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We present the first direct observation of temperature-dependent Fermi wave vectors in the quasi-one-dimensional conductor K_0.3 MoO₃ . By monitoring the position of these vectors through the Peierls transition using angle-resolved photoemission spectroscopy, we demonstrate that the temperature dependence of the charge-density-wave vector observed in earlier experiments is actually that of the Fermi wave vectors.

Since the appearance of Berry's seminal paper in 1984, geometric phases have been discovered in virtually all fields of physics. Here we address molecules and solids, and we limit our scope to the Berry's phases of the many-electron wavefunction. Many advances have occurred in very recent years relating to the theory of such phases and their observable consequences. After discussing the basic features of Berry's phases in a generic quantum system, we specialize to selected examples taken from molecular physics and condensed matter physics; in each of these cases, a Berry's phase of the electronic wavefunction leads to measurable effects.

The process of dehydration for the molybdenum trioxide hydrates, MoO₃ 2H₂ O 2-MoO₃ H₂ O -MoO₃ , was studied for the first time by in situMo K-edge x-ray absorption spectroscopy. A regularization procedure was applied to reconstruct the radial distribution functions around the molybdenum at all stages of the process. Strong variations in the local environment of the molybdenum were found at both phase transitions and were attributed to the changes in direction of the off-centre Mo displacement and in the degree of symmetrization of the Mo-O bonds. Also, an increase of the Mo-O-Mo angle, leading to the appearance of strong multiple-scattering effects in the x-ray absorption fine structure, was observed at the second phase transition -MoO₃ H₂ O -MoO₃ . The final -MoO₃product had nanocrystalline structure with an average grain size of about 15 Å.

Shubnikov-de Haas effect measurements were performed on a high-quality whisker single crystal of UAs₂ . Complementary electrical resistivity and specific heat experiments were also carried out. Five of the seven detected branches exhibit angular dependences clearly indicating cylindrical Fermi surfaces. The effective cyclotron masses were determined to range from 0.33 to 4.5 m₀ , which yielded an electronic specific heat coefficient of 10 mJ K^-2mol^-1 . This is in very good agreement with the value 11.7 mJ K^-2mol^-1determined directly by the low-temperature specific heat measurement. The two-dimensional Fermi surfaces are formed in a strongly flattened magnetic Brillouin zone and the conduction electrons are confined mainly in the U planes, indicating an important contribution of itinerant 5f electrons.

The magnetic and transport properties of layered manganese oxides La_1.2 Sr_1.8 Mn_{2 - x} T_x O₇(T = Fe, Co, Cr) are reported. The undoped oxide La_1.2 Sr_1.8 Mn₂ O₇is ferromagnetic (FM) below 125 K where an insulating-metallic (I-M) transition occurs. Fe and Co doping have similar effects on the properties of La_1.2 Sr_1.8 Mn₂ O₇ , resulting in quick weakening of FM ordering, occurrence of a cluster-glass or spin-glass state for x> 0.07, immediate disappearance of the I-M transition and enhancement of low-temperature magnetoresistance. But a different effect is observed for Cr doping. In La_1.2 Sr_1.8 Mn_{2 - x} Cr_x O₇ , although the I-M transition also disappears quickly, FM ordering is maintained even for x= 0.5. These doping effects could be understood by considering the different exchange interactions between Mn and T (Fe, Co, Cr) and the corresponding change of the double-exchange interaction in La_1.2 Sr_1.8 Mn_{2 - x} T_x O₇ .

The excitation spectra of and emissions of self-trapped excitons (3.4 eV and 5.4 eV, respectively) as well as the excitation spectra of 5.17 eV luminescence of Ag⁺impurity centres were measured in NaCl and NaCl:Ag crystals using synchrotron radiation of 5 - 38 eV. Fast and slow components of these emissions were detected. An analysis of the differences in the excitation spectra measured at 8 and 295 K allowed us to separate the excitonic and electron-hole (e-h) mechanisms of the multiplication of electronic excitations. A photon of 17 - 19 eV forms an e-h pair and a secondary exciton, while the absorption of a 21 - 27 eV photon causes the creation of two e-h pairs. Using luminescent and photoelectric methods, it was shown that a 2p3s Na⁺cation exciton, formed at the absorption of a 33.4 eV photon, decays with the creation of an anion exciton with a 3p hole component and two e-h pairs. Three e-h pairs are formed after the absorption of a 31 eV photon by a chlorine ion.

We study the effective mass and the Landé factor for the three-dimensional as well as the two-dimensional degenerate electron gas. The influences of specific approximations to the self-energy and the vertex, equivalently formulated in terms of static and local effective interactions, are examined, with the aim of developing a legitimate and straightforward description of realistic low-dimensional semiconductor structures. The results obtained are tested against reference data from the literature. We further apply the formalism to a Si-SiO₂metal oxide-semiconductor (MOS) structure and compare the predictions with experiment.

The effect of temporal and spatial potential-energy fluctuations on low-temperature dark dc conductivity in disordered materials is considered. Analytical models are formulated to treat the variable-range hopping in a disordered system of localized states whose energies are subjected to 1/ftemporal fluctuations. Long-range spatial potential fluctuations are described as a random distribution of intrinsic electric field. Temporal 1/ffluctuations are assumed either to be independent of carrier hopping or to be caused by the latter process. Both spatial and independent 1/ftemporal fluctuations are shown to yield temperature dependences of the conductivity much weaker than those predicted by the Mott law. Self-sustaining 1/ftemporal fluctuations caused by the carrier random walk lead to the crossover from Mott's T^-1/4to T^-1/3dependence with increasing temperature.

We study the band-gap renormalization in a model semiconductor quantum wire due to the exchange-correlation effects among the charge carriers. We construct a two-subband model for the quantum wire, and employ the GW -approximation to obtain the renormalized quasi-particle energies at the optical band edge. The renormalization is calculated as a function of electron-hole plasma density and the wire radius. Our results show that the very presence of the second subband affects the renormalization process even in the absence of occupation by the carriers. We compare the fully dynamical random-phase approximation results to the quasi-static case in order to emphasize the dynamical correlation effects. Effects of electron-phonon interaction within the two-subband model are also considered.

Upon closely examining the Seebeck behaviour of Hg-1201 we have discovered that the system, whether stoichiometric or mercury deficient, appears to behave in similar fashion to the LBCO and YBCO-123 systems in that a dip appears in T_c ( p ) at p ~1/8. We have earlier indicated that this concentration of holes marks the point at which stripe phase formation in the organization of charge and spin in the two-subsystem, mixed-valent, HTSC materials passes from one charge wall loading scheme to another.

We observe a strong difference in doping characteristics between the mercury-deficient and mercury-stoichiometric systems, and assemble here a plausible scenario of what is involved. While HgBa₂ CuO₄₊indeed does offer very considerable potential advantages as an archetype to the cuprate HTSC phenomenon, excess anion systems like this clearly introduce materials complexities. These will always demand careful attention and perhaps not endear the system to many accustomed to the relative simplicities of a cation-doped system.

We obtain the logarithmic corrections to the dynamic response function and nuclear magnetic resonance (NMR) T₁ - and T_2G -rates in the spin-1/2 antiferromagnetic Heisenberg chain using the perturbative renormalization group for the leading irrelevant operator. The result is compared with NMR experiments on Sr₂ CuO₃ .

The metamagnetic-like transition at an external field (H_ext) ~7.7 T (H_M) in CeRu₂ Si₂has been probed by ¹⁰¹ Ru NMR/NQR and ²⁹ Si NMR measurements in an external field up to 16.4 T. The nuclear spin-lattice relaxation rate (T₁^-1 ) of ¹⁰¹ Ru is field dependent at low temperatures (T). The value of (T₁T)^-1remains T -independent below 7 K in zero field, whereas the value of (T₁T)^-1at H_Mcontinues to increase down to 1.4 K. At a field of 16.4 T (a field much higher than H_M), (T₁T)^-1behaves independently of Tagain, as expected for the Fermi liquid state, where the value of (T₁T)^-1is much smaller than that in zero field. The field dependence of the spin-echo decay rate (T₂^-1) measured at 4.2 K by means of NMR for both ¹⁰¹ Ru and ²⁹ Si exhibits no anomaly near H_M , suggesting the absence of any enhancement of the ferromagnetic spin fluctuations.

Magnetization and differential susceptibility of manganese tartrate dihydrate were measured in a single crystal from room temperature down to 0.4 K and in the presence of magnetic fields up to 70 kOe. Nearly antiferromagnetic order occurs below T_N= 1.83 K, with spins oriented close to the caxis. Sharp peaks in the susceptibility and the presence of a remanent magnetization in the plane perpendicular to the caxis suggest a weak ferromagnetic structure of spins with a canting angle 0.6°. The magnetic phase diagram below T_Nwas determined. The exchange and anisotropy field were obtained respectively as H_E= 19.4 kOe and H_A= 0.6 kOe.

The Mössbauer effect in Fe₆₅ Ni_{35 - x} Pt_x(x= 0, 5.6 and 10.8) has been measured at temperatures between 4.2 K and 300 K to study local electronic and magnetic properties of the system. The variation of the hyperfine-field distribution with temperature has confirmed the transitions of Fe atoms from a high-magnetic-moment (HM) state to a low-magnetic-moment (LM) state with elevating temperature and a simple model based on a two-state transition interpreted well the changes of the hyperfine-field distribution with Pt content and temperature. The substitution of Pt for Ni makes the HM LM transition of Fe atoms more difficult and decreases the chemical disorder in the sample. By using the composition dependence of the lattice volume, the volume effect and the pure substitution effect of the Pt atom on the average hyperfine field were further separated. Moreover, the Debye temperatures of the samples were evaluated from the decrease of the isomer shift with increasing temperature.

The effect of antiphase domain walls in NH₄ Cl was studied by quantum mechanical calculations using a plane-wave pseudopotential code. The calculated structural parameters for the low-temperature ferro-phase are in good agreement with experimental data. Calculations based on a supercell approach show that, to a first approximation, a model with non-interacting infinitesimally thin domain walls is applicable to ammonium chloride.

Low-temperature (20-290 K) thermoluminescence spectra of Bi₄ Ge₃ O₁₂reveal a range of trapping levels, some of which are common to both undoped and doped material. The emission spectra for undoped and transition-metal-doped Bi₄ Ge₃ O₁₂indicate that at low temperatures, intrinsic luminescence centres result in broad-band emission typical of signals from relaxed excitons or possibly excited bismuth ions. For material containing rare-earth ions, the signals are characteristic of the rare-earth dopants, even when the rare-earth ions are present in concentrations as low as 3 ppm. The temperatures of the glow peaks seen at ~54, 105 and 141 K for undoped material are strongly modified by the rare-earth ions. The trapping and recombination sites, monitored by rare-earth emission, are intimately linked, probably within large complex structures. For these three glow peaks the peak temperature varies smoothly with the ionic radii of the rare-earth impurities. These movements are substantial, with changes of up to 50 K, as a function of the rare-earth radii. Of all the rare-earth ions, europium forms the most stable recombination centres. This is probably because the trivalent europium ion is similar in size to the host (bismuth) ion for which it substitutes. Tentative models for trapping sites and thermoluminescence mechanisms are proposed.

The magnetic circular dichroism in the perpendicular geometry of the resonant 2p3p3p photoemission (PE) spectroscopy has been investigated in metallic Ni as a function of the photon energy across the Ni L₃absorption edge. Within the experimental error bars, the photon energy dependence of the PE dichroism signal is the same as the one shown by the magnetic circular dichroism of the corresponding x-ray absorption (XMCD), obtained in the collinear geometry. This is attributed to the fact that, in metal Ni, the orbital L_z and dipolar T_z moments are smaller than the spin angular moment S_z . The latter is the dominating term in both the expressions that give the integrated values of the PE dichroism or XMCD intensities. Although the respective photon energy dependence is very similar, the normalized PE dichroism intensity is a factor ~5.6 smaller than the normalized XMCD signal, while only a factor ~1.6 is expected from theoretical considerations. This factor is observed even below the L₃threshold, thus we exclude that the small intensity of the perpendicular geometry dichroism in the Ni 2p3p3p resonant photoemission is due to fast relaxation processes in the intermediate state.