William W. Warren

Nuclear magnetic resonance in semiconducting liquid alloys

Abstract NMR studies of Ga69, 71, Tl205 in the Ga1−xTex and Tl1−xTex liquid alloy systems are described. Attention is focused on the temperature and concentration dependences of the Knight shifts, magnetic relaxation rate enhancements and quadrupolar relaxation rates. At temperatures near the liquidus each of these quantities exhibits extremum behavior at the concentrations for which chemical valence requirements are satisfied (Ga2Te3 and Tl2Te). Knight shift data show that the density of states at the Fermi level reaches a minimum value characteristic of a deep pseudogap at the concentration of the compounds. Strong peaks are observed in the magnetic relaxation enhancements and quadrupolar relaxation rates at the same concentrations. The results in Ga1−xTex are interpreted in terms of the occurrence of Ga2Te3 molecular associations in the range 0.50 ⪅ × ⪅ 0.70. Qualitatively similar conclusions apply to Tl1−xTex near Tl2Te. All the data are in good agreement with a model involving weakly localized states in a pseudogap which develops as a consequence of the formation of molecular clusters.

Evidence for the existence of a pseudogap in a semiconducting liquid: Temperature dependent In115 knight shift in In2Te3

Abstract We report the observation of the In115 NMR in solid and liquid In2Te3, measurement of the Knight shift from 725 K to 1500 K, and measurement of the spin-spin relaxation rate from the melting point (Tm = 940 K) to 1400 K. The Knight shift, K, increases continuously from a value K = 0.05 ± 0.02% below 800 K to K = 1.30 ± 0.02% above 1400 K. The data are interpreted in terms of the pseudogap model proposed by Mott. It is shown that the parameter g relating the density of states N(EF) to the free electron value (g≡N(EF)/N(EF)f.e) varies continuously from a value g⩽0.02 below 800 K to a value g∼ 1 2 in the liquid above 1400 K. The increase in g coincides with increasing disorder due to random distribution of the vacancies in the solid and to structural changes in the liquid. The relaxation rate, 1/T2, decreases from a value 1/T2=(2.0 ± 0.3) × 105 sec−1 at Tm to 1/T2 = (0.8±0.1) × 105 sec−1 at 1400 K. Comparison of these relaxation rates with the observed Knight shifts suggests that electronic states in the pseudogap become localized where g ⩽ 0.25±0.08.

Observation of localized electronic states in a liquid semiconductor: NMR in liquid Ga2Te3

Abstract We report observation of the Ga69 and Ga71 NMR in solid and liquid Ga2Te3, measurement of the Knight shifts from 900 to 1500°K and measurement of the nuclear relaxation rates from the melting point (Tm = 1065°K to 1400°K. It is shown that in a liquid semiconductor the nuclear relaxation rate is strongly influenced by the microscopic mobility of the conduction electrons. As a result, in Ga2Te3 near Tm the magnetic component of the relaxation rate is enhanced by more than two orders of magnitude over the usual Korringa rate observed for metallic systems. The data are compared with theoretical proposals for the electronic structure of liquid semiconductors and are shown to support Motts prediction of a band of localized states near the Fermi level.

Solid immiscibility and liquid structure in the Ga2(SexTe1−x)3 alloy system

Abstract Differential thermal analysis and X-ray diffraction studies of the system Ga2(SexTe1−x)3 are reported and correlated with measurements of the electrical conductivity in the liquid state. The experiments reveal that Ga2Te3 and Ga2Se3 do not form a continuous series of pseudo-binary solid solutions but exhibit solid state immiscibility in the range 0.50 ≲ x ≲ 0.90. The composition dependence of the conductivity of the liquid alloys exhibits structure in the range of the solid state phase separation. Such “memory” in the liquid of a solid phase separation has not been reported previously and suggests the importance of concentration fluctuations in determining the electronic properties of the liquid alloys.

NMR studies of expanded liquid cesium

Abstract We report measurements of the 133Cs Knight shift and nuclear relaxation rate in expanded liquid cesium up to 1400°C at 120 bars. The data cover a density range of 1.03

Nuclear relaxation and Cu spin dynamics in YBa2Cu3O6+x

Abstract We report the 63 Cu NQR spectrum and nuclear relaxation rates for Cu(2) sites in 63 Cu-enriched YBa 2 Cu 3 O 6.64 (T c ∼ 60 K). Nuclear spin-lattice relaxation rates exhibit a strong reduction of (T 1 T) −1 between 100 K and T c . Cu(2) Knight shifts measured in a YBa 2 Cu 3 O 6.7 sample of natural isotopic abundance are reduced substantially relative to YBa 2 Cu 3 O 7.0 and show little change below T c .

NMR and NQR studies of high Tc superconductors

Abstract 63,65 Cu NMR and NQR studies of YBa 2 Cu 3 O 7−δ are described for compositions, δ, of 0.0, 0.3, and 0.9. Spin-lattice relaxation rates in YBa 2 Cu 3 O 7 reveal significantly different electron dynamics at the two Cu sites in both the normal and superconducting states. In YBa 2 Cu 3 O 6.7 , the relaxation rates of the sites giving a 31 MHz NQR line are dramatically lower (by a factor ∼ 1/1500) than in YBa 2 Cu 3 O 7 indicating the essential vanishing of mobile carrier density on these sites. In contrast, relaxation at the 22 MHz sites is reduced only about 30%. Interpretation of these and other NMR and NQR data is discussed in relation to the question of assignment of the NQR lines to the appropriate crystalline sites.

Low density electron dynamics in liquids

Abstract Weak screening of electron-electron and electron-ion interactions inevitably modifies electron dynamics at densities well below those of ordinary metals. These effects can be studied indirectly by probing the dynamic electron spin susceptibility with nuclear magnetic resonance (NMR) techniques and directly in real time using pulsed optical techniques. This paper discusses two examples -- a dilute metal-molten salt solution in which electrons are localized as F-center analogues, and expanded liquid cesium in which strong correlation effects lead to incipient antiferromagnetic character.

NMR and NQR studies of copper oxide superconductors (invited)

63,65Cu NMR and NQR measurements reveal remarkably different electronic character at the Cu(1) chain and Cu(2) plane sites in YBa2Cu3O7.0. In the normal state, spin‐lattice relaxation rates at both sites are strongly enhanced relative to estimated d‐band Korringa (T1) rates, but have sharply different temperature dependencies. Moreover, T1 anisotropy is greater than an enhanced Korringa model can account for. Below Tc, the temperature dependencies reveal significantly different spectra for quasiparticle excitation at the two sites with the strongest apparent energy gap occurring for the planar Cu(2) sites. At reduced oxygen content (YBa2Cu3O6.7) the relaxation at the Cu(1) sites is only slightly (∼30%) less than observed for YBa2Cu3O7.0 while the rates for the resonance at the frequency of the Cu(2) sites in YBa2Cu3O7.0 exhibit a large (∼2000 ×) decrease for a minority of planar Cu(2) sites indicates that these sites are electronically inactive. NMR shift measurements are used to analyze the susceptibilit...