arXiv subjects

R. Valdés Aguilar

Publications and source records attributed to R. Valdés Aguilar.

At least 19 recordsLinked to original sources

arXiv2024

Spin-Orbital-Lattice Coupling and the Phonon Zeeman Effect in the Dirac Honeycomb Magnet CoTiO$_3$

The entanglement of electronic spin and orbital degrees of freedom is often the precursor to emergent behaviors in condensed matter systems. With considerable spin-orbit coupling strength, the cobalt atom on a honeycomb lattice offers a platform that can make accessible the study of novel magnetic ground states. Using temperature-dependent Raman spectroscopy and high-magnetic field Raman and infrared (IR) spectroscopy, we studied the lattice and spin-orbital excitations in CoTiO$_3$, an antiferromagnetic material that exhibits topologically protected magnon Dirac crossings in the Brillouin zone. Under the application of an external magnetic field up to 22 T along the crystal's $c$-axis, we observed the splitting of both the spin-orbital excitations and a phonon nearby in energy. Using density functional theory (DFT), we identify a number of new modes that below the antiferromagnetic (AFM) transition become Raman-active due to the zone-folding of the Brillouin zone caused by the doubling of the magnetic unit cell. We use a model that includes both the spin and orbital degrees of freedom of the Co$^{2+}$ ions to explain the spin-orbital excitation energies and their behavior in an applied field. Our experimental observations along with several deviations from the model behavior point to significant coupling between the spin-orbital and the lattice excitations.

cond-mat.str-el↗

arXiv2024

Ring-Exchange Interaction Effects on Magnons in Dirac Magnet CoTiO$_3$

The magnetic interactions that determine magnetic order and magnon energies typically involve only two spins. While rare, multi-spin interactions can also appear in quantum magnets and be the driving force in the ground state selection and in the nature of its excitations. By performing time-domain terahertz and magneto-Raman spectroscopy measurements combined with theoretical modeling, we determine the origin of the magnon excitation gap in Dirac antiferromagnet CoTiO$_3$. By adding a ring-exchange interaction in a hexagonal plaquette of the honeycomb lattice to both an XXZ spin model and to a low energy spin-orbital flavor wave model, a gap is generated in the magnon spectrum at the Brillouin zone center. With this addition, the flavor wave model reproduces a large swath of experimental results including terahertz, Raman, inelastic neutron scattering, and magnetization experiments.

cond-mat.str-el↗

arXiv2021

Numerical Simulation of the Coupling between Split-Ring Resonators and Antiferromagnetic Magnons

We report on the results of simulations of the terahertz response of a split ring resonator (SRR) metamaterial coupled to a hypothetical antiferromagnetic material characterized by a magnon resonance. The simulations were done using finite difference time domain (FDTD) techniques. By adjusting the magnon frequency we find a hybridization of the resonant normal modes of the SRR and the magnon manifested as an avoided crossing. By varying the physical separation between the metamaterial and the antiferromagnet with a dielectric spacer, we evaluated the coupling strength between the two.

cond-mat.other↗

arXiv2019

Broadband Circular Polarization Time-Domain Terahertz Spectroscopy

Light-matter interactions are key in providing fundamental information about materials. Even in the linear-response regime, the spectroscopic response of a material encodes in it many properties of the ground state as well as of its excitations. This knowledge has been critical in our understanding of novel quantum materials, and the further improvement and extensions of linear spectroscopy will continue to be key in the exploration of novel states of matter. We report the development of broadband circular polarization spectroscopy in the terahertz range of the electromagnetic spectrum. We take advantage of a recent design of a broadband quarter wave plate, based on the Fresnel rhomb concept, and use it in conjunction with a polarization modulation technique to provide direct information of the response of a material to circularly polarized THz radiation; a new capability shown here for the first time. As an example of this technique we study the cyclotron resonance of a 2D electron gas from a AlGaAs-GaAs quantum well. We demonstrate the unique advantages that this technique will bring in the study of novel quantum materials.

cond-mat.mtrl-sci↗

arXiv2018

Size and Periodicity Effects on Terahertz Properties of Gammadion Metamaterials

We studied the effects on the terahertz transmission of gammadion metamatials by adjusting the size and periodicity of the metamaterial, while keeping the periodicity and size fixed, respectively. The terahertz transmission responses of these metamaterials were analyzed using terahertz time domain spectroscopy as well as finite difference time domain electromagnetic simulations. We have found that increasing size or periodicity results in a red shift of the resonant frequency, and that increasing size or decreasing periodicity results in the formation of a second resonant frequency. We associate these effects with the changing lengths of the microstrip lines that make up the metamaterial, as well as the changing strength of nearest-neighbor coupling between gammadions.

physics.optics↗

arXiv2017

Corrective Re-gridding Techniques for Non-Uniform Sampling in Time Domain Terahertz Spectroscopy

Time domain terahertz spectroscopy typically uses mechanical delay stages that inherently suffer from non-uniform sampling positions. We review, simulate, and experimentally test the ability of corrective cubic spline and Shannon re-gridding algorithms to mitigate the inherent sampling position noise. We present simulations and experimental results that show re-gridding algorithms can increase the signal to noise ratio within the frequency range of 100 GHz to 2 THz. We also predict that re-gridding corrections will become increasingly important to both spectroscopy and imaging as THz technology continues to improve and higher frequencies become experimentally accessible.

physics.optics↗

arXiv2017

Terahertz Dielectric Analysis and Spin-Phonon Coupling in Multiferroic GeV$_4$S$_8$

We present an investigation of the multiferroic lacunar spinel compound GeV$_4$S$_8$ using time-domain terahertz spectroscopy. We find three absorptions which either appear or shift at the antiferromagnetic transition temperature, T$_N = 17$ K, as S=1 magnetic moments develop on vanadium tetrahedra. Two of these absorptions are coupled to the magnetic state and one only appears below the Néel temperature, and is interpreted as a magnon. We also observe isosbestic points in the dielectric constant in both the temperature and frequency domains. Further, we perform analysis on the isosbestic features to reveal an interesting collapse into a single curve as a function of both frequency and temperature, behavior which exists throughout the phase transitions. This analysis suggests the importance of spectral changes in the terahertz range which are linear in frequency and temperature.

cond-mat.str-el↗

arXiv2016

Terahertz Spin-Orbital Excitations in the paramagnetic state of multiferroic Sr$_2$FeSi$_2$O$_7$

We studied the novel multiferroic material Sr$_2$FeSi$_2$O$_7$, and found 3 absorption modes above the magnetic ordering transition temperature using time-domain terahertz spectroscopy. These absorption modes can be explained as the optical transitions between the spin-orbit coupling and crystal field split 3d$^6$ Fe$^{2+}$ ground state term in this material. Consideration of the compressed tetrahedral environment of the Fe$^{2+}$ site is crucial to understand the excitations. We point out, however, discrepancies between the single-site atomic picture and the experimental results.

cond-mat.str-el↗

arXiv2016

High-Resolution Faraday Rotation and Electron-Phonon Coupling in Surface States of the Bulk-Insulating Topological Insulator Cu$_{0.02}$Bi$_2$Se$_3$

We have utilized time-domain magneto-terahertz spectroscopy to investigate the low frequency optical response of topological insulator Cu$_{0.02}$Bi$_2$Se$_3$ and Bi$_2$Se$_3$ films. With both field and frequency dependence, such experiments give sufficient information to measure the mobility and carrier density of multiple conduction channels simultaneously. We observe sharp cyclotron resonances (CRs) in both materials. The small amount of Cu incorporated into the Cu$_{0.02}$Bi$_2$Se$_3$ induces a true bulk insulator with only a \textit{single} type of conduction with total sheet carrier density $\sim4.9\times10^{12}/$cm$^{2}$ and mobility as high as 4000 cm$^{2}/$V$\cdot$s. This is consistent with conduction from two virtually identical topological surface states (TSSs) on top and bottom of the film with a chemical potential $\sim$145 meV above the Dirac point and in the bulk gap. The CR broadens at high fields, an effect that we attribute to an electron-phonon interaction. This assignment is supported by an extended Drude model analysis of the zero field Drude conductance. In contrast, in normal Bi$_2$Se$_3$ films two conduction channels were observed and we developed a self-consistent analysis method to distinguish the dominant TSSs and coexisting trivial bulk/2DEG states. Our high-resolution Faraday rotation spectroscopy on Cu$_{0.02}$Bi$_2$Se$_3$ paves the way for the observation of quantized Faraday rotation under experimentally achievable conditions to push chemical potential in the lowest Landau Level.

cond-mat.mtrl-sci↗

arXiv2014

Time-resolved terahertz dynamics in thin films of the topological insulator Bi$_{2}$Se$_3$

We use optical pump--THz probe spectroscopy at low temperatures to study the hot carrier response in thin Bi$_2$Se$_3$ films of several thicknesses, allowing us to separate the bulk from the surface transient response. We find that for thinner films the photoexcitation changes the transport scattering rate and reduces the THz conductivity, which relaxes within 10 picoseconds (ps). For thicker films, the conductivity increases upon photoexcitation and scales with increasing both the film thickness and the optical fluence, with a decay time of approximately 5 ps as well as a much higher scattering rate. These different dynamics are attributed to the surface and bulk electrons, respectively, and demonstrate that long-lived mobile surface photo-carriers can be accessed independently below certain film thicknesses for possible optoelectronic applications.

cond-mat.mes-hall↗

arXiv2014

A hierarchy of bound states in the 1D ferromagnetic Ising chain CoNb$_2$O$_6$ investigated by high resolution time-domain terahertz spectroscopy

Kink bound states in the one dimensional ferromagnetic Ising chain compound CoNb$_2$O$_6$ have been studied using high resolution time-domain terahertz spectroscopy in zero applied magnetic field. When magnetic order develops at low temperature, nine bound states of kinks become visible. Their energies can be modeled exceedingly well by the Airy function solutions to a 1D Schrödinger equation with a linear confining potential. This sequence of bound states terminates at a threshold energy near two times the energy of the lowest bound state. Above this energy scale we observe a broad feature consistent with the onset of the two particle continuum. At energies just below this threshold we observe a prominent excitation that we interpret as a novel bound state of bound states -- two pairs of kinks on neighboring chains.

cond-mat.str-el↗

arXiv2013

A sudden collapse in the transport lifetime across the topological phase transition in $(Bi_{1-x}In_x)_2 Se_3$

Topological insulators (TIs) are newly discovered states of matter with robust metallic surface states protected by the topological properties of the bulk wavefunctions. A quantum phase transition (QPT) from a TI to a conventional insulator and a change in topological class can only occur when the bulk band gap closes. In this work, we have utilized time-domain terahertz spectroscopy (TDTS) to investigate the low frequency conductance in (Bi$_{1-x}$In$_x$)$_2$Se$_3$ as we tune through this transition by indium substitution. Above certain substitution levels we observe a collapse in the transport lifetime that indicates the destruction of the topological phase. We associate this effect with the threshold where states from opposite surfaces hybridize. The substitution level of the threshold is thickness dependent and only asymptotically approaches the bulk limit $x \approx 0.06$ where a maximum in the mid-infrared absorption is exhibited. This absorption can be identified with the bulk band gap closing and a change in topological class. The correlation length associated with the QPT appears as the evanescent length of the surface states. The observation of the thickness-dependent collapse of the transport lifetime shows the unusual role that finite size effects play in this topological QPT.

cond-mat.str-el↗

arXiv2012

Polarization modulation time-domain terahertz polarimetry

We present high precision measurements of polarization rotations in the frequency range from 0.1 to 2.5 THz using a polarization modulation technique. A motorized stage rotates a polarizer at ~80 Hz, and the resulting modulation of the polarization is measured by a lock-in technique. We achieve an accuracy of 0.05° (900 μrad) and a precision of 0.02° (350 μrad) for small rotation angles. A detailed mathematical description of the technique is presented, showing its ability to fully characterize elliptical polarizations from 0.1 to 2.5 THz.

physics.optics↗

arXiv2012

Aging and reduced bulk conductance in thin films of the topological insulator Bi$_2$Se$_3$

We report on the effect of exposure to atmospheric conditions on the THz conductivity of thin films of the topological insulator Bi$_2$Se$_3$. We find: 1) two contributions of mobile charge carriers to the THz conductivity immediately after growth, and 2) the spectral weight of the smaller of these decays significantly over a period of several days as the film is exposed to ambient conditions, while the other remains relatively constant. We associate the former with a bulk response, and the latter with the surface. The surface response exhibits the expected robustness of the carriers from 2D topological surface states. We find no evidence for a third spectral feature derived from topologically trivial surface states.

cond-mat.str-el↗

arXiv2012

Low energy electrodynamics of the Kondo-lattice antiferromagnet CeCu$_2$Ge$_2$

We present time-domain THz spectroscopy data of a thin film of the Kondo-lattice antiferromagnet CeCu$_2$Ge$_2$. The low frequency complex conductivity has been obtained down to temperatures below the onset of magnetic order. At low temperatures a narrow Drude-like peak forms, which is similar to ones found in other heavy fermion compounds that do not exhibit magnetic order. Using this data in conjunction with DC resistivity measurements, we obtain the frequency dependence of the scattering rate and effective mass through an extended Drude model analysis. The zero frequency limit of this analysis yields evidence for large mass renormalization even in the magnetic state, the scale of which agrees closely with that obtained from thermodynamic measurements.

cond-mat.str-el↗

arXiv2012

Broadband microwave and time-domain terahertz spectroscopy of CVD grown graphene

We report a study of the complex AC impedance of CVD grown graphene. We measure the explicit frequency dependence of the complex impedance and conductance over the microwave and terahertz range of frequencies using our recently developed broadband microwave Corbino and time domain terahertz spectrometers (TDTS). We demonstrate how one may resolve a number of technical difficulties in measuring the intrinsic impedance of the graphene layer that this frequency range presents, such as distinguishing contributions to the impedance from the substrate. From our microwave measurements, the AC impedance has little dependance on temperature and frequency down to liquid helium temperatures. The small contribution to the imaginary impedance comes from either a remaining residual contribution from the substrate or a small deviation of the conductance from the Drude form.

cond-mat.mtrl-sci↗

arXiv2011

Temporal correlations of superconductivity above the transition temperature in La$_{2-x}$Sr$_x$CuO$_4$ probed by terahertz spectroscopy

The nature of the underdoped pseudogap regime of the high-temperature superconductors has been a matter of long-term debate. On quite general grounds, one expects that due to their low superfluid densities and short correlation lengths, superconducting fluctuations will be significant for transport and thermodynamic properties in this part of the phase diagram. Although there is ample experimental evidence for such correlations, there has been disagreement about how high in temperature they may persist, their role in the phenomenology of the pseudogap, and their significance for understanding high-temperature superconductivity. In this work we use THz time-domain spectroscopy (TTDS) to probe the temporal fluctuations of superconductivity above the critical temperature (T$_c$) in La$_{2-x}$Sr$_x$CuO$_4$ thin films over a doping range that spans almost the entire superconducting dome (x = 0.09 to 0.25). Signatures of the fluctuations persist in the conductivity in a comparatively narrow temperature range, at most 16 K above Tc. Our measurements show that superconducting correlations do not make an appreciable contribution to the charge transport anomalies of the pseudogap in LSCO at temperatures well above T$_c$.

cond-mat.supr-con↗

arXiv2011

THz response and colossal Kerr rotation from the surface states of the topological insulator Bi$_2$Se$_3$

Ordered states of matter are typically categorized by their broken symmetries. With the ordering of spins in a ferromagnet or the freezing of a liquid into a solid, the loss of symmetry distinguishes the ordered state from the disordered one. In contrast, topological states are distinguished by specific \textit{topological} properties that are encoded in their quantum mechanical wavefunctions. Frequently, a consequence of these properties is that there are robust "topologically protected" states on the sample's boundaries. The edge states of the quantum Hall effect (QHE) are the classic example. In the last few years, it was realized that another class of such topological matter may exist in 3D band insulators with large spin-orbit interaction. These so-called topological insulators are predicted to host robust surface states, which exhibit a number of interesting properties including spin helicity, immunity to back-scattering, and weak \textit{anti}--localization. There are predictions of a number of unusual phenomena associated with these surface states, including a proximity-effect-induced exotic superconducting state with Majorana fermions bound to a vortex and an `axion' electromagnetic response, and proposals for applications, such as their use in terahertz devices. In this Letter we report the observation of an unprecedentedly large rotation of the polarization plane of linearly polarized light reflected from thin films of the topological insulator Bi$_2$Se$_3$ in magnetic fields. This Kerr rotation can be as large as 65$^\circ$ and is due almost entirely to the surface states. Our results are evidence for the intrinsic response of the topologically protected surface states and provide a benchmark for the large magneto-electric effect predicted for these materials.

cond-mat.str-el↗