Optical evidence of the chiral magnetic anomaly in the Weyl semimetal TaAs.

Chiral pumping from optical electric fields oscillating at terahertz frequencies is observed in the Weyl material TaAs with electric and magnetic fields aligned along both the a and c axes. Free-carrier spectral weight enhancement is measured directly, confirming theoretical expectations of chiral pumping. A departure from linear field dependence of the Drude weight is observed at the highest fields in the quantum limit, providing evidence of field-dependent Fermi velocity of the chiral Landau level. Implications for the chiral magnetic effect in Weyl semimetals from the optical f -sum rule are discussed.

Infrared phonons as a probe of spin-liquid states in herbertsmithite ZnCu3(OH)6Cl2.

We report on temperature dependence of the infrared reflectivity spectra of a single crystalline herbertsmithite in two polarizations-parallel and perpendicular to the kagome plane of Cu atoms. We observe anomalous broadening of the low frequency phonons possibly caused by fluctuations in the exotic dynamical magnetic order of the spin liquid.

Photothermal response in dual-gated bilayer graphene.

The photovoltaic and bolometric photoresponse in gapped bilayer graphene was investigated by optical and transport measurements. A pulse coincidence technique at 1.5 µm was used to measure the response times as a function of temperature. The bolometric and photovoltaic response times were found to be identical implying that the photovoltaic response is also governed by hot electron thermal relaxation. Response times of τ ∼ 100-20 ps were found for temperatures from 3-100 K. Above 10 K, the relaxation time was observed to be τ = 25 ± 5 ps, independent of temperature within noise.

Dual-gated bilayer graphene hot-electron bolometer.

Graphene is an attractive material for use in optical detectors because it absorbs light from mid-infrared to ultraviolet wavelengths with nearly equal strength. Graphene is particularly well suited for bolometers-devices that detect temperature-induced changes in electrical conductivity caused by the absorption of light-because its small electron heat capacity and weak electron-phonon coupling lead to large light-induced changes in electron temperature. Here, we demonstrate a hot-electron bolometer made of bilayer graphene that is dual-gated to create a tunable bandgap and electron-temperature-dependent conductivity. The bolometer exhibits a noise-equivalent power (33 fW Hz(-1/2) at 5 K) that is several times lower, and intrinsic speed (>1 GHz at 10 K) three to five orders of magnitude higher than commercial silicon bolometers and superconducting transition-edge sensors at similar temperatures.

Carbon nanotube quantum dots as highly sensitive terahertz-cooled spectrometers.

Terahertz technology has recently emerged as a highly sought-after and versatile scientific tool in many fields, including medical imaging, security screening, and wireless communication. However, scientific progress has been hindered by the lack of sources and detectors in this frequency range, thereby known as the terahertz gap. Here, we show that carbon nanotube quantum dots coupled to antennas are extremely sensitive, broad-band, terahertz quantum detectors with spectral resolution. Their response is due to photon-assisted single-electron tunneling and it is substantially enhanced by a novel radiation-induced nonequilibrium cooling of the electrons, causing a sharp height increase of the Coulomb oscillation peaks.

Bianisotropy and spatial dispersion in highly anisotropic near-infrared resonator arrays.

We measure, simulate, and analyze the optical transmission through arrays of Ag nanorod pairs and U-shaped nanostructures as a function of polarization and angle of incidence. The bianisotropic nature of the metamaterials is exhibited in data and in simulations, and we argue that the electric field rather than the magnetic field excites the low frequency "magnetic" mode. We also observe spatial dispersion in the form of frequency shifts as a function of incident angle which we attribute to coupling effects between neighboring structures. A simple model based upon coupled electromagnetic dipoles is found to provide a qualitative description for the main features observed in the spectra.

Simultaneous measurement of circular dichroism and Faraday rotation at terahertz frequencies utilizing electric field sensitive detection via polarization modulation.

A far-infrared system measures the full complex Faraday angle, the rotation as well as the ellipticity, with an unprecedented accuracy of 10 microrad/T. The system operates on several far-infrared laser lines in the spectral range from 0.3 to 6 THz and produces results as a continuous function of temperature from 10 to 310 K and applied fields between +/-8 T. Performance of the instrument is demonstrated by measurements on a GaAs two-dimensional electron gas heterostructure and Bi(2)Sr(2)CaCu(2)O(8+x) single crystal.

Origin of electromagnon excitations in multiferroic RMnO3.

Electromagnon excitations in multiferroic orthorhombic RMnO3 are shown to result from the Heisenberg coupling between spins despite the fact that the static polarization arises from the much weaker Dzyaloshinskii-Moriya exchange interaction. We present a model incorporating the structural characteristics of this family of manganites that is confirmed by far infrared transmission data as a function of temperature and magnetic field and inelastic neutron scattering results. A deep connection is found between the magnetoelectric dynamics of the spiral phase and the static magnetoelectric coupling in the collinear E phase of this family of manganites.

Optical plasmonic resonances in split-ring resonator structures: an improved LC model.

We systematically investigate the resonant behavior of arrays of Ag nano-structures ranging from isolated simple rods, to U-shapes, to single split ring structures. We show that the lowest order plasmonic resonance associated with a rod red shifts as we create a U and SRR into the position normally associated with a simple LC mode. A second mode red shifts and grows in intensity as we extend the arms of the U-shape, and a third mode appears in the spectra as we close the arms and form a split ring structure. We performed simulations of the structures and examine the E-field and current density. The simulations show that the current path is different for these modes. We examine the behavior of the lowest order mode in detail, discuss the effects of skin depth, and present an improved LC model to describe this resonance.

Effective masses in a strongly anisotropic fermi liquid.

Motivated by the recent experimental observation of quantum oscillations in the underdoped cuprates, we study the cyclotron and infrared Hall effective masses in an anisotropic Fermi liquid characterized by an angle-dependent quasiparticle residue Z_{q}. Our primary motivation is to explain the relatively large value of the cyclotron mass observed experimentally and its relation with the effective Hall mass. Using a phenomenological model of an anisotropic Fermi liquid, we find that the cyclotron mass is enhanced by a factor 1/Z_{q}, while the effective Hall mass is proportional to Z_{q}/Z_{q};{2}, where cdots, three dots, centered implies an averaging over the Fermi surface. If the Z-factor becomes small in some part of the Fermi surface (e.g., in the case of a Fermi arc), the cyclotron mass is enhanced sharply while the infrared Hall mass may remain small.

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 steplike 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.

Probing spin correlations with phonons in the strongly frustrated magnet ZnCr2O4.

The spin-lattice coupling plays an important role in strongly frustrated magnets. In ZnCr2O4, 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 ZnCr2O4 tracks the nearest-neighbor spin correlations above T(c). The splitting of the phonon triplet below T(c) provides a way to measure the spin-Peierls order parameter.

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 T(c) superconductors and used to study the magnetotransport. YBa2Cu3O6+x thin films are investigated in the temperature range 10-300 K in magnetic fields up to 8 T 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 density wave models.

High temperature ferromagnetism with a giant magnetic moment in transparent co-doped SnO(2-delta).

The occurrence of room temperature ferromagnetism is demonstrated in pulsed laser deposited thin films of Sn(1-x)Co(x)O(2-delta) (x<0.3). Interestingly, films of Sn(0.95)Co(0.05)O(2-delta) grown on R-plane sapphire not only exhibit ferromagnetism with a Curie temperature close to 650 K, but also a giant magnetic moment of 7.5+/-0.5 micro(B)/Co, not yet reported in any diluted magnetic semiconductor system. The films are semiconducting and optically highly transparent.

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 blueshift ( approximately 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 the temperature dependence of the quasistatic dielectric constant epsilon(0) below T(N) approximately 90 K is overwhelmingly dominated by the phonon contributions.

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.

Spectral measurement of the Hall angle response in normal state cuprate superconductors.

We measure the temperature and frequency dependence of the complex Hall angle for normal state YBa(2)Cu(3)O(7) films from dc to far-infrared frequencies (20-250 cm(-1)) using a new modulated polarization technique. We determine that the functional dependence of the Hall angle on scattering does not fit the expected Lorentzian response. We find spectral evidence supporting models of the Hall effect where the scattering Gamma(H) is linear in T, suggesting that a single relaxation rate, linear in temperature, governs transport in the cuprates.

Inducing superconductivity at a nanoscale: photodoping with a near-field scanning optical microscope.

The local modification of an insulating GdBa2Cu3O6.5 thin film, made superconducting by illumination with a near-field scanning optical microscope (NSOM), is reported. A 100-nm aperture NSOM probe acts as a sub-wavelength light source of wavelength lambda(exc) = 480-650 nm, locally generating photocarriers in an otherwise insulating GdBa2-Cu3O6.5 thin film. Of the photogenerated electron-hole pairs, electrons are trapped in the crystallographic lattice, defining an electrostatic confining potential to enable the holes to move. Reflectance measurements at lambda = 1.55 microm at room temperature show that photocarriers can be induced and constrained to move on a approximately 200 nm scale for all investigated lambda(exc). Photogenerated wires present a superconducting critical temperature Tc= 12 K with a critical current density Jc = 10(4) A cm(-2). Exploiting the flexibility provided by photodoping through a NSOM probe, a junction was written by photodoping a wire with a narrow (approximately 50 nm) under-illuminated gap. The strong magnetic field modulation of the critical current provides a clear signature of the existence of a Josephson effect in the junction.

Residual esterase activity of lima bean inhibitor-binding anhydrochymotrypsin preparations.

A search for the source of the residual esterase activity of crude lima bean protease inhibitor-binding anhydrochymotrypsin preparations was undertaken. The preparations were found to contain about 40% of protein that possesses 1% (kc/Km) to 12% (kc) of the esterase activity of alpha-chymotrypsin. The active protein was isolated by affinity chromatography on soybean trypsin inhibitor-Sepharose. It appears to be an anhydroenzyme or a mixture of a limited number of anhydroenzymes in which a serine other than the catalytically essential serine-195 of the native enzyme has been converted to dehydroalanine.

A (3)he-cooled bolometer for the far infrared.

A germanium bolometer cooled to 0.37 K by liquid (3)He is described. This detector has a NEP of about 3 x 10(-14) W /Hz(1/2) which is more than three decades greater sensitivity than the Golay cell. The factors that determine the sensitivity of the detector are discussed.