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In this Letter, we present resonance properties in terahertz metamaterials consisting of a split-ring resonator array made from high-temperature superconducting films. By varying the temperature, we observe efficient metamaterial resonance switching and frequency tuning. The results are well reproduced by numerical simulations of metamaterial resonance using the experimentally measured complex conductivity of the superconducting film. We develop a theoretical model that explains the tuning features, which takes into account the resistive resonance damping and additional split-ring inductance contributed from both the real and imaginary parts of the temperature-dependent complex conductivity. The theoretical model further predicts more efficient resonance tuning in metamaterials consisting of a thinner superconducting split-ring resonator array, which are also verified in subsequent experiments.

We explore the properties of the bipolaron in a 1D Holstein-Hubbard model with dynamical quantum phonons. Using a recently developed variational method combined with analytical strong coupling calculations, we compute correlation functions, effective mass, bipolaron isotope effect, and the phase diagram. The two site bipolaron has a significantly reduced mass and isotope effect compared to the on-site bipolaron, and is bound in the strong coupling regime up to twice the Hubbard U naively expected. The model can be described in this regime as an effective t-J-V model with nearest neighbor repulsion. These are the most accurate bipolaron calculations to date.

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A decrease in the rotational period observed in torsional oscillator measurements was recently taken as a possible indication of a putative supersolid state of helium. We reexamine this interpretation and note that the decrease in the rotation period is also consistent with a solidification of a small liquidlike component into a lowtemperature glass. Such a solidification may occur by a low-temperature quench of topological defects (e.g., grain boundaries or dislocations) which we examined in an earlier work. The low-temperature glass can account for not only a monotonic decrease in the rotation period as the temperature is lowered but also explains the peak in the dissipation occurring near the transition point. Unlike the non-classical rotational inertia scenario, which depends on the supersolid fraction, the dependence of the rotational period on external parameters, e.g., the oscillator velocity, provides an alternate interpretation of the oscillator experiments.

We present a terahertz spectroscopic study of magnetic excitations in ferroelectric antiferromagnet BiFeO 3 . We interpret the observed spectrum of long-wavelength magnetic resonance modes in terms of the normal modes of the material's cycloidal antiferromagnetic structure. We find that the modulated Dzyaloshinski-Moriya interaction leads to a splitting of the out-of-plane resonance modes. We also assign one of the observed absorption lines to an electromagnon excitation that results from the magnetoelectric coupling between the ferroelectric polarization and the cycloidal magnetic structure of BiFeO 3 .

We report a comprehensive study of ultrafast carrier dynamics in single crystals of multiferroic BiFeO3. Using femtosecond optical pump-probe spectroscopy, we find that the photoexcited electrons relax to the conduction band minimum through electron-phonon coupling with a ∼1 picosecond time constant that does not significantly change across the antiferromagnetic transition. Photoexcited electrons subsequently leave the conduction band and primarily decay via radiative recombination, which is supported by photoluminescence measurements. We find that despite the coexisting ferroelectric and antiferromagnetic orders in BiFeO3, the intrinsic nature of this chargetransfer insulator results in carrier relaxation similar to that observed in bulk semiconductors. Bismuth ferrite (BFO) is one of the most actively studied multiferroic materials due to its room temperature coexistence of ferroelectric (FE) (T c ∼1100 K) and antiferromagnetic (AFM) (T N ∼640 K) orders. Much research has focused on enhancing their weak mutual coupling, particularly by using growth techniques that vary the structure or strain in BFO films. 1-6 This could allow both control of magnetism with electric fields and control of electric polarization with magnetic fields, which would lead to a variety of potential applications in optoelectronics, spintronics, and data storage.Despite the intense research on this material, relatively few studies of its optical properties have been done to date. However, these studies have uncovered several unique phenomena that are linked to FE order in BFO. For example, calculated and measured absorption spectra reveal the optical band gap to be ∼2.6-2.8 eV at 300 K, 7-9 arising from the dipole-allowed O 2p to Fe 3d charge transfer (CT) transition. These measurements have also revealed strong absorption edge "smearing" 7,8,10 which was attributed to low-lying electronic 8,11 or defect states, 11,12 both of which can strongly impact the ferroelectric response. Terahertz (THz) emission spectroscopy indicates that ultrafast depolarization of the FE order causes the observed emission, 13,14 although the detailed mechanism is not entirely clear. A substantial zero-bias photovoltaic effect has also been observed in BFO with near/above band gap illumination, and the photocurrent preferentially moves along the direction of FE polarization. 15 Deeper insight into these and other phenomena, as well as their potential for applications (e.g., in determining switching speeds in BFO-based devices), can be gained by tracking carrier dynamics in BFO on an ultrafast timescale, 16 particularly after excitation of the p-d CT transitions that dominate the near band gap optical response and have been linked to FE properties. 8 This can be done using ultrafast optical spectroscopy (UOS), a technique that is capable of tracking the interplay between carrier, spin and lattice degrees of freedom by using femtosecond optical pulses to photoexcite materials and probing the response in the time domain. [17][18][19] In this letter, we use UOS, in...

We present the first ultrafast time-resolved optical measurements, to the best of our knowledge, on ensembles of germanium nanowires. Vertically aligned germanium nanowires with mean diameters of 18 and 30 nm are grown on (111) silicon substrates through chemical vapor deposition. We optically inject electron-hole pairs into the nanowires and exploit the indirect band structure of germanium to separately probe electron and hole dynamics with femtosecond time resolution. We find that the lifetime of both electrons and holes decreases with decreasing nanowire diameter, demonstrating that surface effects dominate carrier relaxation in semiconductor nanowires.