of vibrojet magnetic activation on the rheological properties of highparaffin petroleums has been investigated. The maximum decrease in the rheological characteristics of the petroleums was obtained in 20min vibrojet treatment. It has been noted that the maximum effect of vibroaction is observed for petroleums with a higherthanaverage content of paraffins. The role of the magnetic component contributing to the decrease in the congelation point and the amount of the basic sediment in the petroleums under study has been shown.     

Pseudogap in Fermionic Density of States in the BCS-BEC Crossover of Atomic Fermi Gases

We study pseudogap behaviors of ultracold Fermi gases in the BCS-BEC crossover region. We calculate the density of states (DOS), as well as the single-particle spectral weight, above the superfluid transition temperature T _c including pairing fluctuations within a T-matrix approximation. We find that DOS exhibits a pseudogap structure in the BCS-BEC crossover region, which is most remarkable near the unitarity limit. We determine the pseudogap temperature T ^* at which the pseudogap structure in DOS disappears. We also introduce another temperature T ^** at which the BCS-like double-peak structure disappears in the spectral weight. While one finds T ^*>T ^** in the BCS regime, T ^** becomes higher than T ^* in the crossover and BEC regime. We also determine the pseudogap region in the phase diagram in terms of temperature and pairing interaction.     

Effect of High Magnetic Fields on the Charge–Density–Wave State in NbSe3

We present a systematic study on a large magnetoresistance in NbSe₃ as a function of temperature under various pressures. The large magnetoresistance below T _c2 is observed as long as the T _c2-CDW phase exists. However, when the T _c2-CDW phase is suppressed totally by high pressure and the superconducting phase is induced at the same time, no anomaly, including the large magnetoresistance, is induced by a magnetic field up to 12 T. The origin of the large magnetoresistance is discussed by considering normal carriers on the ungapped Fermi surface created on the CDW transition and comparing this magnetoresistance with the pressure-induced magnetoresistance that is newly observed above T _c2.     

Design and Realization of the Control and Measurement System of the Wuhan Pulsed High Magnetic Field Facility

A user-friendly Control and Measurement System (CMS) is designed and realized at the Wuhan High Magnetic Field Center (WHMFC). Structure and functions of the CMS system are described in detail. Three kernel parts of CMS are discussed. The success of the comprehensive system test shows that the CMS is effective and reliable.     

Electrical Conductivity of Thin Polycrystalline La0.83Sr0.17MnO3 Films in Pulsed High Electric and Magnetic Fields

The electrical conductivity of thin polycrystalline La_0.83Sr_0.17MnO₃ films was investigated. These films were simultaneously affected by a single electrical pulse having a pulse length of several ns and a half-sinus magnetic pulse with a pulse length of 1 ms and amplitude ranging up to 20 T. The influence of the high magnetic field on the electroresistance was studied at electric field strengths up to 80 kV/cm when the temperature of the film ranged from 100 K to 300 K. It was found that depending on the temperature and magnetic field value the electroresistance (ER) can be enhanced or suppressed by the applied magnetic field. These results are explained by assuming that the ER appears due to inelastic multistep tunneling through grain boundaries and by taking into account the redistribution of voltage across crystalline grains and grain boundary regions.     

The Influence of the Morphology on the Magnetic Properties of Poly(3-hexylthiophene)

The influence of the morphology on the magnetic properties of poly(3-hexylthiophene) has been studied and the results reported. The changes in the saturation magnetization were explained considering the polymer morphology prepared with different amounts of water in the solution during sample synthesis. The results show that the saturation magnetization is maximized for 200 ppm of the water in acetonitrile solution.     

Magnetic Orders of Highly Frustrated Spinel, ZnCr2O4 in Magnetic Fields up to 400 T

We investigated the magnetization process of the highly frustrated spinel ZnCr₂O₄ using the Faraday rotation method at 6 K under the ultra-high magnetic fields up to 400 T by the electro-magnetic flux compression method. We successfully observed the full magnetization process in this material. We found some anomalies which show new phase transitions at 240 T, 290 T, 350 T, and 390 T.     

Pseudogap Ground State and Its Doping Evolution in Bi2Sr2?x La x CuO6+δ : Removing the Superconducting State by the Application of Very High Magnetic Fields

We present results for the pseudogap ground state and its doping evolution in single-layered copper-oxide Bi₂Sr_2?x La _x CuO_6+?? . We apply very high magnetic fields up to 44 T to remove the superconducting state and reveal the hidden low temperature (T) normal state. Through ⁶³Cu-NMR Knight shift and spin-lattice relaxation rate measurements, we find that there remains a finite density of states (DOS) at the Fermi level in the zero-T limit when the superconductivity is removed, which indicates that the pseudogap ground state is a metallic state with a finite volume of Fermi surface. The residual DOS in the pseudogap ground state decreases with decreasing doping (increasing x), but remains quite large even at the vicinity of the magnetically ordered phase of x?? 0.8. The result indicates that the superconductivity emerges from the remaining Fermi surface and coexists with the pseudogap.     

Unusual Hysteresis Loop of (Tb0.2Pr0.8)(Fe0.4Co0.6)1.9−x C x in Pulsed High Magnetic Field

The magnetization properties of (Tb_0.2Pr_0.8)(Fe_0.4Co_0.6)_1.9?x C _x ( $x=0$ –0.5) were investigated in a pulsed high magnetic field. Obvious unsymmetry magnetic hysteresis loops were observed, in which the magnetization measured in the second(negative) magnetic field route was larger than that measured in the first(positive) one. Meanwhile, the unsymmetry of the hysteresis loops was C doping levels, magnetic fields and temperatures dependent. The experimental results suggest that the unusual hysteresis phenomenon result mainly from the collapse of the pinning center and the formation of ferromagnetic(FM) pseudo-domain.