arXiv subjects

H. D. Drew

Publications and source records attributed to H. D. Drew.

At least 37 records · Page 2Linked to original sources

arXiv2006

Infrared Hall Effect in the electron-doped high Tc cuprate Pr(2-x)Ce(x)CuO(4)

The electron-doped cuprate Pr(2-x)Ce(x)CuO(4) is investigated using infrared magneto-optical measurements. The optical Hall conductivity sigma_{xy} shows a strong doping, frequency and temperature dependence consistent with the presence of a temperature and doping-dependent coherent backscattering amplitude which doubles the electronic unit cell. The data suggest that the coherent backscattering vanishes at a quantum critical point inside the superconducting dome and is associated with the commensurate antiferromagnetic order observed by other workers. Using a spectral weight analysis we have further investigated the Fermi-liquid like behavior of the overdoped sample. The observed Hall-conductance spectral weight is about 10 times less than that predicted by band theory, raising the fundamental question concerning the effect of Mott and antiferromagnetic correlations on the Hall conductance of strongly correlated materials.

cond-mat.supr-con↗

arXiv2006

Infrared Hall conductivity of Na$_{0.7}$CoO$_2$

We report infrared Hall conductivity $σ_{xy}(ω)$ of Na$_{0.7}$CoO$_2$ thin films determined from Faraday rotation angle $θ_{F}$ measurements. $σ_{xy}(ω)$ exhibits two types of hole conduction, Drude and incoherent carriers. The coherent Drude carrier shows a large renormalized mass and Fermi liquid-like behavior of Hall scattering rate, $γ_{H} \sim aT^{2}$. The spectral weight is suppressed and disappears at T = 120K. The incoherent carrier response is centered at mid-IR frequency and shifts to lower energy with increasing T. Infrared Hall constant is positive and almost independent of temperature in sharp contrast with the dc-Hall constant.

cond-mat.str-el↗

arXiv2006

The infrared Hall conductivity in optimally-doped BSCCO

he frequency dependence in the mid IR from 900 to 1100 cm$^{-1}$ and far IR at 84 cm$^{-1}$ and temperature dependence from 35 to 330 K of the normal state Hall transport is reported in single crystal, optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. The results show a nearly Drude behavior in the Hall conductivity $σ_{xy}$, which stands in contrast to the more complex extended Drude behavior for the longitudinal conductivity $σ_{xx}$. The mid IR Hall scattering rate increases linearly with temperature and has a small, positive, projected intercept of 200 cm$^{-1}$ at T=0. In contrast, the longitudinal scattering rate is much larger and exhibits very little temperature dependence. The spectral weight of $σ_{xy}$ is suppressed to 0.09 times the band value, whereas the spectral weight of $σ_{xx}$ is 0.33 times the band value. These disparate behaviors are consistent with calculations based on the fluctuation-exchange interaction when current vertex corrections are included.

cond-mat.supr-con↗

arXiv2006

Electromagnons in multiferroic YMn2O5 and TbMn2O5

Based on temperature dependent far infrared transmission spectra of YMn2O5 and TbMn2O5 single crystals, we report the observation of electric dipole-active magnetic excitations, or electromagnons, in these multiferroics. Electromagnons are found to be directly responsible for the step-like anomaly of the static dielectric constant at the commensurate--incommensurate magnetic transition and are the origin of the colossal magneto-dielectric effect reported in these multiferroics.

cond-mat.str-el↗

arXiv2006

IR Phonon signatures of multiferroicity in TbMn$_{2}$O$_{5}$

The infrared (IR) active phonons in multiferroic TbMn$_2$O$_5$ are studied as a function of temperature. Most of the symmetry allowed IR modes polarized along the \textit{a} and \textit{b} crystal axes are observed and the behavior of several \textit{b} polarized phonons is correlated with the magnetic and ferroelectric transitions. A high frequency \textit{b} polarized phonon only Raman allowed in the paraelectric phase becomes IR active at the ferroelectric transition. The IR strength of this mode is proportional to the square of the ferroelectric order parameter and gives a sensitive measure of the symmetry lowering lattice distortions in the ferroelectric phase.

cond-mat.mtrl-sci↗

arXiv2006

The mid-infrared Hall effect in optimally-doped Bi_2 Sr_2 CaCu_2 O_{8+δ}

Heterodyne polarometry is used to measure the frequency dependence in the mid IR from 900 to 1100 cm$^-1$ and temperature dependence from 35 to 330 K of the normal state Hall transport in single crystal, optimally doped Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$. The results show a simple Drude behavior in the Hall conductivity $σ_{xy}$ which stands in contrast to the more complex, extended Drude behavior for the longitudinal conductivity $σ_{xx}$. The mid IR Hall scattering rate $γ_{xy}$ increases linearly with temperature and has a small, positive, projected intercept at T=0. The longitudinal scatter rate, in contrast, is much larger and exhibits very little temperature dependnece. The Hall frequency indicates a carrier mass which is 6.7 times the band mass and which decreases slighly with increasing frequency. These disparate behaviors are consistent with calculations based on the fluctuation-exchange interaction using current vertex corrections.

cond-mat.str-el↗

arXiv2005

Infrared Hall effect in underdoped and optimally doped LSCO

We report a study of magneto-optical properties in the mid-infrared region of a series of La$_{2-x}$Sr$_x$CuO$_4$ samples with hole doping level ranging from severely underdoped ($x=0.03$)to optimally doped ($x=0.15$). The Faraday rotation and circular dichroism are measured in a magnetic field of 8 Tesla and in a temperature range between 30K and 300K. The doping and temperature dependence of infrared Hall angle is found to be understood within a simple Drude model. A significant increase of Hall frequency is observed when the hole doping level is reduced from optimal doping, which is compared with models of the pseudogap.

cond-mat.str-el↗

arXiv2004

Probing spin correlations with phonons in the strongly frustrated magnet ZnCr2O4

The spin-lattice coupling plays an important role in strongly frustrated magnets. In ZnCr$_2$O$_4$, an excellent realization of the Heisenberg antiferromagnet on the ``pyrochlore'' network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at $T_c = 12.5$ K. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of an optical phonon triplet in ZnCr$_2$O$_4$ tracks the nearest-neighbor spin correlations above $T_c$. The splitting of the phonon triplet below $T_c$ provides a way to measure of the spin-Peierls order parameter.

cond-mat.str-el↗

arXiv2004

Infrared study of giant dielectric constant in Li and Ti doped NiO

We have measured optical reflectivity of Li and Ti doped NiO (LTNO) in the infrared range at various temperatures. A Drude-like absorption is found at low energy, $ω<$ 100 cm$^{-1}$ and its spectral weight increases substantially as temperature decreases. This observation and DC-resistivity result show that LTNO has a conductive grain and resistive boundary. Such composite structure provides evidence of the Maxwell-Wagner (MW) mechanism as the origin of the high dielectric constant $ε_{o}$. We propose a three-phase granular structure and show that this extended MW model explains the observed frequency and temperature dependence of the dielectric constant as well as the giant value of $ε_{o}$.

cond-mat.str-el↗

arXiv2003

Magneto-optical evidence for a gapped Fermi surface in underdoped YBa2Cu3O6+x

The infrared (900-1100 cm-1) Faraday rotation and circular dichroism are measured in the normal state of underdoped High Tc superconductors and used to study the magneto-transport. YBa2Cu3O6+x thin films are investigated in the temperature range 10-300 K in magnetic fields up to 8 Tesla and as a function of oxygen concentration. A dramatic increase of the Hall frequency is observed for underdoped samples which is not consistent with the approach to a Mott transition but is consistent with a partial gapping of the Fermi surface as predicted in charge density wave models.

cond-mat.supr-con↗

arXiv2003

Optical band edge shift of anatase cobalt-doped titanium dioxide

We report on the optical properties of magnetic cobalt-doped anatase phase titanium dioxide Ti_{1-x}Co_{x}O_{2-d} films for low doping concentrations, 0 <= x <= 0.02, in the spectral range 0.2 to 5 eV. For well oxygenated films (d << 1) the optical conductivity is characterized by an absence of optical absorption below an onset of interband transitions at 3.6 eV and a blue shift of the optical band edge with increasing Co concentration. The absence of below band gap absorption is inconsistent with theoretical models which contain midgap magnetic impurity bands and suggests that strong on-site Coulomb interactions shift the O-band to Co-level optical transitions to energies above the gap.

cond-mat.str-el↗

arXiv2003

Exchange interaction effects on the optical properties of LuMnO3

We have measured the optical conductivity of single crystal LuMnO3 from 10 to 45000 cm-1 at temperatures between 4 and 300 K. A symmetry allowed on-site Mn $d$-$d$ transition near 1.7 eV is observed to blue shift (~0.1 eV) in the antiferromagnetic state due to Mn-Mn superexchange interactions. Similar anomalies are observed in the temperature dependence of the TO phonon frequencies which arise from spin-phonon interaction. We find that the known anomaly in temperature dependence of the quasi-static dielectric constant epsilon_0 below the T_N ~90 K is overwhelmingly dominated by the phonon contributions.

cond-mat.str-el↗

arXiv2003

Temperature dependent optical studies of Ti$_{1-x}$Co$_x$O$_2$

We present the results of Raman and photoluminescence (PL) studies on epitaxial anatase phase Ti$_{1-x}$Co$_x$O$_2$ films for $x$ = 0-0.07, grown by pulsed laser deposition. The low doped system ($x$=0.01 and 0.02) shows a Curie temperature of ~700 K in the as-grown state. The Raman spectra from the doped and undoped films confirm their anatase phase. The photoluminescence spectrum is characterized by a broad emission from self-trapped excitons (STE) at 2.3 eV at temperatures below 120 K. This peak is characteristic of the anatase-phase TiO$_2$ and shows a small blueshift with increasing doping concentration. In addition to the emission from STE, the Co-doped samples show two emission lines at 2.77 eV and 2.94 eV that are absent in the undoped film indicative of a spin-flip energy.

cond-mat.mtrl-sci↗

arXiv2003

Quasiparticles in high temperature superconductors: consistency of angle resolved photoemission and optical conductivity

The consistency of angle-resolved photoemission and optical conductivity experiments on high temperature superconductors is examined. In the limit (apparently consistent with angle-resolved photoemission data) of an electron self energy with a weak momentum dependence and a strong frequency dependence formulae are derived which directly related quantities measured in the two experiments. Application of the formuale to optimally and overdoped $Bi_{2}Ca_{2}SrCu_{2}O_{8+δ}$ shows that the total self energy inferred from photoemission measurements cannot be interpreted as a transport scattering rate (in agreement with work of Varma and Abrahams), but that the inelastic part may be so interpreted, if Landau parameter effects are non-negligible.

cond-mat.str-el↗

arXiv2003

Disentangling the exchange coupling of entangled donors in the Si quantum computer architecture

We develop a theory for micro-Raman scattering by single and coupled two-donor states in silicon. We find the Raman spectra to have significant dependence on the donor exchange splitting and the relative spatial positions of the two donor sites. In particular, we establish a strong correlation between the temperature dependence of the Raman peak intensity and the interdonor exchange coupling. Micro-Raman scattering can therefore potentially become a powerful tool to measure interqubit coupling in the development of a Si quantum computer architecture.

cond-mat.mes-hall↗

arXiv2003

High Temperature Ferromagnetism with Giant Magnetic Moment in Transparent Co-doped SnO2-d

Occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn1-xCoxO2-d (x<0.3). Interestingly, films of Sn0.95Co0.05O2-d grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of about 7 Bohr-Magneton/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

cond-mat↗

arXiv2002

Ferromagnetism in laser deposited anatase Ti$_{1-x}$Co$_{x}$O$_{2-$δ$}$ films

Pulsed laser deposited films of Co doped anatase TiO2 are examined for Co substitutionality, ferromagnetism, transport, magnetotransport and optical properties. Our results show limited solubility (up to ~ 2 %) of Co in the as-grown films and formation of Co clusters thereafter. For Ti0.93Co0.07O2-d sample, which exhibits a Curie temperature (Tc) over 1180 K, we find the presence of 20-50 nm Co clusters as well as a small concentration of Co incorporated into the remaining matrix. After being subjected to the high temperature anneal during the first magnetization measurement, the very same sample shows a Tc ~ 650 K and almost full matrix incorporation of Co. This Tc is close to that of as-grown Ti0.99Co0.01O2-d sample (~ 700 K). The transport, magnetotransport and optical studies also reveal interesting effects of the matrix incorporation of Co. These results are indicative of an intrinsic Ti1-xCoxO2-d diluted magnetic semiconductor with Tc of about 650-700 K.

cond-mat↗

arXiv2002

The mid-infrared AC Hall effect in optimally-doped BSCCO

A novel heterodyne polarimetry system determines the frequency dependence from 900 to 1100 cm^-1 and temperature dependence from 35 to 330 K of the Hall transport in single crystal, optimally doped BSCCO. The results show a significant disconnect from the behavior of the Hall angle in the existing data for YBCO in the far-infrared, which indicate a negative value for the real part of the Hall angle above 250 cm^-1, whereas that of the current work at 1000 cm^-1 is positive. The current work when analyzed using an extended Drude formalism results in a Hall mass comparable to the ARPES Fermi mass and a scattering rate comparable to the low frequency conductivity and Hall effect scattering rates, which, however, are only 1/4 of the ARPES values.

cond-mat.supr-con↗