arXiv subjects

X. Wei

Publications and source records attributed to X. Wei.

29 records · Page 2Linked to original sources

arXiv2006

Observation of 300 K High Energy MagnetoDielectric Response in the Bilayer Manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We observed a large HEMD effect in the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$, a direct consequence of field driven spin-glass insulator to ferromagnetic metal transition. The remnants of the transition can be used to achieve dielectric contrast at room temperature. This discovery suggests that electronic mechanisms such as the metal-insulator transition, charge ordering, and orbital ordering can be exploited to give substantial dielectric contrast in other materials.

cond-mat.mtrl-sci↗

arXiv2006

Optical Aharonov-Bohm effect in stacked type-II quantum dots

Excitons in vertically stacked type-II quantum dots experience the topological magnetic phase and demonstrate the Aharonov-Bohm oscillations in the emission intensity. Photoluminescence of vertically stacked ZnTe/ZnSe quantum dots is measured in magnetic fields up to 31 T. The Aharonov-Bohm oscillations are found in the magnetic-field dependence of emission intensity. The positions of the peaks of the emission intensity are in a good agreement with numerical simulations of excitons in stacked quantum dots.

cond-mat.mes-hall↗

arXiv2006

Magneto-optical investigation of the field-induced spin-glass insulator to ferromagnetic metallic transition of the bilayer manganite (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$

We measured the magneto-optical response of (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$ in order to investigate the microscopic aspects of the magnetic field driven spin-glass insulator to ferromagnetic metal transition. Application of a magnetic field recovers the ferromagnetic state with an overall redshift of the electronic structure, growth of the bound carrier localization associated with ferromagnetic domains, development of a pseudogap, and softening of the Mn-O stretching and bending modes that indicate a structural change. We discuss field- and temperature-induced trends within the framework of the Tomioka-Tokura global electronic phase diagram picture and suggest that controlled disorder near a phase boundary can be used to tune the magnetodielectric response. Remnants of the spin-glass insulator to ferromagnetic metallic transition can also drive 300 K color changes in (La$_{0.4}$Pr$_{0.6}$)$_{1.2}$Sr$_{1.8}$Mn$_2$O$_7$.

cond-mat.str-el↗

arXiv2006

Cooperative Recombination of a Quantized High-Density Electron-Hole Plasma

We investigate photoluminescence from a high-density electron-hole plasma in semiconductor quantum wells created via intense femtosecond excitation in a strong perpendicular magnetic field, a fully-quantized and tunable system. At a critical magnetic field strength and excitation fluence, we observe a clear transition in the band-edge photoluminescence from omnidirectional output to a randomly directed but highly collimated beam. In addition, changes in the linewidth, carrier density, and magnetic field scaling of the PL spectral features correlate precisely with the onset of random directionality, indicative of cooperative recombination from a high density population of free carriers in a semiconductor environment.

cond-mat.other↗

arXiv2006

Magneto-Dielectric Effect in the S = 1/2 Quasi-Two Dimensional Antiferromagnet K2V3O8

We report the optical and magneto-optical properties of K2V3O8, an S=1/2 quasi-two-dimensional Heisenberg antiferromagnet. Local spin density approximation electronic structure calculations are used to assign the observed excitations and analyze the field dependent features. Two large magneto-optical effects, centered at ~1.19 and 2.5 eV, are attributed to field-induced changes in the V 4+ d to d on-site excitations due to modification of the local crystal field environment of the VO5 square pyramids with applied magnetic field. Taken together, the evidence for a soft lattice, the presence of vibrational fine structure on the sharp 1.19 eV magneto-optical feature,and the fact that these optical excitations are due to transitions from a nearly pure spin polarized V d state to hybridized states involving both V and O, suggest that the magneto-dielectric effect in K2V3O8 is driven by strong lattice coupling.

cond-mat.str-el↗

arXiv2005

Excitons in Carbon Nanotubes with Broken Time-Reversal Symmetry

Near-infrared magneto-optical spectroscopy of single-walled carbon nanotubes reveals two absorption peaks with an equal strength at high magnetic fields ($>$ 55 T). We show that the peak separation is determined by the Aharonov-Bohm phase due to the tube-threading magnetic flux, which breaks the time-reversal symmetry and lifts the valley degeneracy. This field-induced symmetry breaking thus overcomes the Coulomb-induced intervalley mixing which is predicted to make the lowest exciton state optically inactive (or ``dark'').

cond-mat.mes-hall↗

arXiv2004

Dark-bright magneto-exciton mixing induced by Coulomb interaction in strained quantum wells

Coupled magneto-exciton states between allowed (`bright') and forbidden (`dark') transitions are found in absorption spectra of strained In$_{0.2}$Ga$_{0.8}$As/GaAs quantum wells with increasing magnetic field up to 30 T. We found large (~ 10 meV) energy splittings in the mixed states. The observed anticrossing behavior is independent of polarization, and sensitive only to the parity of the quantum confined states. Detailed experimental and theoretical investigations indicate that the excitonic Coulomb interaction rather than valence band complexity is responsible for the splittings. In addition, we determine the spin composition of the mixed states.

cond-mat.other↗

arXiv2002

Photo-response of the conductivity in functionalized pentacene compounds

We report the first investigation of the photo-response of the conductivity of a new class of organic semiconductors based on functionalized pentacene. These materials form high quality single crystals that exhibit a thermally activated resistivity. Unlike pure pentacene, the functionalized derivatives are readily soluble in acetone, and can be evaporated or spin-cast as thin films for potential device applications. The electrical conductivity of the single crystal materials is noticeably sensitive to ambient light changes. The purpose, therefore, of the present study, is to determine the nature of the photo-response in terms of carrier activation vs. heating effects, and also to measure the dependence of the photo-response on photon energy. We describe a new method, involving the temperature dependent photo-response, which allows an unambiguous identification of the signature of heating effects in materials with a thermally activated conductivity. We find strong evidence that the photo-response in the materials investigated is predominantly a highly localized heating mechanism. Wavelength dependent studies of the photo-response reveal resonant features and cut-offs that indicate the photon energy absorption is related to the electronic structure of the material.

cond-mat.mtrl-sci↗

arXiv2002

Optical Evidence of Multiphase Coexistence in Single Crystalline (La,Pr,Ca)MnO3

We investigated temperature (T)- and magnetic field-dependent optical conductivity spectra (\s\w) of a La_5/8-yPr_yCa_3/8MnO_3 (y~0.35) single crystal, showing intriguing phase coexistence at low T. At T_C < T < T_CO, a dominant charge-ordered phase produces a large optical gap energy of ~0.4 eV. At T < T_C, at least two absorption bands newly emerge below 0.4 eV. Analyses of (\s\w) indicate that the new bands should be attributed to a ferromagnetic metallic and a charge-disordered phase that coexist with the charge-ordered phase. This optical study clearly shows that La_5/8-yPrCa_3/8MnO_3 (y~0.35) is composed of multiphases that might have different lattice strains.

cond-mat.str-el↗

arXiv2000

Magnetic Field Dependent Optical Studies of a Layered Antiferromagnet Pr$%_{1/2}$Sr$_{1/2}$MnO$_3$

Magnetic field ($H$)-dependent optical conductivity spectra $σ(ω) $ and dielectric constant spectra $ε(ω)$ of a layered antiferromagnet ($A$-type) Pr$_{1/2}$Sr$_{1/2}$MnO$_3$ were presented. At 0.0 T in 4.2 K, the $σ(ω)$ showed a small Drude-like peak in the far-infrared region and a broad absorption peak in the mid-infrared region. With increasing $H$, large spectral weights were transferred from high to low energy regions with increase of the Drude-like peak. We found that a polaron picture could explain most of the $H$-dependent spectral changes. By comparing with the $H$-dependent spectral weight changes and $ε(ω)$ of Nd$_{1/2}$Sr$_{1/2}$MnO$_3$, which has the $CE$-type spin ordering, we showed that a dimensional crossover from a 2-dimensional to a 3-dimensional metal occurs in Pr$_{1/2}$Sr$_{1/2}$MnO$_3$.

cond-mat.str-el↗

arXiv1999

Melting of Charge/Orbital Ordered States in Nd$_{1/2}$Sr$_{1/2}$MnO$_3$: Temperature and Magnetic Field Dependent Optical Studies

We investigated the temperature ($T=$ 15 $\sim $ 290 K) and the magnetic field ($H=$ 0 $\sim $ 17 T) dependent optical conductivity spectra of a charge/orbital ordered manganite, Nd$_{1/2}$Sr$_{1/2}$MnO$_3$. With variation of $T$ and $H$, large spectral weight changes were observed up to 4.0 eV. These spectral weight changes could be explained using the polaron picture. Interestingly, our results suggested that some local ordered state might remain above the charge ordering temperature, and that the charge/orbital melted state at a high magnetic field (i.e. at $H=$ 17 T and $% T=$ 4.2 K) should be a three dimensional ferromagnetic metal. We also investigated the first order phase transition from the charge/orbital ordered state to ferromagnetic metallic state using the $T$- and $H$% -dependent dielectric constants $ε_1$. In the charge/orbital ordered insulating state, $ε_1$ was positive and $dε_1/dω\approx 0$. With increasing $T$ and $H$, $ε_1$ was increased up to the insulator-metal phase boundaries. And then, $ε_1$ abruptly changed into negative and $dε_1/dω>0$, which was consistent with typical responses of a metal. Through the analysis of $% ε_1$ using an effective medium approximation, we found that the melting of charge/orbital ordered states should occur through the percolation of ferromagnetic metal domains.

cond-mat.str-el↗