Kui Jin

Link between spin fluctuations and electron pairing in copper oxide superconductors.

Although it is generally accepted that superconductivity is unconventional in the high-transition-temperature copper oxides, the relative importance of phenomena such as spin and charge (stripe) order, superconductivity fluctuations, proximity to a Mott insulator, a pseudogap phase and quantum criticality are still a matter of debate. In electron-doped copper oxides, the absence of an anomalous pseudogap phase in the underdoped region of the phase diagram and weaker electron correlations suggest that Mott physics and other unidentified competing orders are less relevant and that antiferromagnetic spin fluctuations are the dominant feature. Here we report a study of magnetotransport in thin films of the electron-doped copper oxide La2 − xCexCuO4. We show that a scattering rate that is linearly dependent on temperature—a key feature of the anomalous normal state properties of the copper oxides—is correlated with the electron pairing. We also show that an envelope of such scattering surrounds the superconducting phase, surviving to zero temperature when superconductivity is suppressed by magnetic fields. Comparison with similar behaviour found in organic superconductors strongly suggests that the linear dependence on temperature of the resistivity in the electron-doped copper oxides is caused by spin-fluctuation scattering.

Anomalous magnetoresistance in the spinel superconductor LiTi2O4.

LiTi2O4 is a unique compound in that it is the only known spinel oxide superconductor. The lack of high quality single crystals has thus far prevented systematic investigations of its transport properties. Here we report a careful study of transport and tunnelling spectroscopy in epitaxial LiTi2O4 thin films. An unusual magnetoresistance is observed which changes from nearly isotropic negative to prominently anisotropic positive as the temperature is decreased. We present evidence that shows that the negative magnetoresistance likely stems from the suppression of local spin fluctuations or spin-orbit scattering centres. The positive magnetoresistance suggests the presence of an orbital-related state, also supported by the fact that the superconducting energy gap decreases as a quadratic function of magnetic field. These observations indicate that the spin-orbital fluctuations play an important role in LiTi2O4 in a manner similar to high-temperature superconductors.

Phase Diagram of (Li1–xFex)OHFeSe: A Bridge between Iron Selenide and Arsenide Superconductors

Previous experimental results have shown important differences between iron selenide and arsenide superconductors which seem to suggest that the high-temperature superconductivity in these two subgroups of iron-based families may arise from different electronic ground states. Here we report the complete phase diagram of a newly synthesized superconducting (SC) system, (Li1-xFex)OHFeSe, with a structure similar to that of FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ∼127 K. This is the first example to demonstrate such an SDW phase in an FeSe-based superconductor system. Transmission electron microscopy shows that a well-known √5×√5 iron vacancy ordered state, resulting in an AFM order at ∼500 K in AyFe2-xSe2 (A = metal ions) superconductor systems, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q = (1)/2(1,1,0), identical to that seen in the SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors.

(Li0.84Fe0.16)OHFe0.98Se superconductor: Ion-exchange synthesis of large single-crystal and highly two-dimensional electron properties

A large and high-quality single crystal (Li0.84Fe0.16)OHFe0.98Se, the optimal superconductor of a reported (Li1-xFex)OHFe1-ySe system, has been successfully synthesized via a hydrothermal ion-exchange technique. The superconducting transition temperature (T-c) of 42 K is determined by magnetic susceptibility and electric resistivity measurements, and the zero-temperature upper critical magnetic fields are evaluated as 79 and 313 T for the field along the c axis and the ab plane, respectively. The ratio of out-of-plane to in-plane electric resistivity rho(c)/rho(ab) is found to increase with decreasing temperature and to reach a high value of 2500 at 50 K, with an evident kink occurring at a characteristic temperature T* = 120 K. The negative in-plane Hall coefficient indicates that electron carriers dominate in the charge transport, and the hole contribution is significantly reduced as the temperature is lowered to approach T *. From T * down to T-c we observe the linear temperature dependencies of the in-plane electric resistivity and the magnetic susceptibility for the FeSe layers. Our findings thus reveal that the normal state of (Li0.84Fe0.16)OHFe0.98Se becomes highly two dimensional and anomalous prior to the superconducting transition, providing an insight into the mechanism of high-T-c superconductivity.

The phase diagram of electron-doped La 2− x Ce x CuO 4− δ

Superconductivity is a striking example of a quantum phenomenon in which electrons move coherently over macroscopic distances without scattering. The high-temperature superconducting oxides (cuprates) are the most studied class of superconductors, composed of two-dimensional CuO2 planes separated by other layers that control the electron concentration in the planes. A key unresolved issue in cuprates is the relationship between superconductivity and magnetism. Here we report a sharp phase boundary of static three-dimensional magnetic order in the electron-doped superconductor La(2-x)Ce(x)CuO(4-δ), where small changes in doping or depth from the surface switch the material from superconducting to magnetic. Using low-energy spin-polarized muons, we find that static magnetism disappears close to where superconductivity begins and well below the doping level at which dramatic changes in the transport properties are reported. These results indicate a higher degree of symmetry between the electron and hole-doped cuprates than previously thought.

Quantum critical scaling at the edge of Fermi liquid stability in a cuprate superconductor

In the high-temperature cuprate superconductors, the pervasiveness of anomalous electronic transport properties suggests that violation of conventional Fermi liquid behavior is closely tied to superconductivity. In other classes of unconventional superconductors, atypical transport is well correlated with proximity to a quantum critical point, but the relative importance of quantum criticality in the cuprates remains uncertain. Here, we identify quantum critical scaling in the electron-doped cuprate material La2-xCexCuO4 with a line of quantum critical points that surrounds the superconducting phase as a function of magnetic field and charge doping. This zero-temperature phase boundary, which delineates a metallic Fermi liquid regime from an extended non-Fermi liquid ground state, closely follows the upper critical field of the overdoped superconducting phase and gives rise to an expanse of distinct non-Fermi liquid behavior at finite temperatures. Together with signatures of two distinct flavors of quantum fluctuations, these facts suggest that quantum criticality plays a significant role in shaping the anomalous properties of the cuprate phase diagram.

Evidence for antiferromagnetic order inLa2−xCexCuO4from angular magnetoresistance measurements

We investigated the in-plane angular magnetoresistivity (AMR) of

Anomalous enhancement of the superconducting transition temperature of electron-doped La 2 − x Ce x CuO 4 and Pr 2 − x Ce x CuO 4 cuprate heterostructures

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Electric-field control of ferromagnetism through oxygen ion gating

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Evolution of electronic states in n-type copper oxide superconductor via electric double layer gating

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