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X. H. Chen

Publications and source records attributed to X. H. Chen.

At least 199 records · Page 11Linked to original sources
arXiv2007

Origin of superconductivity in nominally "undoped" T'-La$_{2-x}$Y$_{x}$CuO$_{4}$ films

We have systematically studied the transport properties of the La$_{2-x}$Y$_{x}$CuO$_{4}$(LYCO) films of T'-phase ($0.05\leq x \leq 0.30$). In this nominally "undoped" system, superconductivity was acquired in certain Y doping range ($0.10\leq x \leq 0.20$). Measurements of resistivity, Hall coefficients in normal states and resistive critical field ($H^ρ_{c2}$)in superconducting states of the T'-LYCO films show the similar behavior as the known Ce-doped n-type cuprate superconductors, indicating the intrinsic electron-doping nature. The charge carriers are induced by oxygen deficiency. Non-superconducting Y-doped Pr- or Nd-based T'-phase cuprate films were also investigated for comparison, suggesting the crucial role of the radii of A-site cations in the origin of superconductivity in the nominally "undoped" cuptates. Based on a reasonable scenario in the microscopic reduction process, we put forward a self-consistent interpretation of these experimental observations.

cond-mat.supr-con↗
arXiv2007

Oxygen isotope effect on the superconductivity and stripe phase in La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$

The oxygen isotope effect on the superconductivity, stripe phase and structure transition is systematically investigated in La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$ with static stripe phase. Substitution of $^{16}$O by $^{18}$O leads to a decrease in superconducting transition temperature T$_C$, while enhances the temperature of the structural transition from low-temperature-orthorhombic (LTO) phase to low-temperature-tetragonal (LTT) phase. Compared to the Nd free sample, a larger isotope effect on $T_C$ is observed in La$_{1.6-x}$Nd$_{0.4}$Sr$_{x}$CuO$_4$. These results indicate that the distortion of CuO$_2$ plane suppresses the superconductivity, giving a direct evidence for the competing of stripe phase and superconductivity because the distortion of CuO$_2$ plane enhances the stripe phase.

cond-mat.supr-con↗
arXiv2007

Magnetic field induced spin-flop transition in Na$_x$CoO$_2$ (0.5$<$x$<$0.55)

The isothermal magnetoresistance (MR) with magnetic field (H) parallel to and perpendicular to ab plane is systematically studied on the single crystal Na$_{0.52}$CoO$_2$ with charge ordering at $\sim 50$ K and an in-plane ferromagnetism below 25 K. The isothermal MR behavior with H $\parallel$ ab plane and H $\perp$ ab plane is quite different. When H $\parallel$ ab plane, the MR is always negative and the in-plane ferromagnetic behavior is enhanced. While the MR with H $\perp$ ab plane changes from negative to positive with decreasing temperature or increasing H, and the in-plane ferromagnetic behavior is suppressed. A striking feature is that the MR with H $\perp$ ab plane shows a hysteresis behavior below 25 K, which is absent for the case of H $\parallel$ ab plane. These results provide strong evidence for a spin-flop transition of small moments of Co$^{3.5-δ}$ sites induced by H $\perp$ ab plane, leading to a metamagnetic transition for small moments of Co$^{3.5-δ}$ sites. These complex magnetism suggests an unconventional superconductivity in Na$_x$CoO$_2$ system because the Na$_x$CoO$_2$ around x=0.5 is considered to be the parent compound of superconductivity.

cond-mat.str-el↗
arXiv2007

Novel dynamical effects and glassy response in strongly correlated electronic system

We find an unconventional nucleation of low temperature paramagnetic metal (PMM) phase with monoclinic structure from the matrix of high-temperature antiferromagnetic insulator (AFI) phase with tetragonal structure in strongly correlated electronic system $BaCo_{0.9}Ni_{0.1}S_{1.97}$. Such unconventional nucleation leads to a decease in resistivity by several orders with relaxation at a fixed temperature without external perturbation. The novel dynamical process could arise from the competition of strain fields, Coulomb interactions, magnetic correlations and disorders. Such competition may frustrate the nucleation, giving rise to a slow, nonexponential relaxation and "physical aging" behavior.

cond-mat.str-el↗
arXiv2007

Single-gap s-wave superconductivity in Cu$_x$TiSe$_2$

The in-plane thermal conductivity $κ$ of the layered superconductor Cu$_x$TiSe$_2$ was measured down to temperatures as low as $T_c$/40, at $x$ = 0.06 near where the CDW order vanishes. The absence of a residual linear term at $ T \to 0$ is strong evidence for conventional s-wave superconductivity in this system. This is further supported by the slow magnetic field dependence, also consistent with a single gap, of uniform magnitude across the Fermi surface. Comparison with the closely related material NbSe$_2$, where the superconducting gap is three times larger on the Nb 4$d$ band than on the Se 4$p$ band, suggests that in Cu$_{0.06}$TiSe$_2$ the Se 4$p$ band is below the Fermi level and the Ti 3$d$ band is alone responsible for the superconductivity.

cond-mat.supr-con↗
arXiv2006

Intrinsic electron-doping in nominal "non-doped" superconducting (La,Y)$_2$CuO$_4$ thin films grown by dc magnetron sputtering

The superconducting nominal "non-doped" $La_{1.85}Y_{0.15}CuO_4$ (LYCO) thin films are successfully prepared by dc magnetron-sputtering and in situ post-annealing in vacuum. The best $T_{C0}$ more than 13K is achieved in the optimal LYCO films with highly pure c-axis oriented T'-type structure. In the normal state, the quasi-quadratic temperature dependence of resistivity, the negative Hall coefficient and effect of oxygen content in the films are quite similar to the typical Ce-doped T'-214 cuprates, suggesting that T'-LYCO shows the electron-doping nature like known n-type cuprates, and is not a band superconductor as proposed previously. The charge carriers are considered to be induced by oxygen deficiency.

cond-mat.supr-con↗
arXiv2006

Anisotropic Magnetoresistance in Charge-Ordering $Na_{0.34}(H_3O)_{0.15}CoO_2$:Strong Spin-Charge Coupling and Spin Ordering

Angular-dependent in-plane magnetoresistance (AMR) for single crystal $Na_{0.34}(H_3O)_{0.15}CoO_2$ with charge ordering is studied systematically. The anisotropic magnetoresistance shows a twofold symmetry at high temperature with rotating H in the Co-O plane, while a sixfold symmetry below a certain temperature ($T_ρ$). At $T_ρ$, the symmetry of AMR changes from twofold to fourfold with rotating magnetic field (H) in the plane consisting of the current and c-axis. The variation of AMR symmetry with temperature arises from the subtle changes of the spin structure. These results give a direct evidence for the itinerant electrons directly coupled to the localized spins.

cond-mat.str-el↗
arXiv2006

Giant isotope effect and spin state transition induced by oxygen isotope exchange in ($Pr_{1-x}Sm_x)_{0.7}Ca_{0.3}CoO_3$

We systematically investigate effect of oxygen isotope in $(Pr_{1-x}Sm_x)_{0.7}Ca_{0.3}CoO_3$ which shows a crossover with x from ferromagnetic metal to the insulator with spin-state transition. A striking feature is that effect of oxygen isotope on the ferromagnetic transition is negligible in the metallic phase, while replacing $^{16}O$ with $^{18}O$ leads to a giant up-shift of the spin-state transition temperature ($T_s$) in the insulating phase, especially $T_s$ shifts from 36 to 54 K with isotope component $α_S=-4.7$ for the sample with x=0.175. A metal-insulator transition is induced by oxygen isotope exchange in the sample x=0.172 being close to the insulating phase. The contrasting behaviors observed in the two phases can be well explained by occurrence of static Jahn-Teller distortions in the insulating phase, while absence of them in the metallic phase.

cond-mat.str-el↗
arXiv2006

Magnetic and transport properties in Gd$_{1-x}$Sr$_x$CoO$_{3}$ ($x$= 0.10-0.70)

Magnetic and transport properties of polycrystalline Gd$_{1-x}$Sr$_x$CoO$_{3}$ ($x$= 0.10-0.70) are investigated systemically. Cluster-glass magnetism for $x \leq$ 0.45 and long-range ferromagnetic order in higher doping level are observed. Transport measurements indicate insulator-like behaviors for the samples with $x \leq$ 0.30, and an insulator-metal (IM) transition around $x$ = 0.35, and metallic behaviors for higher $x$ samples. In contrast to La$_{1-x}$Sr$_{x}$COO$_{3}$, the striking feature is that the system reenters insulator-like state for $x \geq$ 0.60, and the long-range ferromagnetic order and IM transition take place in samples with higher Sr content. It could be attributed to the enhancement of low-spin state stability for the trivalent Co ions due to the small radius of Gd$^{3+}$ ion.

cond-mat.str-el↗
arXiv2005

Charge ordering in charge-compensated $Na_{0.41}CoO_2$ by oxonium ions

Charge ordering behavior is observed in the crystal prepared through the immersion of the $Na_{0.41}CoO_2$ crystal in distilled water. Discovery of the charge ordering in the crystal with Na content less than 0.5 indicates that the immersion in water brings about the reduction of the $Na_{0.41}CoO_2$. The formal valence of Co changes from +3.59 estimated from the Na content to +3.5, the same as that in $Na_{0.5}CoO_2$. The charge compensation is confirmed to arise from the intercalation of the oxonium ions as occurred in the superconducting sodium cobalt oxide bilayer-hydrate.\cite{takada1} The charge ordering is the same as that observed in $Na_{0.5}CoO_2$. It suggests that the Co valence of +3.5 is necessary for the charge ordering.

cond-mat.mtrl-sci↗
arXiv2005

In-plane ferromagnetism in charge-ordering $Na_{0.55}CoO_2$

The magnetic and transport properties are systematically studied on the single crystal $Na_{0.55}CoO_2$ with charge ordering and divergency in resistivity below 50 K. A long-range ferromagnetic ordering is observed in susceptibility below 20 K with the magnetic field parallel to Co-O plane, while a negligible behavior is observed with the field perpendicular to the Co-O plane. It definitely gives a direct evidence for the existence of in-plane ferromagnetism below 20 K. The observed magnetoresistance (MR) of 30 % at the field of 6 T at low temperatures indicates an unexpectedly strong spin-charge coupling in triangle lattice systems.

cond-mat.str-el↗
arXiv2005

Influence of doping level on the Hall coefficient and on the thermoelectric power in $Nd_{2-x}Ce_xCuO_4$

Hall coefficient $R_H$ and thermoelectric power TEP are studied systematically in the single crystals $Nd_{2-x}Ce_xCuO_{4+δ}$ (NCCO) with different x from underdoped to overdoped regime. With increasing doping level, both $R_H$ and TEP decrease and change their sign from negative to positive. A striking feature is that the temperature dependence of the Hall angle follows a $T^4$ behavior in the underdoped regime, while a $T^2$ in the overdoped regime. These behaviors are closely related to the evolution of Fermi surface with doping level observed by ARPES.

cond-mat.str-el↗
arXiv2005

Oxygen Isotope Effect on the Spin State Transition in (Pr$_{0.7}$Sm$_{0.3}$)$_{0.7}$Ca$_{0.3}$CoO${_3}$

Oxygen isotope substitution is performed in the perovskite cobalt oxide (Pr$_{0.7}$Sm$_{0.3}$)$_{0.7}$Ca$_{0.3}$CoO${_3}$ which shows a sharp spin state transition from the intermediate spin (IS) state to the low spin (LS) state at a certain temperature. The transition temperature of the spin state up-shifts with the substitution of $^{16}O$ by $^{18}$O from the resistivity and magnetic susceptibility measurements. The up-shift value is 6.8 K and an oxygen isotope exponent ($α_S$) is about -0.8. The large oxygen isotope effect indicates strong electron-phonon coupling in this material. The substitution of $^{16}$O by $^{18}$O leads to a decrease in the frequency of phonon and an increase in the effective mass of electron ($m$$^\ast$), so that the bandwidth W is decreased and the energy difference between the different spin states is increased. This is the reason why the $T_s$ is shifted to high temperature with oxygen isotopic exchange.

cond-mat.str-el↗
arXiv2005

Doping evolution of chemical potential, spin-correlation gap, and charge dynamics in Nd$_{2-x}$Ce$_x$CuO$_4$

We report optical reflectivity study on Nd$_{2-x}$Ce$_x$CuO$_4$ over a broad doping range 0$\leq x\leq$0.20. The study reveals a systematic shift of chemical potential with doping. A pronounced peak structure specific to the electron-doped cuprate is observed in optical scattering rate based on the extended Drude model analysis. The energy scale of the peak could be very different from that of the peak in conductivity spectrum. We clarify that the "hot spots" gap probed in photoemission experiments correlates directly with the sharp suppression feature in scattering rate rather than conductivity spectra. The gap is associated with the short-range antiferromagnetic correlation, and disappears in a manner of fading away with increasing doping and temperature. In the heavily overdoped region, a dominant $ω^2$-dependence of the scattering rate is identified up to very high energy.

cond-mat.supr-con↗
arXiv2005

Effect of iron-doping on spin-state transition and ferromagnetism in Pr$_{0.5}$Ca$_{0.5}$CoO$_{3-δ}$ cobalt oxides

Resistivity and dc magnetization measurements were performed for the polycrystalline Pr$_{0.5}$Ca$_{0.5}$Co$_{1-x}$Fe$_{x}$O$_{3-δ}$ ($x$ = 0, 0.05, 0.10 and 0.15) samples. The as-fabricated samples exhibit ferromagnetic (FM) transition and the transition temperature increases with increasing the iron doping level. Annealing under high oxygen pressure induces a spin-state transition of Co ions in the \emph{iron-free} sample and such transition is reinforced with increasing the annealing oxygen pressure, while the annealing under high oxygen pressure suppresses the ferromagnetic ordering. Contrary to the case of the \emph{iron-free} sample, no spin-state transition is induced by the annealing under high oxygen pressure for the \emph{iron-doped} samples, and the ferromagnetic transition temperature is nearly independent of the annealing procedures. The enhancement of the spin-state transition in the \emph{iron-free} sample after annealing under high oxygen pressure should be attributed to the reduction of the cell volume. The suppression of the spin-state transition by the Fe doping is related to the enlargement of the cell volume and the stronger Fe-O bonds than Co-O bonds. The enhancement of the ferromagnetism by the iron-doping might arise from the ferromagnetic exchange interaction between Fe$^{3+}$ and Co$^{4+}$ through oxygen (Fe$^{3+}$-O-Co$^{4+}$).

cond-mat.str-el↗
arXiv2005

Ballistic magnon transport and phonon scattering in the antiferromagnet Nd$_2$CuO$_4$

The thermal conductivity of the antiferromagnet Nd$_2$CuO$_4$ was measured down to 50 mK. Using the spin-flop transition to switch on and off the acoustic Nd magnons, we can reliably separate the magnon and phonon contributions to heat transport. We find that magnons travel ballistically below 0.5 K, with a thermal conductivity growing as $T^3$, from which we extract their velocity. We show that the rate of scattering of acoustic magnons by phonons grows as $T^3$, and the scattering of phonons by magnons peaks at twice the average Nd magnon frequency.

cond-mat.supr-con↗
arXiv2004

Anomalous Magnetoresistance in Pb-doped Bi$_2$Sr$_2$Co$_2$O$_y$ Single Crystals

Magnetoresistance (MR) of the Bi$_{2-x}$Pb$_x$Sr$_2$Co$_2$O$_y$ ($x$=0, 0.3, 0.4) single crystals is investigated systematically. A nonmonotonic variation of the isothermal in-plane and out-of-plane MR with the field is observed. The out-of-plane MR is positive in high temperatures and increases with decreasing $T$, and exhibits a pronounced hump, and changes the sign from positive to negative at a centain temperature. These results strongly suggest that the observed MR consists of two contributions: one \emph{negative} and one \emph{positive} component. The isothermal MR in high magnetic fields follows a $H^2$ law. While the negative contribution comes from spin scattering of carriers by localized-magnetic-moments based on the Khosla-Fischer model.

cond-mat.str-el↗