arXiv subjects

B. S. Dennis

Publications and source records attributed to B. S. Dennis.

At least 19 recordsLinked to original sources

arXiv2015

Small footprint nano-mechanical plasmonic phase modulators

The authors' recent Nature Photonics article titled "Compact Nano-Mechanical Plasmonic Phase Modulators" [1] is reviewed which reports a new phase modulation principle with experimental demonstration of a 23 μm long non-resonant modulator having 1.5 π rad range with 1.7 dB excess loss at 780 nm. Analysis showed that by decreasing all dimensions, a low loss, ultra-compact π rad phase modulator is possible. Application of this type of nano-mechanical modulator in a miniature 2 x 2 switch is suggested and an optical design numerically validated. The footprint of the switch is 0.5 μm x 2.5 μm.

physics.optics↗

arXiv2011

Observation of non-conventional spin waves in composite fermion ferromagnets

We find unexpected low energy excitations of fully spin-polarized composite-fermion ferromagnets in the fractional quantum Hall liquid, resulting from a complex interplay between a topological order manifesting through new energy levels and a magnetic order due to spin polarization. The lowest energy modes, which involve spin reversal, are remarkable in displaying unconventional negative dispersion at small momenta followed by a deep roton minimum at larger momenta. This behavior results from a nontrivial mixing of spin-wave and spin-flip modes creating a spin-flip excitonic state of composite-fermion particle-hole pairs. The striking properties of spin-flip excitons imply highly tunable mode couplings that enable fine control of topological states of itinerant two-dimensional ferromagnets.

cond-mat.str-el↗

arXiv2009

Spin Order and Excitations in Triangular Antiferromagnet La2Ca2MnO7

We report a spin S = 3/2 triangular antiferromagnet with nearest-neighbor coupling J = 0.29 meV in La2Ca2MnO7. A genuinely two-dimensional, three-sublattice order develops below 2.80 K << the Weiss constant (25 K). The spin excitations deviate substantially from linear spin-wave theory, suggesting that magnon breakdown occurs in the material. Such a breakdown has been anticipated in recent theoretical studies, although the excitation spectrum remains to be accounted for.

cond-mat.str-el↗

arXiv2007

Multi-Gap Superconductivity in MgB2: Magneto-Raman Spectroscopy

Electronic Raman scattering studies on MgB2 single crystals as a function of excitation and polarization have revealed three distinct superconducting features: a clean gap below 37 cm-1 and two coherence peaks at 109 cm-1 and 78 cm-1 which we identify as the superconducting gaps in π- and σ-bands and as the Leggett's collective mode arising from the fluctuation in the relative phase between two superconducting condensates residing on corresponding bands. The temperature and field dependencies of the superconducting features have been established. A phononic Raman scattering study of the E2g boron stretching mode anharmonicity and of superconductivity induced self-energy effects is presented. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ~2.5% hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization.

cond-mat.supr-con↗

arXiv2007

Observation of Leggett's collective mode in a multi-band MgB2 superconductor

We report observation of Leggett's collective mode in a multi-band MgB2 superconductor with T_c=39K arising from the fluctuations in the relative phase between two superconducting condensates. The novel mode is observed by Raman spectroscopy at 9.4 meV in the fully symmetric scattering channel. The observed mode frequency is consistent with theoretical considerations based on the first principle computations.

cond-mat.supr-con↗

arXiv2007

Anharmonicity and self-energy effects of the E2g phonon in MgB2

We present a Raman scattering study of the E2g phonon anharmonicity and of superconductivity induced self-energy effects in MgB2 single crystals. We show that anharmonic two phonon decay is mainly responsible for the unusually large linewidth of the E2g mode. We observe ~ 2.5 % hardening of the E2g phonon frequency upon cooling into the superconducting state and estimate the electron-phonon coupling strength associated with this renormalization.

cond-mat.supr-con↗

arXiv2006

Absorption in the fractional quantum Hall regime: trion dichroism and spin polarization

We present measurements of optical interband absorption in the fractional quantum Hall regime in a GaAs quantum well in the range 0 < nu < 1. We investigate the mechanism of singlet trion absorption, and show that its circular dichroism can be used as a probe of the spin polarization of the ground state of the two-dimensional electron system (2DES). We find that at nu = 1/3 the 2DES is fully spin-polarized. Increasing the filling factor results in a gradual depolarization, with a sharp minimum in the dichroism near nu = 2/3. We find that in the range 0.5 < nu < 0.85 the 2DES remains partially polarized for the broad range of magnetic fields from 2.75 to 11 Tesla. This is consistent with the presence of a mixture of polarized and depolarized regions.

cond-mat.mes-hall↗

arXiv2006

Field dependence of the magnetic spectrum in anisotropic and Dzyaloshinskii-Moriya antiferromagnets: II. Raman spectroscopy

We compare the theoretical predictions of the previous article [L. Benfatto and M. B. Silva Neto, cond-mat/0602419], with Raman spectroscopy experiments in Sr(2)CuO(2)Cl(2) and untwinned La(2)CuO(4) single crystals. We construct the magnetic point group for the magnetically ordered phase of the two compounds, Sr(2)CuO(2)Cl(2) and La(2)CuO(4), and we classify all the Raman active one-magnon excitations according to the irreducible co-representations for the associated magnetic point group. We then measure the evolution of the one-magnon Raman energies and intensities for low and moderate magnetic fields along the three crystallographic directions. In the case of La(2)CuO(4), we demonstrate that from the jump of the Dzyaloshinskii-Moriya gap at the critical magnetic field H_c ~ 6.6 T for the weak-ferromagnetic transition one can determine the value of the interlayer coupling J_\perp/J ~ 3.2 x 10^-5. We furthermore determine the components of the anisotropic gyromagnetic tensor as g_s^a=2.0, g_s^b=2.08, and the upper bound g_s^c=2.65. For the case of Sr(2)CuO(2)Cl(2), we compare the Raman data obtained in an in-plane magnetic field with previous magnon-gap measurements done by ESR. Using the very low magnon gap estimated by ESR (~ 0.05 meV), the data for the one-magnon Raman energies agree reasonably well with the theoretical predictions for the case of a transverse field (only hardening of the gap).

cond-mat.str-el↗

arXiv2006

Transition from Free to Interacting Composite Fermions away from $ν$=1/3

Spin excitations from a partially populated composite fermion level are studied above and below $ν=1/3$. In the range $2/7<ν<2/5$ the experiments uncover significant departures from the non-interacting composite fermion picture that demonstrate the increasing impact of interactions as quasiparticle Landau levels are filled. The observed onset of a transition from free to interacting composite fermions could be linked to condensation into the higher order states suggested by transport experiments and numerical evaluations performed in the same filling factor range.

cond-mat.mes-hall↗

arXiv2006

Superconductivity in Cu_xTiSe_2

Charge density waves (CDWs) are periodic modulations of the conduction electron density in solids. They are collective states that arise from intrinsic instabilities often present in low dimensional electronic systems. The layered dichalcogenides are the most well-studied examples, with TiSe_2 one of the first CDW-bearing materials known. The competition between CDW and superconducting collective electronic states at low temperatures has long been held and explored, and yet no chemical system has been previously reported where finely controlled chemical tuning allows this competition to be studied in detail. Here we report how, upon controlled intercalation of TiSe_2 with Cu to yield Cu_xTiSe_2, the CDW transition is continuously suppressed, and a new superconducting state emerges near x = 0.04, with a maximum T_c of 4.15 K found at x = 0.08. Cu_xTiSe_2 thus provides the first opportunity to study the CDW to Superconductivity transition in detail through an easily-controllable chemical parameter, and will provide new insights into the behavior of correlated electron systems.

cond-mat.supr-con↗

arXiv2006

Resonant Rayleigh scattering from quantum phases of cold electrons in semiconductor heterostructures

Resonant Rayleigh scattering of light from electrons confined in gallium arsenide double quantum wells displays significant changes at temperatures that are below one degree Kelvin. The Rayleigh resonance occurs for photon energies that overlap a quantum well exciton and when electron bilayers condense into a quantum-Hall state. Marked changes in Rayleigh scattering intensities that occur in response to application of an in-plane magnetic field indicate that the unexpected temperature dependence is linked to formation of non-uniform electron fluids in a disordered quantum-Hall phase. These results demonstrate a new realm of study in which resonant Rayleigh scattering methods probe quantum phases of cold electrons in semiconductor heterostructures.

cond-mat.mes-hall↗

arXiv2005

Evolution of Superconductivity in Electron-Doped Cuprates: Magneto-Raman Spectroscopy

The electron-doped cuprates Pr_{2-x}Ce_xCuO_4 and Nd_{2-x}Ce_xCuO_4 have been studied by electronic Raman spectroscopy across the entire region of the superconducting (SC) phase diagram. The SC pairing strength is found to be consistent with a weak-coupling regime except in the under-doped region where we observe an in-gap collective mode at 4.5 k_{B}T_c while the maximum amplitude of the SC gap is ~8 k_{B}T_{c}. In the normal state, doped carriers divide into coherent quasi-particles (QPs) and carriers that remain incoherent. The coherent QPs mainly reside in the vicinity of (π/2, π/2) regions of the Brillouin zone (BZ). We find that only coherent QPs contribute to the superfluid density in the B_{2g} channel. The persistence of SC coherence peaks in the B_{2g} channel for all dopings implies that superconductivity is mainly governed by interactions between the hole-like coherent QPs in the vicinity of (π/2, π/2) regions of the BZ. We establish that superconductivity in the electron-doped cuprates occurs primarily due to pairing and condensation of hole-like carriers. We have also studied the excitations across the SC gap by Raman spectroscopy as a function of temperature (T) and magnetic field (H) for several different cerium dopings (x). Effective upper critical field lines H*_{c2}(T, x) at which the superfluid stiffness vanishes and H^{2Δ}_{c2}(T, x) at which the SC gap amplitude is suppressed by field have been determined; H^{2Δ}_{c2}(T, x) is larger than H*_{c2}(T, x) for all doping concentrations. The difference between the two quantities suggests the presence of phase fluctuations that increase for x< 0.15. It is found that the magnetic field suppresses the magnitude of the SC gap linearly at surprisingly small fields.

cond-mat.supr-con↗

arXiv2005

Symmetry and light coupling to phononic and collective magnetic excitations in SrCu_2(BO_3)_2

We perform a low temperature Raman scattering study of phononic and collective spin excitations in the orthogonal dimers compound SrCu_2(BO_3)_2, focussing on the symmetry and the effects of external fields on the magnetic modes. The zero field symmetry and the behavior in magnetic fields of the elementary and bound magnetic triplet states are experimentally determined. We find that a minimal 4-spin cluster forming the unit cell is able to describe the symmetry as well as the anisotropic dispersions in external fields of the spin gap multiplet branches around 24 cm^{-1}. We identify two Raman coupling mechanisms responsible for the distinct resonance behavior of these magnetic modes and we show that one of these can be ascribed to an effective intra-dimer Dzyaloshinskii-Moriya spin interaction. Our data also suggest a possible explanation for the existence of a strongly bound two-triplet state in the singlet sector which has an energy below the spin gap. The low temperature phononic spectra suggest strong spin-phonon coupling and show intriguing quasi-degeneracy of modes in the context of the present crystal structure determination.

cond-mat.str-el↗

arXiv2005

Splitting of Long-Wavelength Modes of the Fractional Quantum Hall Liquid at $ν=1/3$

Resonant inelastic light scattering experiments at $ν=1/3$ reveal a novel splitting of the long wavelength modes in the low energy spectrum of quasiparticle excitations in the charge degree of freedom. We find a single peak at small wavevectors that splits into two distinct modes at larger wavevectors. The evidence of well-defined dispersive behavior at small wavevectors indicates a coherence of the quantum fluid in the micron length scale. We evaluate interpretations of long wavelength modes of the electron liquid.

cond-mat.mes-hall↗

arXiv2005

Evolution of Coherence and Superconductivity in Electron-Doped Cuprates

The electron-doped cuprates were studied by electronic Raman spectroscopy across the entire region of the superconducting (SC) phase diagram. We determined that the magnitude of the SC order parameter varies between 4.6 and 3.5 k_BT_c, consistent with weak coupling BCS theory. Using a ``Raman conductivity" sum rule, we found that doped carriers divide into coherent quasi-particles (QPs) and carriers that remain incoherent. The coherent QPs mainly reside in the vicinity of (pi/2a, pi/2a) regions of the Brillouin zone. The carriers doped beyond optimal doping remain incoherent. Only coherent QPs contribute to the superfluid density in the SC state.

cond-mat.supr-con↗

arXiv2004

Magnetic order in lightly doped La_{2-x}Sr_{x}CuO_{4}

We study long wavelength magnetic excitations in lightly doped La_{2-x}Sr_{x}CuO_{4} (x < 0.03) detwinned crystals. The lowest energy magnetic anisotropy induced gap can be understood in terms of the antisymmetric spin interaction inside the antiferromagnetic (AF) phase. The second magnetic resonace, analyzed in terms of in-plane spin anisotropy, shows unconventional behavior within the AF state and led to the discovery of collective spin excitations pertaining to a field induced magnetically ordered state. This state persists in a 9 T field to more than 100 K above the Néel temperature in x = 0.01.

cond-mat.str-el↗

arXiv2003

Light Scattering by Low Lying Quasiparticle Excitations in the Fractional Quantum Hall Regime

Low lying excitations of electron liquids in the fractional quantum Hall (FQH) regime are studied by resonant inelastic light scattering methods. We present here results from charge and spin excitations of FQH states in the lowest spin-split Landau levels that are of current interest. In the range of filling factors $2/5 \geq ν\geq 1/3$, we find evidence that low energy quasiparticle excitations can be interpreted with spin-split composite fermion quasi-Landau levels. At FQH states around $ν=3/2$, we find well-defined excitations at 4/3 and 8/5 that are consistent with a spin-unpolarized population of quasi-Landau levels.

cond-mat.mes-hall↗

arXiv2003

Inhomogeneous CuO_{6} Tilt Distribution and Charge/Spin Correlations in La_{2-x-y}Nd_{y}Sr_{x}CuO$_{4} around commensurate hole concentration

Phononic and magnetic Raman scattering are studied in La$_{2-x-y}$Nd$_{y}$Sr$_{x}$CuO$_{4}$ with three doping concentrations: x ~ 1/8, y = 0; x ~ 1/8, y = 0.4; and x = 0.01, y = 0. We observe strong disorder in the tilt pattern of the CuO_{6} octahedra in both the orthorhombic and tetragonal phases which persist down to 10 K and are coupled to bond disorder in the cation layers around 1/8 doping independent of Nd concentration. The weak magnitude of existing charge/spin modulations in the Nd doped structure does not allow us to detect the specific Raman signatures on lattice dynamics or two-magnon scattering around 2200 cm-1.

cond-mat.str-el↗