Thermoelectric power of amorphous alloys near the metal-insulator transition

The thermoolectric powerS and the electrical conductivity σ of amorphous Au_xSb_100-x and Cu_xSb_100-x films have been measured in the temperature range between about 2 K and 350 K for concentrations close to the eetal-insulator transition. In both systems the transition occurs at a critical concentrationx _c≈8 at.% noble metal content. A characteristic feature of the transition is in both cases a strong increase of the low temperature slope of the thermopower, i.e.S/T| _t»0 , when approachingx _c from the metallic side. The results are compared with different theoretical predictions for the metal-insulator transition. Furthermore we report on the changes ofS and σ during annelaing. It will be shown that especiallyS(T) of the samples withx close tox _c depends strongly on the annealing state of the films.     

Spin-glass freezing and RKKY interactions near the metal-insulator transition in amorphous Gd-Si alloys

Magnetization measurements of giant negative magnetoresistive amorphous GdxSi1-x ( 0.04相似文献   

Anisotropic metal-insulator transition in epitaxial thin films

By comparing the properties of In and Pb quantum wells in a scanning tunneling microscopy subsurface imaging experiment, we found the existence of lateral bound states, a 2D Mott-Hubbard correlation gap, induced by transverse confinement. Its formation is attributed to spin or charge overscreening of quasi-2D excitations. The signature of the 2D confinement-deconfinement transition is also experimentally observed, with the correlation gap being pinned in the middle of the conduction band. A self-organized 2D Anderson lattice is suggested as a new ground state.     

Electronic properties and superconductivity of amorphous Si1-xAux alloys near the metal-insulator transition

We report on specific-heat and resistivity measurements on quench-condensed Si_1-xAu_x films for 0.11 ⩽ x 0.36 in the temperature range 0.35 K ⩽ T ⩽ 6 K. A distinct increase of the specificheat derived electronic density of states at the Fermi level is observed at x_b ≈ 0.2, i.e., in the vicinity of the metal-insulator transition occurring for our samples at x_c = 0.16. This suggests a different type of bonding between Au and Si for x < x_b and x > x_b. While resistive transitions to superconductivity are observed for x⩾0.21, the absence of a specific-heat anomaly at the transition points to filamentary superconductivity except for × = 0.35 where a sizable anomaly is seen. The difference in various electronic properties between differently prepared samples of these metastable alloys, in particular the influence of different preparation and annealing temperatures is emphasized. It is suggested that these differences are caused by incipient phase separation in the room-temperature prepared samples.     

The metal-insulator transition and the phase transition in metal-ammonia solutions

The ground state of impurity metal (sodium) atoms in liquid ammonia close to the solvated state of the free electrons is considered. It is shown that the critical solubility point lying on the metal side of the metal-insulator transition is determined by the Coulomb interaction between the ions and electrons in the overlapping impurity states, classically accessible spheres of which form an infinite percolation cluster. The percolation conductivity via the impurity states is estimated. The estimate agrees with the experimental data near the critical solubility point. Zh. éksp. Teor. Fiz. 111, 938–948 (March 1997)     

On the mean-field theory of the metal-insulator phase transition

Two mean-field models describing the metal-insulator phase transition are studied. In the model due to Keldysh and Kopaev (1964) different effective masses in the valence and conduction bands are considered and in the model due to Mattis and Langer (1970) the exact s.c. and b.c.c. densities of states are taken into account. It is shown that these modifications do not change the qualitative features of the phase diagrams. In each case a special attention is paid to the domain of the degenerate semiconductor, which has been of primary interest in many attempts to explain conjectured enhancement of superconductivity in systems undergoing dielectric transition.     

Magnetic field diffusion during a metal-insulator phase transition

An analysis is made of the diffusion of a static (slowly varying) magnetic field in a conductor in which a first-order phase transition to the insulating state takes place under the action of the Joule heating. An investigation is made of the case of subsonic propagation of the phase boundary. A (V_1−X Cr_X)₂O₃ solid solution is analyzed as a model substance. The application of this effect in pulsed high-current circuit breakers is discussed. Zh. Tekh. Fiz. 68, 43–48 (December 1998)     

Density of states of amorphous GdxSi1-x at the metal-insulator transition

We have determined the electronic density of states of amorphous Gd xSi (1-x), N(GdSi)(E), in the vicinity of the metal-insulator transition by measuring the tunneling conductance dI/dV across a Gd xSi (1-x)/oxide/Pb tunnel junction at low T (T approximately 100 mK). By applying a magnetic field we can tune through the metal-insulator transition and simultaneously measure the transport and N(E) on a single sample. We find a smooth transition from a metal with strong Coulomb interactions to a developing Coulomb gap in the insulating regime. In the metallic region N(GdSi)(0) scales approximately with sigma(2).     

Magnetovolume effect in transition metal-metalloid amorphous alloys

The thermal expansion, spontaneous volume magnetostriction ω_s, forced volume magnetostriction $\text{(}\text{?ω}\text{?H}\text{)}$ and Young's modulus of amorphous Fe-B, Fe-P, Co-B and (Fe-M)₇₇Si₁₀B₁₃ (M = Cr, Mn, Co, Ni) alloys have been measured to make clear the magnetovolume effect in transition metal-metalloid amorphous alloys. The thermal expansion coefficient α, ω_s and $\text{(}\text{?ω}\text{?H}\text{)}$ are dependent on the number of d-electrons per transition metal atom n_eff calculated based on the charge transfer model. The n_eff vs. α, ω_s and $\text{(}\text{?ω}\text{?H}\text{)}$ curves are quite similar to the corresponding curves in fcc alloys. The maxima in those curves are, however, found at n_eff ≈ 8.2 for the amorphous alloys in contrast with n_eff ≈ 8.7 for the fcc Fe-Ni alloys. On the other hand, Young's modulus measured under the saturation of magnetization is governed by the molar volume, irrespective of n_eff. The magnetovolume effect in transition metal-metalloid amorphous alloys is discussed in connection with the instability of ferromagnetism of amorphous Fe.     

An exactly solvable model for metal-insulator phase transition

A simple model of electrons interacting with photons which displays a metal-insulator phase transition in the case of s.c. and b.c.c. tight-binding bands is studied. A proof is given that the model is exactly solvable in the thermodynamic limit.     

Magnetic-field-induced metal-insulator quantum phase transition in CaFeAsF near the quantum limit

The quantum limit, where only the lowest Landau level is occupied by electrons, can be achieved under a high magnetic field when the Landau level splitting is comparable with the Fermi energy. The rather small Fermi pockets and Fermi energy in CaFeAsF reported recently make this compound a good candidate for investigating the electrical transport near the quantum limit.Here, we report high-field experiments up to 65 T on a single-crystalline CaFeAsF, which shows a metal-insulator quantum phase transition tuned by the out-of-plane magnetic field. The obtained critical exponent zν through the finite-size scaling analysis is very close to 4/3. This transition is closely associated with the evolution of electronic states approaching the quantum limit.The resistivity behaviors as a function of field and temperature were evaluated based on Adams-Holstein theory(A-H theory).Moreover, the in-plane component of the field, which does not affect the transport behavior in the classical region, suppressed the magnetoresistance near the quantum limit.     

Atomic-scale phase coexistence and fluctuation at the quasi-one-dimensional metal-insulator transition

Scanning tunneling microscopy of a quasi-one-dimensional (1D) metal-insulator transition in an In nanowire array on the Si(111) surface reveals unprecedented details in the transition dynamics. The transition proceeds in microscopic first order, namely, through the domain-by-domain conversion at the nanoscale, from the metallic to the insulating phase or vice versa. The definition of domains and their effective transition temperatures (Tc) are strongly correlated with the distribution of defects. Below Tc, the condensation and the fluctuation of 1D charge density waves are observed within the isolated metallic domains, as well as at the domain boundaries. The appearance of such isolated condensates suggests a strong intrawire coupling: a manifestation of the 1D nature of the critical fluctuation, as well as the origin of the first-order transition.     

Quasiparticle dynamics in the vicinity of metal-insulator phase transition in NaxCoO2

Layered cobaltates embody novel realizations of correlated matter on a spin-1/2 triangular lattice. We report a high-resolution systematic photoemission study of the insulating cobaltates. The observation of a single-particle gap opening and band folding provides direct evidence of anisotropic particle-hole instability on the Fermi surface due to its unique topology. Overlap of the measured Fermi surface is observed with the square root 3xsquare root 3 charge-order Brillouin zone near x=1/3 but not at x=1/2 where the insulating transition is actually observed. Unlike conventional density waves, charge stripes, or band insulators, the onset of the gap depends on the quasiparticle's quantum coherence which is found to occur well below the disorder-order symmetry breaking temperature of the crystal (the first known example of its kind).     

Magnetic phases of amorphous transition metal-metalloid alloys

Hysteresis loop and ac susceptibility measurements were performed on three series of amorphous alloys: (A_wB_1-w)₇₅P₁₆B₆Al₃, where (A, B) are (Fe, Ni), (Co, Ni) and (Fe, Mn). Upon cooling, low w alloys undergo paramagne t to spin glass transitions. Alloys with higher w first experience a Curie transition to a ferromagnetic state, and then a spin freezing transition to a spin glass state. the T dependence of the width of the ac hysteresis loop is used to determine the spin freezing transition temperature. A magnetic phase diagram is presented for each alloy series and the value of w required for ferromagnetism, w_C, is determined. When measured in the presence of small constant fields, the ac susceptibility of alloys with w just above w_C has maxima near both transition temperatures. The field and temperature dependences of the peaks are explained by scaling arguments, used to determine the critical exponent δ for the Curie transition, and suggest that a similar scaling law holds for the ferromagnet to spin glass transition.     

Superconducting properties of amorphous transition metal alloys

We have co-sputtered amorphous films of several Mo and W-based superconducting alloys. Measurements of T_c, ($\text{d}\text{H}{}_{\mathrm{c2}}\text{d}\text{T}$)_Tc and the normal state resistivity near T_c were made on a number of these alloys. Our results and other data from the literature are studied to examine the correlation between T_c and the dressed density of states at the Fermi level.     

Electronic structure of amorphous transition metal alloys

The electrical resistivity, the Hall coefficient and the magnetic susceptibility of amorphous alloys (Metglass 2826A, Allied Chemical) have been measured above room temperature. The experimental data show strong similarities with liquid alloys.