Transport properties of diluted magnetic semiconductors: Dynamical mean field theory and Boltzmann theory

The transport properties of diluted magnetic semiconductors (DMS) are calculated using dynamical mean field theory (DMFT) and Boltzmann transport theory. Within DMFT we study the density of states and the dc-resistivity, which are strongly parameter dependent such as temperature, doping, density of the carriers, and the strength of the carrier-local impurity spin exchange coupling. Characteristic qualitative features are found distinguishing weak, intermediate, and strong carrier-spin coupling and allowing quantitative determination of important parameters defining the underlying ferromagnetic mechanism. We find that spin-disorder scattering, formation of bound state, and the population of the minority spin band are all operational in DMFT in different parameter range. We also develop a complementary Boltzmann transport theory for scattering by screened ionized impurities. The difference in the screening properties between paramagnetic ($T>T_c$) and ferromagnetic ($T<T_c$) states gives rise to the temperature dependence (increase or decrease) of resistivity, depending on the carrier density, as the system goes from the paramagnetic phase to the ferromagnetic phase. The metallic behavior below $T_c$ for optimally doped DMS samples can be explained in the Boltzmann theory by temperature dependent screening and thermal change of carrier spin polarization.Comment: 15 pages, 15 figure

Lifetime of Gapped Excitations in a Collinear Quantum Antiferromagnet

We demonstrate that local modulations of magnetic couplings have a profound effect on the temperature dependence of the relaxation rate of optical magnons in a wide class of antiferromagnets in which gapped excitations coexist with acoustic spin waves. In a two-dimensional collinear antiferromagnet with an easy-plane anisotropy, the disorder-induced relaxation rate of the gapped mode, Gamma_imp=Gamma_0+A(TlnT)^2, greatly exceeds the magnon-magnon damping, Gamma_m-m=BT^5, negligible at low temperatures. We measure the lifetime of gapped magnons in a prototype XY antiferromagnet BaNi2(PO4)2 using a high-resolution neutron-resonance spin-echo technique and find experimental data in close accord with the theoretical prediction. Similarly strong effects of disorder in the three-dimensional case and in noncollinear antiferromagnets are discussed.Comment: 4.5 pages + 2.5 pages supplementary material, published versio

Expression, Purification, Crystallization and Preliminary X-ray Studies of Histamine Dehydroganase from Nocardioides simplex

This is the publisher's version, also available electronically from http://scripts.iucr.org/cgi-bin/paper?S1744309108023336.Histamine dehydrogenase (HADH) from Nocardioides simplex catalyzes the oxidative deamination of histamine to produce imidazole acetaldehyde and an ammonium ion. HADH is functionally related to trimethylamine dehydrogenase (TMADH), but HADH has strict substrate specificity towards histamine. HADH is a homodimer, with each 76 kDa subunit containing two redox cofactors: a [4Fe-4S] cluster and an unusual covalently bound flavin mononucleotide, 6-S-cysteinyl-FMN. In order to understand the substrate specificity of HADH, it was sought to determine its structure by X-ray crystallography. This enzyme has been expressed recombinantly in Escherichia coli and successfully crystallized in two forms. Diffraction data were collected to 2.7 Å resolution at the SSRL synchrotron with 99.7% completeness. The crystals belonged to the orthorhombic space group P212121, with unit-cell parameters a = 101.14, b = 107.03, c = 153.35 Å

Ultrafast demagnetization in the sp-d model: a theoretical study

We propose and analyze a theoretical model of ultrafast light-induced magnetization dynamics in systems of localized spins that are coupled to carriers' spins by sp-d exchange interaction. A prominent example of a class of materials falling into this category are ferromagnetic (III,Mn)V semiconductors, in which ultrafast demagnetization has been recently observed. In the proposed model light excitation heats up the population of carriers, taking it out of equilibrium with the localized spins. This triggers the process of energy and angular momentum exchange between the two spin systems, which lasts for the duration of the energy relaxation of the carriers. We derive the Master equation for the density matrix of a localized spin interacting with the hot carriers and couple it with a phenomenological treatment of the carrier dynamics. We develop a general theory within the sp-d model and we apply it to the ferromagnetic semiconductors, taking into account the valence band structure of these materials. We show that the fast spin relaxation of the carriers can sustain the flow of polarization between the localized and itinerant spins leading to significant demagnetization of the localized spin system, observed in (III,Mn)V materials.Comment: 15 pages, 8 figure

Interlayer Magnetic Frustration in Quasi-stoichiometric Li1-xNi1+xO2

Susceptibility, high-field magnetization and submillimeter wave electron spin resonance measurements of layered quasi-stoichiometric Li1-xNi1+xO2 are reported and compared to isomorphic NaNiO2. A new mechanism of magnetic frustration induced by the excess Ni ions always present in the Li layers is proposed. We finally comment on the possible realization of an orbital liquid state in this controversial compound.Comment: 4 pages, 5 figures, submitted to Phys.Rev.B, Rapid Com

Comparison of formaldehyde and methanol fixatives used in the detection of ion channel proteins in isolated rat ventricular myocytes by immunofluorescence labelling and confocal microscopy

In this study, a fixation protocol using a 10% neutral buffered formalin (FA) solution and another protocol using a methanol (MeOH) solution were compared for detection of ion channels, Kv1.5, Kv4.2, Cav1.2, Kir6.2, Nav1.5 and Nav1.1 in rat myocytes by immunolabelling. Kv1.5 and Kv4.2 at intercalated discs and Cav1.2 at transverse tubules were not detected by FA but were detected by MeOH. Kir6.2 at transverse tubules and Nav1.5 at sarcolemma were detected by FA but not by MeOH. It is suggested that both FA and MeOH fixation protocols should be used for the detection of cardiac ion channels by immunolabellin

Optical properties of metallic (III,Mn)V ferromagnetic semiconductors in the infrared to visible range

We report on a study of the ac conductivity and magneto-optical properties of metallic ferromagnetic (III,Mn)V semiconductors in the infrared to visible spectrum. Our analysis is based on the successful kinetic exchange model for (III,Mn)V ferromagnetic semiconductors. We perform the calculations within the Kubo formalism and treat the disorder effects pertubatively within the Born approximation, valid for the metallic regime. We consider an eight-band Kohn-Luttinger model (six valence bands plus two conduction bands) as well as a ten-band model with additional dispersionless bands simulating phenomenologically the upper-mid-gap states induced by antisite and interstitial impurities. These models qualitatively account for optical-absorption experiments and predict new features in the mid-infrared Kerr angle and magnetic-circular-dichroism properties as a function of Mn concentration and free carrier density.Comment: 10 pages, 7 figures, some typos correcte

Interactions in high-mobility 2D electron and hole systems

Electron-electron interactions mediated by impurities are studied in several high-mobility two-dimensional (electron and hole) systems where the parameter $k_BT\tau /\hbar$ changes from 0.1 to 10 ($\tau$ is the momentum relaxation time). This range corresponds to the \textit{intermediate} and \textit {ballistic} regimes where only a few impurities are involved in electron-electron interactions. The interaction correction to the Drude conductivity is detected in the temperature dependence of the resistance and in the magnetoresistance in parallel and perpendicular magnetic fields. The effects are analysed in terms of the recent theories of electron interactions developed for the ballistic regime. It is shown that the character of the fluctuation potential (short-range or long-range) is an important factor in the manifestation of electron-electron interactions in high-mobility 2D systems.Comment: 22 pages, 11 figures; to appear in proceedings of conference "Fundamental Problems of Mesoscopic Physics", Granada, Spain, 6-11 September, 200

Orbital Configurations and Magnetic Properties of Double-Layered Antiferromagnet Cs3_3Cu2_2Cl4_4Br3_3

We report the single-crystal X-ray analysis and magnetic properties of a new double-layered perovskite antiferromagnet, Cs$_3$Cu$_2$Cl$_4$Br$_3$. This structure is composed of Cu$_2$Cl$_4$Br$_3$ double layers with elongated CuCl$_4$Br$_2$ octahedra and is closely related to the Sr$_3$Ti$_2$O$_7$ structure. An as-grown crystal has a singlet ground state with a large excitation gap of $\Delta/k_{\rm B}\simeq 2000$ K, due to the strong antiferromagnetic interaction between the two layers. Cs$_3$Cu$_2$Cl$_4$Br$_3$ undergoes a structural phase transition at $T_{\rm s}\simeq330$ K accompanied by changes in the orbital configurations of Cu$^{2+}$ ions. Once a Cs$_3$Cu$_2$Cl$_4$Br$_3$ crystal is heated above $T_{\rm s}$, its magnetic susceptibility obeys the Curie-Weiss law with decreasing temperature even below $T_{\rm s}$ and does not exhibit anomalies at $T_{\rm s}$. This implies that in the heated crystal, the orbital state of the high-temperature phase remains unchanged below $T_{\rm s}$, and thus, this orbital state is the metastable state. The structural phase transition at $T_{\rm s}$ is characterized as an order-disorder transition of Cu$^{2+}$ orbitals.Comment: 6pages. 6figures, to appear in J. Phys. Soc. Jpn. Vol.76 No.