arXiv subjects

R. Merlin

Publications and source records attributed to R. Merlin.

At least 19 recordsLinked to original sources

arXiv2024

Magnetophononics and the Chiral Phonon Misnomer

The direct, ultrafast excitation of polar phonons with electromagnetic radiation is a potent strategy for controlling the properties of a wide range of materials, particularly in the context of influencing their magnetic behavior. Here, we show that, contrary to common perception, the origin of phonon-induced magnetic activity does not stem from the motion of ions themselves; instead, it arises from the effect their motion exerts on the electron subsystem. Through the mechanism of electron-phonon coupling, a coherent state of circularly polarized phonons generates substantial non-Maxwellian fields that disrupt time reversal symmetry, effectively emulating the behavior of authentic magnetic fields. Notably, the effective field can reach magnitudes as high as 100 T, surpassing by several orders of magnitude the Maxwellian field resulting from the circular motion of the ions. Because the light-induced non-reciprocal fields depend on the square of the phonon displacements, the chirality the photons transferred to the ions plays no role in magnetophononics.

cond-mat.mes-hall↗

arXiv2023

Unraveling the Effect of Circularly Polarized Light on Reciprocal Media: Breaking Time Reversal Symmetry with Non-Maxwellian Magnetic-esque Fields

Optical rectification of intense, circularly polarized light penetrating a material generates a static magnetic field aligned with the light's direction and proportional to its intensity. Recent experiments have unveiled a substantial, orders-of-magnitude gap between the observed magnetization and theoretical predictions. In this study, we demonstrate that circularly polarized light creates large non-Maxwellian fields that disrupt time-reversal symmetry, effectively mimicking authentic magnetic fields within the material while eluding detection externally. These unconventional fields give rise to Faraday-rotation-like phenomena, which are a high-frequency manifestation of Berry's curvature.

physics.optics↗

arXiv2023

Phonon Bloch Oscillations

The classical motion of a one-dimensional chain of atoms coupled through a specific force function that depends on position shows features very similar to the Wannier-Stark problem of a quantum particle under the combined effects of a periodic lattice potential and a constant electric field. Both problems exhibit localized modes and a ladder of equally-spaced eigenfrequencies, leading to temporal dynamics characterized by periodic oscillations.

physics.class-ph↗

arXiv2023

Perturbed Periodic Lattices: Sharp Crossover Between Effective-Mass-Like States and Wannier-Stark-Like Ladders

The concept of Wannier-Stark ladders, describing the equally spaced spectrum of a tightly-bound particle in a constant electric field, is generalized to account for arbitrary slowly-varying potentials. It is shown that an abrupt transition exists that separates Wannier-Stark-like from effective-mass-like behavior when the depth of the perturbation becomes equal to the width of the band of extended states. For potentials bounded from below, the spectrum bifurcates above the critical energy while the wavefunctions detach from the effective-mass region and split into two pieces.

cond-mat.mes-hall↗

arXiv2021

Photo-induced plasmon-phonon coupling in PbTe

We report the observation of photo-induced plasmon-phonon coupled modes in the group IV-VI semiconductor PbTe using Fourier-transform inelastic X-ray scattering at the Linac Coherent Light Source (LCLS). We measure the near-zone-center dispersion of the heavily screened longitudinal optical (LO) phonon branch as extracted from differential changes in x-ray diffuse scattering intensity following above band gap photoexcitation.

cond-mat.mtrl-sci↗

arXiv2021

Intensity correlation speckles as a technique for removing Doppler broadening

A method involving intensity correlation measurements is described, which allows for the complete removal of Doppler broadening in the emission of electromagnetic radiation from far-away sources that are inaccessible to conventional Doppler-free measurements. The technique, relying on a correction to g(2) of order N-1, probes the separation between neighboring spectral lines and is also applicable to the elimination of broadening due to collisions (N is the number of emitting particles and g(2) is the second-order field correlation function). Possible applications include a determination of cosmological parameters from red shifts of gravitationally-lensed quasars.

physics.optics↗

arXiv2017

Phonon-assisted oscillatory exciton dynamics in monolayer MoSe2

In monolayer semiconductor transition metal dichalcogenides, the exciton-phonon interaction is expected to strongly affect the photocarrier dynamics. Here, we report on an unusual oscillatory enhancement of the neutral exciton photoluminescence with the excitation laser frequency in monolayer MoSe2. The frequency of oscillation matches that of the M-point longitudinal acoustic phonon, LA(M). Oscillatory behavior is also observed in the steady-state emission linewidth and in timeresolved photoluminescence excitation data, which reveals variation with excitation energy in the exciton lifetime. These results clearly expose the key role played by phonons in the exciton formation and relaxation dynamics of two-dimensional van der Waals semiconductors.

cond-mat.mtrl-sci↗

arXiv2016

Terahertz Spectrometer of Wavelength Dimensions Based on Extraordinary Transmission

Subwavelength-slotted parallel plate waveguides exhibit a localized electromagnetic resonance bound to the slits at a frequency slightly below the transverse electric cutoff [R. Merlin, Phys. Rev. X 2, 031015 (2012)]. The resonance is long-lived and, as opposed to the vanishingly small transmission shown by a single sub-wavelength aperture, it gives perfect transmission for perfectly-conducting plates. We show that the aperture-supported resonances of a pair of slotted copper plates have long lifetimes at THz frequencies. Finite element method calculations show that these bound resonances can have quality factors greater than 100. Effects of plate length and imperfect parallel alignment are also discussed. Using THz time domain spectroscopy, we measured the transmission of a broadband pulse through a test structure for several plate separations. These results suggest that the slotted waveguide can function as a highly compact THz spectrometer.

physics.optics↗

arXiv2016

Surface vibrational modes of the topological insulator Bi$_2$Se$_3$ observed by Raman spectroscopy

We present polarization resolved Raman scattering study of surface vibration modes in the topological insulator Bi$_2$Se$_3$ single crystal and thick films. Besides the four Raman active bulk phonons, we observed four additional modes with much weaker intensity and slightly lower energy than the bulk counterparts. Using symmetry analysis, we assigned these additional modes to out-of-plane surface phonons. Comparing with first principle calculations, we conclude that the appearance of these modes is due to $c$-axis lattice distortion and van der Waals gap expansion near the crystal surface. Two of the surface modes at 60 and 173 cm$^{-1}$ are associated with Raman active $A_{1g}$ bulk phonon modes, the other two at 136 and 158 cm$^{-1}$ are associated with infrared active bulk phonons with $A_{2u}$ symmetry. The latter become Raman allowed due to reduction of crystalline symmetry from $D_{3d}$ in the bulk to $C_{3v}$ on the crystal surface. In particular, the 158 cm$^{-1}$ surface phonon mode shows a Fano lineshape under resonant excitation, suggesting interference in the presence of electron-phonon coupling of the surface excitations.

cond-mat.mtrl-sci↗

arXiv2016

Orthogonality Catastrophes in Quantum Electrodynamics

The insertion of a small polarizable particle in an arbitrarily large optical cavity significantly alters the quantum-mechanical state of the electromagnetic field in that the photon ground state of the empty cavity and that of the cavity with the particle become mutually orthogonal and, thus, cannot be connected adiabatically in the infinite limit. The photon problem can be mapped exactly onto that of a many-body system of fermions, which is known to exhibit an orthogonality catastrophe when a finite-range local potential is introduced. We predict that the motion of polarizable objects inside a cavity, no matter how slow, as well as their addition and removal from the cavity, will generate a macroscopic, diverging number of low-energy photons. The significance of these results in regard to the quantum measurement problem and the dynamical Casimir effect are also discussed.

quant-ph↗

arXiv2015

An effective magnetic field from optically driven phonons

Light fields at THz and mid-infrared frequencies allow for the direct excitation of collective modes in condensed matter, which can be driven to large amplitudes. For example, excitation of the crystal lattice, has been shown to stimulate insulator-metal transitions, melt magnetic order, or enhance superconductivity. Here, we generalize these ideas and explore the simultaneous excitation of more than one lattice mode, which are driven with controlled relative phases. This nonlinear mode mixing drives rotations as well as displacements of the crystal-field atoms, mimicking the application of a magnetic field and resulting in the excitation of spin precession in the rare-earth orthoferrite $ErFeO_3$. Coherent control of lattice rotations may become applicable to other interesting problems in materials research, for example as a way to affect the topology of electronic phases.

cond-mat.mtrl-sci↗

arXiv2015

Photoinduced suppression of the ferroelectric instability in PbTe

The interactions between electrons and phonons drive a large array of technologically relevant material properties including ferroelectricity, thermoelectricity, and phase-change behaviour. In the case of many group IV-VI, V, and related materials, these interactions are strong and the materials exist near electronic and structural phase transitions. Their close proximity to phase instability produces a fragile balance among the various properties. The prototypical example is PbTe whose incipient ferroelectric behaviour has been associated with large phonon anharmonicity and thermoelectricity. Experimental measurements on PbTe reveal anomalous lattice dynamics, especially in the soft transverse optical phonon branch. This has been interpreted in terms of both giant anharmonicity and local symmetry breaking due to off-centering of the Pb ions. The observed anomalies have prompted renewed theoretical and computational interest, which has in turn revived focus on the extent that electron-phonon interactions drive lattice instabilities in PbTe and related materials. Here, we use Fourier-transform inelastic x-ray scattering (FT-IXS) to show that photo-injection of free carriers stabilizes the paraelectric state. With support from constrained density functional theory (CDFT) calculations, we find that photoexcitation weakens the long-range forces along the cubic direction tied to resonant bonding and incipient ferroelectricity. This demonstrates the importance of electronic states near the band edges in determining the equilibrium structure.

cond-mat.mtrl-sci↗

arXiv2015

A Heuristic Approach to the Quantum Measurement Problem: How to Distinguish Particle Detectors from Ordinary Objects

Elementary particle detectors fall broadly into only two classes: phase-transformation devices, such as the bubble chamber, and charge-transfer devices like the Geiger-Mueller tube. Quantum measurements are seen to involve transitions from a long-lived metastable state (e. g., superheated liquid or a gas of atoms between charged capacitor plates) to a thermodinamically stable condition. A detector is then a specially prepared object undergoing a metastable-to-stable transformation that is significantly enhanced by the presence of the measured particle, which behaves, in some sense, as the seed of a process of heterogeneous nucleation. Based on this understanding of the operation of a conventional detector, and using results of orthogonality-catastrophe theory, we argue that, in the thermodynamic limit, the pre-measurement Hamiltonian is not the same as that describing the detector during or after the interaction with a particle and, thus, that superpositions of pointer states (Schroedinger cats) are unphysical because their time evolution is ill defined. Examples of particle-induced changes in the Hamiltonian are also given for ordinary systems whose macroscopic parameters are susceptible to radiation damage, but are not modified by the interaction with a single particle.

quant-ph↗

arXiv2012

Possible observation of parametrically amplified coherent phasons in K0.3MoO3 using time-resolved extreme-ultraviolet ARPES

We use time- and angle-resolved photoemission spectroscopy (tr-ARPES) in the Extreme Ultraviolet (EUV) to measure the time- and momentum-dependent electronic structure of photo-excited K0.3MoO3. Prompt depletion of the Charge Density Wave (CDW) condensate launches coherent oscillations of the amplitude mode, observed as a 1.7-THz-frequency modulation of the bonding band position. In contrast, the anti-bonding band oscillates at about half this frequency. We attribute these oscillations to coherent excitation of phasons via parametric amplification of phase fluctuations.

cond-mat.str-el↗

arXiv2010

Comment on "Perfect imaging with positive refraction in three dimensions"

Leonhard and Philbin [Phys. Rev. A 81, 011804(R) (2010)] have recently constructed a mathematical proof that the Maxwell's fish-eye lens provides perfect imaging of electromagnetic waves without negative refraction. In this comment, we argue that the unlimited resolution is an artifact of having introduced an unphysical drain at the position of the geometrical image. The correct solution gives focusing consistent with the standard diffraction limit.

physics.optics↗

arXiv2007

Near-Field Focusing Plates and Their Design

This paper describes the design of near-field focusing plates, which are grating-like structures that can focus electromagnetic radiation to spots or lines of arbitrarily small subwavelength dimension. A general procedure is outlined for designing a near-field plate given a desired image, and its implementation at microwave frequencies is discussed. Full-wave (method of moments) simulations clearly demonstrate the near-field plate's ability to overcome the diffraction limit. Finally, it is shown that the performance of near-field plates is weakly affected by losses.

physics.class-ph↗

arXiv2007

Generation and remote detection of THz sound using semiconductor superlattices

The authors introduce a novel approach to study the propagation of high frequency acoustic phonons in which the generation and detection involves two spatially separated superlattices $\sim 1 {\rm μm}$ apart. Propagating modes of frequencies up to $\sim 1 {\rm THz}$ escape from the superlattice where they are generated and reach the second superlattice where they are detected. The measured frequency spectrum reveals finite size effects, which can be accounted for by a continuum elastic model.

cond-mat.mtrl-sci↗

arXiv2006

Light-Induced Magnetic Precession in (Ga,Mn)As Slabs: Hybrid Standing-Wave Damon-Eshbach Modes

Coherent oscillations associated with spin precessions were observed in ultrafast optical experiments on ferromagnetic (Ga,Mn)As films. Using a complete theoretical description of the processes by which light couples to the magnetization, values for the exchange and anisotropy constants were obtained from the field-dependence of the magnon frequencies and the oscillation-amplitude ratios. Results reveal a relatively large negative contribution to the energy due to surface anisotropy leading to excitations that are a mixture of bulk waves and surface modes.

cond-mat.mtrl-sci↗