arXiv subjects

Michael C. Martin

Publications and source records attributed to Michael C. Martin.

At least 19 recordsLinked to original sources

arXiv2021

Hybrid Machine Learning for Scanning Near-field Optical Spectroscopy

The underlying physics behind an experimental observation often lacks a simple analytical description. This is especially the case for scanning probe microscopy techniques, where the interaction between the probe and the sample is nontrivial. Realistic modeling to include the details of the probe is always exponentially more difficult than its "spherical cow" counterparts. On the other hand, a well-trained artificial neural network based on real data can grasp the hidden correlation between the signal and sample properties. In this work, we show that, via a combination of model calculation and experimental data acquisition, a physics-infused hybrid neural network can predict the tip-sample interaction in the widely used scattering-type scanning near-field optical microscope. This hybrid network provides a long-sought solution for accurate extraction of material properties from tip-specific raw data. The methodology can be extended to other scanning probe microscopy techniques as well as other data-oriented physical problems in general.

physics.optics↗

arXiv2021

THz Near-Field Imaging of Extreme Subwavelength Metal Structures

Modern scattering-type scanning near-field optical microscopy (s-SNOM) has become an indispensable tool in material research. However, as the s-SNOM technique marches into the far-infrared (IR) and terahertz (THz) regimes, emerging experiments sometimes produce puzzling results. For example, anomalies in the near-field optical contrast have been widely reported. In this Letter, we systematically investigate a series of extreme subwavelength metallic nanostructures via s-SNOM near-field imaging in the GHz to THz frequency range. We find that the near-field material contrast is greatly impacted by the lateral size of the nanostructure, while the spatial resolution is practically independent of it. The contrast is also strongly affected by the connectivity of the metallic structures to a larger metallic ground plane. The observed effect can be largely explained by a quasi-electrostatic analysis. We also compare the THz s-SNOM results to those of the mid-IR regime, where the size-dependence becomes significant only for smaller structures. Our results reveal that the quantitative analysis of the near-field optical material contrasts in the long-wavelength regime requires a careful assessment of the size and configuration of metallic (optically conductive) structures.

physics.optics↗

arXiv2017

Ultrafast Dynamics of Vibrational Symmetry Breaking in a Charge-ordered Nickelate

The ability to probe symmetry breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-THz response of the model stripe compound La$_{1.75}$Sr$_{0.25}$NiO$_{4}$, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen - as witnessed by time-delayed suppression of zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry breaking dynamics in solids.

cond-mat.str-el↗

arXiv2015

Phase transition in bulk single crystals and thin films of VO2 by nano-infrared spectroscopy and imaging

We have systematically studied a variety of vanadium dioxide (VO2) crystalline forms, including bulk single crystals and oriented thin films, using infrared (IR) near-field spectroscopic imaging techniques. By measuring the IR spectroscopic responses of electrons and phonons in VO2 with sub-grain-size spatial resolution (~20 nm), we show that epitaxial strain in VO2 thin films not only triggers spontaneous local phase separations but also leads to intermediate electronic and lattice states that are intrinsically different from those found in bulk. Generalized rules of strain and symmetry dependent mesoscopic phase inhomogeneity are also discussed. These results set the stage for a comprehensive understanding of complex energy landscapes that may not be readily determined by macroscopic approaches.

cond-mat.str-el↗

arXiv2015

Amplitude- and phase-resolved nano-spectral imaging of phonon polaritons in hexagonal boron nitride

Phonon polaritons are quasiparticles resulting from strong coupling of photons with optical phonons. Excitation and control of these quasiparticles in 2D materials offer the opportunity to confine and transport light at the nanoscale. Here, we image the phonon polariton (PhP) spectral response in thin hexagonal boron nitride (hBN) crystals as a representative 2D material using amplitude- and phase-resolved near-field interferometry with broadband mid-IR synchrotron radiation. The large spectral bandwidth enables the simultaneous measurement of both out-of-plane (780 cm-1) and in-plane (1370 cm-1) hBN phonon modes. In contrast to the strong and dispersive in-plane mode, the out-of-plane mode PhP response is weak. Measurements of the PhP wavelength reveal a proportional dependence on sample thickness for thin hBN flakes, which can be understood by a general model describing two-dimensional polariton excitation in ultrathin materials.

cond-mat.mes-hall↗

arXiv2014

Gate-dependent Pseudospin Mixing in Graphene/Boron Nitride Moire Superlattices

Electrons in graphene are described by relativistic Dirac-Weyl spinors with a two-component pseudospin1-12. The unique pseudospin structure of Dirac electrons leads to emerging phenomena such as the massless Dirac cone2, anomalous quantum Hall effect2, 3, and Klein tunneling4, 5 in graphene. The capability to manipulate electron pseudospin is highly desirable for novel graphene electronics, and it requires precise control to differentiate the two graphene sub-lattices at atomic level. Graphene/boron nitride (graphene/BN) Moire superlattice, where a fast sub-lattice oscillation due to B-N atoms is superimposed on the slow Moire period, provides an attractive approach to engineer the electron pseudospin in graphene13-18. This unusual Moire superlattice leads to a spinor potential with unusual hybridization of electron pseudospins, which can be probed directly through infrared spectroscopy because optical transitions are very sensitive to excited state wavefunctions. Here, we perform micro-infrared spectroscopy on graphene/BN heterostructure and demonstrate that the Moire superlattice potential is dominated by a pseudospin-mixing component analogous to a spatially varying pseudomagnetic field. In addition, we show that the spinor potential depends sensitively on the gate-induced carrier concentration in graphene, indicating a strong renormalization of the spinor potential from electron-electron interactions. Our study offers deeper understanding of graphene pseudospin structure under spinor Moire potential, as well as exciting opportunities to control pseudospin in two-dimensional heterostructures for novel electronic and photonic nanodevices.

cond-mat.mes-hall↗

arXiv2014

Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides

Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide (TMDC) heterostructures can be designed and built by assembling individual single-layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single layer WSe2 and MoS2 building blocks. We observe a large Stokes-like shift of ~100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment with spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of this interlayer coupling consequently provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.

cond-mat.mes-hall↗

arXiv2009

A Tunable Phonon-Exciton Fano System in Bilayer Graphene

Interference between different possible paths lies at the heart of quantum physics. Such interference between coupled discrete and continuum states of a system can profoundly change its interaction with light as seen in Fano resonance. Here we present a unique many-body Fano system composed of a discrete phonon vibration and continuous electron-hole pair transitions in bilayer graphene. Mediated by the electron-phonon interactions, the excited state is described by new quanta of elementary excitations of hybrid phonon-exciton nature. Infrared absorption of the hybrid states exhibit characteristic Fano lineshapes with parameters renormalized by many-body interactions. Remarkably, the Fano resonance in bilayer graphene is continuously tunable through electrical gating. Further control of the phonon-exciton coupling may be achieved with an optical field exploiting the excited state infrared activity. This tunable phonon-exciton system also offers the intriguing possibility of a 'phonon laser' with stimulated phonon amplification generated by population inversion of band-edge electrons.

cond-mat.mes-hall↗

arXiv1999

The First Synchrotron Infrared Beamlines at the ALS: Spectromicroscopy and Fast Timing

Two recently commissioned infrared beamlines on the 1.4 bending magnet port at the Advanced Light Source, LBNL, are described. Using a synchrotron as an IR source provides three primary advantages: increased brightness, very fast light pulses, and enhanced far-IR flux. The considerable brightness advantage manifests itself most beneficially when performing spectroscopy on a microscopic length scale. Beamline (BL) 1.4.3 is a dedicated FTIR spectromicroscopy beamline, where a diffraction-limited spot size using the synchrotron source is utilized. BL 1.4.2 consists of a vacuum FTIR bench with a wide spectral range and step-scan capability. This BL makes use of the pulsed nature of the synchrotron light as well as the far-IR flux. Fast timing is demonstrated by observing the pulses from the electron bunch storage pattern at the ALS. Results from several experiments from both IR beamlines will be presented as an overview of the IR research currently being done at the ALS.

cond-mat.mtrl-sci↗

arXiv1997

Spin-Peierls and Antiferromagnetic Phases in Cu{1-x}Zn{x}GeO{3}: A Neutron Scattering Study

Comprehensive neutron scattering studies were carried out on a series of high-quality single crystals of Cu_{1-x}Zn_xGeO_3. The Zn concentration, x, was determined for each sample using Electron Probe Micro-Analysis. The measured Zn concentrations were found to be 40-80% lower than the nominal values. Nevertheless the measured concentrations cover a wide range which enables a systematic study of the effects due to Zn-doping. We have confirmed the coexistence of spin-Peierls (SP) and antiferromagnetic (AF) orderings at low temperatures and the measured phase diagram is presented. Most surprisingly, long-range AF ordering occurs even in the lowest available Zn concentration, x=0.42%, which places important constraints on theoretical models of the AF-SP coexistence. Magnetic excitations are also examined in detail. The AF excitations are sharp at low energies and show no considerable broadening as x increases indicating that the AF ordering remains long ranged for x up to 4.7%. On the other hand, the SP phase exhibits increasing disorder as x increases, as shown from the broadening of the SP excitations as well as the dimer reflection peaks.

cond-mat.str-el↗

arXiv1996

Energy gap in superconducting fullerides: optical and tunneling studies

Tunneling and optical transmission studies have been performed on superconducting samples of Rb3C60. At temperatures much below the superconducting transition temperature Tc the energy gap is 2 Delta=5.2 +- 0.2meV, corresponding to 2 Delta/kB Tc = 4.2. The low temperature density of states, and the temperature dependence of the optical conductivity resembles the BCS behavior, although there is an enhanced ``normal state" contribution. The results indicate that this fulleride material is an s-wave superconductor, but the superconductivity cannot be described in the weak coupling limit.

cond-mat↗

arXiv1996

Motional diminishing of optical activity: a novel method for studying molecular dynamics in liquids and plastic crystals

Molecular dynamics calculations and optical spectroscopy measurements of weakly active infrared modes are reported. The results are qualitatively understood in terms of the "motional diminishing" of IR lines, a process analogous to the motional narrowing of a nuclear magnetic resonance (NMR) signal. In molecular solids or liquids where the appropriate intramolecular resonances are observable, motional diminishing can be used to study the fluctuations of the intermolecular interactions having time scales of 1psec to 100psec.

chem-ph↗

arXiv1996

New phase diagram of Zn-doped CuGeO3

A series of high-quality single crystals of Cu$_{1-x}$Zn$_x$GeO$_3$ have been examined by neutron scattering techniques. An antiferromagnetic (AF) ordering is confirmed for samples with $x\geq 0.02$, in complete agreement with previous reports. We show that the spin-Peierls (SP) phase transition persists to 6\% Zn, whereas previous magnetic susceptibility measurements reported a deterioration of the SP transition above 2\% Zn. We present some details of the successive transitions upon lowering temperature into the spin-Peierls phase which is followed by a transition into an antiferromagnetically ordered phase. Below the Néel temperature we observe the coexistence of the SP lattice dimerization and AF states.

cond-mat↗

arXiv1996

Temperature Dependence of the Spin-Peierls Energy Gap and Anomalous Line Shapes in CuGeO$_3$

Neutron scattering measurements on a large single crystal of CuGeO$_3$ have been used to determine the temperature-dependence of the spin-Peierls energy gap. While the power law behavior of the intensity of structural superlattice peaks is well fit by $I({\rm T})\propto ({\rm T}_c - {\rm T})^{2β}$ with an exponent of $β=0.33$, the exponent for the temperature dependence of the energy gap is significantly smaller than expected for conventional spin-Peierls materials. Usual scaling relations relate the energy gap to the superlattice reflection intensity as $Δ({\rm T}) \propto I^a$ with $a=1/3$; the present results suggest an exponent of $a\approx 1/6$ for CuGeO$_3$. Additional scattering cross-section is observed in constant-$q$ and constant-$E$ scans creating a long `tail' extending to higher energies relating to a proposed scattering continuum.

cond-mat↗

arXiv1995

Magnetism and Structural Distortion in the La0.7Sr0.3MnO3 Metallic Ferromagnet

Neutron scattering studies on a single crystal of the highly-correlated electron system, La1-xSrxMnO3 with x~0.3, have been carried out elucidating both the spin and lattice dynamics of this metallic ferromagnet. We report a large measured value of the spin wave stiffness constant, which directly shows that the electron transfer energy of the d band is large. The spin dynamics, including magnetic critical scattering, demonstrate that this material behaves similar to other typical metallic ferromagnets such as Fe or Ni. The crystal structure is rhombohedral, as previously reported, for all temperatures studied (below ~425K). We have observed new superlattice peaks which show that the primary rhombohedral lattice distortion arises from oxygen octahedra rotations resulting in an R-3c structure. The superlattice reflection intensities which are very sensitive to structural changes are independent of temperature demonstrating that there is no primary lattice distortion anomaly at the magnetic transition temperature, Tc = 378.1 K, however there is a lattice contraction.

cond-mat↗

arXiv1995

Polymeric alkali fullerides are stable in air

Infrared transmission, electron spin resonance, and X-ray diffraction measurements show unambiguously that RbC$_{60}$ and KC$_{60}$ are stable in air, in contrast to Rb$_{6}$C$_{60}$ which decomposes rapidly upon exposure. The specimens studied transform into pure C$_{60}$ and other byproducts when heated above $100\dd $C, approximately the temperature of the orthorhombic-fcc phase transition. The stability of these compounds raises the possibility of applying them as protective layers for the superconducting fullerides.

cond-mat↗

arXiv1994

Vibrational Study of 13C-enriched C60 Crystals

The infrared (IR) spectrum of solid C60 exhibits many weak vibrational modes. Symmetry breaking due to 13C isotopes provides a possible route for optically activating IR-silent vibrational modes. Experimental spectra and a semi-empirical theory on natural abundance and 13C-enriched single crystals of C60 are presented. By comparing the experimental results with the theoretical results, we exclude this isotopic activation mechanism from the explanation for weakly active fundamentals in the spectra.

cond-mat↗

arXiv1994

Infrared and Raman Evidence for Dimers and Polymers in Rb1C60

The infrared- and Raman-active vibrational modes of C60 were measured in the various structural states of Rb1C60. According to earlier studies, Rb1C60 has an $fcc$ structure at temperatures above ~100C, a linear chain polymer orthorhombic structure when slowly cooled, and an as yet undetermined structure when very rapidly cooled ("quenched"). We show that the spectra obtained in the polymer state are consistent with each C60 molecule having bonds to two diametrically opposite neighbors. In the quenched state, we find evidence for further symmetry breaking, implying a lower symmetry structure than the polymer state. The spectroscopic data of the quenched phase are shown to be consistent with Rb2(C60)2, a dimerization of C60.

cond-mat↗