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We present THz range optical conductivity data of a thin film of the near quantum critical heavy fermion compound CeFe$_2$Ge$_2$. Our complex conductivity measurements find a deviation from conventional Drude-like transport in a temperature range previously reported to exhibit unconventional behavior. We calculate the frequency dependent effective mass and scattering rate using an extended Drude model analysis. We find the inelastic scattering rate can be described by a temperature dependent power-law $\omega^{n(T)}$ where $n(T)$ approaches $\sim1.0 \pm 0.2$ at 1.5 K. This is compared to the $\rho \sim T^{1.5}$ behavior claimed in dc resistivity data and the $\rho \sim T^{2}$ expected from Fermi-liquid theory. In addition to a low temperature mass renormalization, we find an anomalous mass renormalization that persists to high temperature. We attribute this to a Hund's coupling in the Fe states in a manner similar to that recently proposed in the ferro-pnictides. CeFe$_2$Ge$_2$ appears to be a very interesting system where one may study the interplay between the usual $4f$ lattice Kondo effect and this Hund's enhanced Kondo effect in the $3d$ states., Comment: 6 pages, 4 figures, Supplemental Material

We present time-domain THz spectroscopy data of a thin film of the Kondo-lattice antiferromagnet CeCu$_2$Ge$_2$. The low frequency complex conductivity has been obtained down to temperatures below the onset of magnetic order. At low temperatures a narrow Drude-like peak forms, which is similar to ones found in other heavy fermion compounds that do not exhibit magnetic order. Using this data in conjunction with DC resistivity measurements, we obtain the frequency dependence of the scattering rate and effective mass through an extended Drude model analysis. The zero frequency limit of this analysis yields evidence for large mass renormalization even in the magnetic state, the scale of which agrees closely with that obtained from thermodynamic measurements., 4 pages, 3 figures

We describe a strategy for using resonant soft x-ray scattering (RSXS) to study the electronic structure of transition metal oxide quantum wires. Using electron beam lithography and ion milling, we have produced periodic, patterned arrays of colossal magnetoresistance (CMR) phase La(1-x)Sr(x)MnO(3) consisting of ~ 5000 wires, each of which is 80 nm in width. The scattered intensity exhibits a series of peaks that can be interpreted as Bragg reflections from the periodic structure or, equivalently, diffraction orders from the grating-like structure. RSXS measurements at the Mn L(2,3) edge, which has a large magnetic cross section, show clear evidence for a magnetic superstructure with a commensurate period of five wires, which we interpret as commensurately modulated antiferromagnetism. This superstructure, which is accompanied by non-trivial reorganization of the magnetization within each wire, likely results from classical dipole interactions among the wires. We introduce a simple, exactly soluble, analytic model of the scattering that captures, semi-quantitatively, the primary features in the RSXS data; this model will act as a foundation for forthcoming, detailed studies of the magnetic structure in these systems., Comment: 8 pages, 10 figures

Photoinduced magnetization dynamics is investigated in chemically ordered $(\mathrm{LaMnO}_3)_{2n}/(\mathrm{SrMnO}_3)_n$ superlattices using the time-resolved magneto-optic Kerr effect. A monotonic frequency-field dependence is observed for the $n=1$ superlattice, indicating a single spin population consistent with a homogeneous hole distribution. In contrast, for $n\geq2$ superlattices, a large precession frequency is observed at low fields indicating the presence of an exchange torque in the dynamic regime. We propose a model that ascribes the emergence of exchange torque to the coupling between two spin populations -- viscous and fast spins., 5 pages, 4 figures

Superlattices of (LaMnO3)2n/(SrMnO3)n (n=1 to 5), composed of the insulators LaMnO3 and SrMnO3, undergo a metal-insulator transition as a function of n, being metallic for n=3. Measurements of transport, magnetization and polarized neutron reflectivity reveal that the ferromagnetism is relatively uniform in the metallic state, and is strongly modulated in the insulating state, being high in LaMnO3 and suppressed in SrMnO3. The modulation is consistent with a Mott transition driven by the proximity between the (LaMnO3)/(SrMnO3) interfaces. Disorder localizes states at the Fermi level at the interfaces for n>=3. We suggest that this disorder is due to magnetic frustration at the interfaces., 19 pages, includes 5 figures

Polarized neutron reflectivity measurements of a ferromagnetic [(LaMnO$_3$)$_{11.8}$/(SrMnO$_3$)$_{4.4}$]$_6$ superlattice reveal a modulated magnetic structure with an enhanced magnetization at the interfaces where LaMnO$_3$ was deposited on SrMnO$_3$ (LMO/SMO). However, the opposite interfaces (SMO/LMO) are found to have a reduced ferromagnetic moment. The magnetic asymmetry arises from the difference in lateral structural roughness of the two interfaces observed via electron microscopy, with strong ferromagnetism present at the interfaces that are atomically smooth over tens of nanometers. This result demonstrates that atomic-scale roughness can destabilize interfacial phases in complex oxide heterostructures., Comment: 5 pages, 4 figures

Using polarized X-rays, we compare the electronic and magnetic properties of a La(2/3)Sr(1/3)MnO(3)(LSMO)/SrTiO(3)(STO) and a modified LSMO/LaMnO(3)(LMO)/STO interface. Using the technique of X-ray resonant magnetic scattering (XRMS), we can probe the interfaces of complicated layered structures and quantitatively model depth-dependent magnetic profiles as a function of distance from the interface. Comparisons of the average electronic and magnetic properties at the interface are made independently using X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD). The XAS and the XMCD demonstrate that the electronic and magnetic structure of the LMO layer at the modified interface is qualitatively equivalent to the underlying LSMO film. From the temperature dependence of the XMCD, it is found that the near surface magnetization for both interfaces falls off faster than the bulk. For all temperatures in the range of 50K - 300K, the magnetic profiles for both systems always show a ferromagnetic component at the interface with a significantly suppressed magnetization that evolves to the bulk value over a length scale of ~1.6 - 2.4 nm. The LSMO/LMO/STO interface shows a larger ferromagnetic (FM) moment than the LSMO/STO interface, however the difference is only substantial at low temperature., Comment: 4 pages, 4 figures

We have used a scanning tunneling microscope to demonstrate that a single CuO_2 plane can form a stable and atomically ordered layer at the surface of Bi_2Sr_2CaCu_2O_{8+delta}. In contrast to previous studies on high-T_c surfaces, the CuO_2-terminated surface exhibits a strongly suppressed tunneling conductance at low voltages. We consider a number of different explanations for this phenomena and propose that it may be caused by how the orbital symmetry of the CuO_2 plane's electronic states affects the tunneling process., to be published in PRL vol. 89, issue 8

We report on a systematic investigation of the critical fluctuations in the complex conductivity of epitaxially-grown Bi$_{2}$Sr$_{2}$CaCu$_{2}$O$_{8+\delta}$ films for ${T \lesssim T_c}$ using a two-coil inductive technique at zero applied field. We observe the static 3D-XY critical exponent in the superfluid density near $T_{c}$. Linear scaling analysis close to the critical temperature yields a dynamic critical exponent of $z = 2.0$ for small drive currents, but non-linear effects are seen to be important. At $T_{c}$, a non-linear scaling analysis also yields a 3D dynamic exponent of $z = 2.0$., Comment: 4 pages, 3 figures

The large flux-flow velocity behavior of the high temperature superconductor BSCCO in the mixed state is studied by measurement of the current-voltage characteristic as a function of temperature and magnetic field direction. We find strong evidence for vortex-antivortex pairs that are excited by quantum fluctuations and contribute to the dissipation when dissociated by a large current., 4 pages, 4 figures

We report measurements of the phase of the conductivity, $\phi_\sigma\equiv arg(\sigma)$, in the normal state of a $Bi_{2}Sr_{2}CaCu_{2}O_{8+\delta}$ (BSCCO) thin film from 0.2-1.0 THz. From $\phi_\sigma$ we obtain the time delay of the current response, $\tau_\sigma\equiv\phi_\sigma/\omega$. After discovering a systematic error in the data analysis, the extracted $\tau_\sigma$ has changed from that reported earlier. The revised data is shown in the sole figure below. Analysis and discussion of these data will follow., Comment: 1 Figure, revised data

We have measured the complex conductivity of a BSCCO(2212) thin film between 0.2 and 1.0 THz. We find the conductivity in the superconducting state to be well described as the sum of contributions from quasiparticles, the condensate, and order parameter fluctuations which draw 30% of the spectral weight from the condensate. An analysis based on this decomposition yields a quasiparticle scattering rate on the order of k_(B)*T/(hbar) for temperatures below Tc., Comment: 4 Pages, 3 Figures, Submitted to Phys. Rev. Lett

We report measurements of anamolously large dissipative conductivities in BiSrCaCuO(2212) at low temperatures. We have measured the complex conductivity of BSCCO thin films at 100-600 GHz as a function of doping from the underdoped to the overdoped state. At low temperatures there exists a residual dissipative conductivity which scales with the T=0 superfluid density as the doping is varied. This residual dissipative conductivity is larger than the possible contribution from a thermal population of quasiparticles at the d-wave gap nodes., Comment: Submitted to the Proceedings of the 22nd International Low Temperature Physics Conference. To be published in Physica B (http://www.elsevier.nl/locate/physb); 2 Pages with 2 Figures

We report angle-resolved photoemission spectroscopy measurements of the excitation gap in underdoped superconducting thin films of Bi_2Sr_2Ca_{1-x}Dy_xCu_2O_{8+d}. As Tc is reduced by a factor of 2 by underdoping, the superconducting state gap \Delta does not fall proportionally, but instead stays constant or increases slightly, in violation of the BCS mean-field theory result. The different doping dependences of \Delta and kT_c indicate that they represent different energy scales. The measurements also show that \Delta is highly anisotropic and consistent with a d_{x^2-y^2} order parameter, as in previous studies of samples with higher dopings. However, in these underdoped samples, the anisotropic gap persists well above T_c. The existence of a normal state gap is related to the failure of \Delta to scale with T_c in theoretical models that predict pairing without phase coherence above T_c., Comment: 10 pages, 4 postscript figures, revtex format