W. Yu

Niobium doped TiO2: Intrinsic transparent metallic anatase versus highly resistive rutile phase

We report on the structural, electrical, and optical properties of 5% niobium doped TiO2 thin films grown on various substrates by pulsed laser deposition. The epitaxial anatase Nb:TiO2 film on LaAlO3 is shown to be an intrinsic transparent metal and its metallic property arises from Nb substitution into Ti site as evidenced by the Rutherford backscattering channeling result. In contrast, the rutile Nb:TiO2 thin films show insulating behaviors with 2–3 orders higher room temperature electrical resistivity and ∼30 times lower mobility. A blueshift in the optical absorption edge is observed in both phases, though of differing magnitude.

Growth parameter-property phase diagram for pulsed laser deposited transparent oxide conductor anatase NB: TiO2

The authors performed a systematic study of the structural and electrical properties of Nb:TiO2 thin films by varying the substrate temperature (TS) and oxygen partial pressure (PO2). Niobium is found to incorporate easily and substitutionally into titanium lattice site as indicated by its low activation energy. By increasing TS, the carrier concentration (n) increases in the same way that niobium substitution fraction (s) increases, and the mobility increases as the structural quality is improved. With increasing PO2, n decreases dramatically though s does not change considerably. This may indicate that a large number of p-type native defects form, which “kill” the electrons produced by the Nb donors.

Transport evidence of a magnetic quantum phase transition in electron-doped high-temperature superconductors

In strongly correlated electron systems, quantum fluctuations close to a quantum critical point lead to many exotic properties of matter [1, 2]. One example is the unconventional superconductivity (SC) and the non-Fermi liquid normal state properties, which appear close to a quantum phase transition (QPT). Such phenomena are found in many heavy Fermion [3, 4] and organic [5] superconductors. However, attempts to apply quantum phase transition ideas to describe the properties of the high-TC cuprate superconductors are controversial. In the hole-doped (p-type) cuprates, whether a superconducting fluctuation scenario [6] or a competing order scenario [7, 8, 9] is an appropriate description of the pseudogap phenomena is still highly debated. In the electron-doped (n-type) cuprates, the existence of an antiferromagnetic to paramagnetic QPT is more plausible, but there is significant disagreement over if, and where, it occurs and its role in the physical properties [10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. Several transport studies [10, 11, 12] on electron-doped Pr2−xCexCuO4 (PCCO) thin films suggest an antiferromagnetic QPT inside the superconducting dome at x≈0.16, which is slightly above the optimal doping. Angle resolved photoemission spectroscopy (ARPES) measurements on Nd2−xCexCuO4 (NCCO) [18] and optical measurements on NCCO and PCCO [19, 20] revealed a normal-state gap which still exists at the optimal doping x=0.15. However, a recent inelastic neutron scattering (INS) measurement on NCCO single crystals suggests that long-range order antiferromagnetism (LROAF) does not coexist with SC and an antiferromagnetic QPT occurs just before the superconducting dome at x≈0.13 [21]. A recent ARPES work on superconducting Sm2−xCexCuO4 (SCCO) single crystals suggests a short-range order antiferromagnetism (SROAF) instead at x=0.14 [22]. In principle, neutron scattering (NS) and μSR could differentiate these different interpretations. But, so far, measurements from different groups are in significant disagreement [13, 14, 15, 16, 21, 23]. The major experimental difficulty is likely caused by a high-temperature oxygen annealing, which is necessary to achieve superconductivity on the n-type cuprates, but 0.00 0.05 0.10 0.15 0.20 0 100 200 300

Magnetic-field dependence of the low-temperature specific heat of the electron-doped superconductor Pr 1.85 Ce 0.15 Cu O 4

We remeasured the magnetic field dependence of the low-temperature specific heat of the electron-doped superconductor Pr1.85Ce0.15CuO4 (T_C=22\pm 2K) under a different measurement procedure. Under field-cooling, the electronic specific heat follows C_{el}(H, T)=\gamma (H)T from 4.5K down to 1.8K. In the field range H_{C1}<H<0.5 H_{C2}, the Sommerfeld coefficient \gamma (H) is well fit by a power-law \gamma (H)\sim H^{1/2}. This result suggests that the pairing symmetry is d-wave-like at all temperatures below 4.5K. Our new measurement shows no evidence for the linear field dependence of \gamma (H) found previously at T=2K.

c -axis longitudinal magnetoresistance of the electron-doped superconductor Pr 1.85 Ce 0.15 Cu O 4

We report c-axis resistivity and longitudinal magnetoresistance measurements of superconducting Pr1.85Ce0.15CuO4 single crystals. In the temperature range 13K<T<32K, a negative magnetoresistance is observed at fields just above Hc2. Our studies suggest that this negative magnetoresistance is caused by superconducting fluctuations. At lower temperatures (T<13K), a different magnetoresistance behavior and a resistivity upturn are observed, whose origin is still unknown.