High-temperature setup for infrared reflection spectroscopy.

We have designed an oven for optical reflection measurements at temperatures as high as 1000 K. The compact setup can be attached to any Fourier-transform infrared spectrometer using a microscope. The details of the layout, operation, and performance are discussed as well as ways for reference measurements and data correction.

Tunable sub-terahertz resonance absorption in high-coercivity magnetodielectric ceramics.

Recently, giant coercivities (20-42 kOe) and sub-terahertz natural ferromagnetic resonance (NFMR) at 100-300 GHz were observed for single-domain M-type hexaferrite particles with high aluminum substitution. Herein, we fabricated dense ceramics of SrCaFeAlO and, for the first time, investigated their magnetostatic and magnetodynamic properties in the temperature range of 5-300 K. It was shown that dense ceramics maintain their high magnetic hardness (a coercivity of 10-20 kOe) and NFMR frequencies of 140-200 GHz durably in the entire temperature range.

Radio frequency dielectric measurements in diamond anvil cells.

We present the modifications, performance, and test of a diamond anvil cell for radio frequency dielectric spectroscopy studies of single crystals that can be used from room temperature down to 4 K and up to pressures of 5-6 GPa. Continuous frequency-dependent measurements between 5 Hz and 1 MHz can be performed with this modified pressure cell. The cell has an excellent performance with temperature-, frequency-, and pressure-independent stray capacitance of around 2 pF, enabling us to use relatively small samples with a weak dielectric response.

Fingerprints of Critical Phenomena in a Quantum Paraelectric Ensemble of Nanoconfined Water Molecules.

We have studied the radio frequency dielectric response of a system consisting of separate polar water molecules periodically arranged in nanocages formed by the crystal lattice of the gemstone beryl. Below = 20-30 K, quantum effects start to dominate the properties of the electric dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature-dependent real dielectric permittivity ε'(). When analyzing in detail the temperature evolution of the reciprocal permittivity (ε') down to ≈ 0.

Gapped magnetic ground state in quantum spin liquid candidate κ-(BEDT-TTF)Cu(CN).

Geometrical frustration, quantum entanglement, and disorder may prevent long-range ordering of localized spins with strong exchange interactions, resulting in an exotic state of matter. κ-(BEDT-TTF)Cu(CN) is considered the prime candidate for this elusive quantum spin liquid state, but its ground-state properties remain puzzling. We present a multifrequency electron spin resonance (ESR) study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at 6 kelvin.

Rise and fall of Landau's quasiparticles while approaching the Mott transition.

Landau suggested that the low-temperature properties of metals can be understood in terms of long-lived quasiparticles with all complex interactions included in Fermi-liquid parameters, such as the effective mass m. Despite its wide applicability, electronic transport in bad or strange metals and unconventional superconductors is controversially discussed towards a possible collapse of the quasiparticle concept. Here we explore the electrodynamic response of correlated metals at half filling for varying correlation strength upon approaching a Mott insulator.

Rotational coherence of encapsulated ortho and para water in fullerene-C revealed by time-domain terahertz spectroscopy.

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C cages by time-domain terahertz (THz) spectroscopy. We employ single-cycle THz pulses to excite the low-frequency rotational motion of water and measure the subsequent coherent emission of electromagnetic waves by water molecules. At temperatures below ~ 100 K, C lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps.

Revealing excess protons in the infrared spectrum of liquid water.

The most common species in liquid water, next to neutral [Formula: see text] molecules, are the [Formula: see text] and [Formula: see text] ions. In a dynamic picture, their exact concentrations depend on the time scale at which these are probed. Here, using a spectral-weight analysis, we experimentally resolve the fingerprints of the elusive fluctuations-born short-living [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text] ions in the IR spectra of light ([Formula: see text]), heavy ([Formula: see text]), and semi-heavy (HDO) water.

Microwave probing of bulk dielectrics using superconducting coplanar resonators in distant-flip-chip geometry.

Dielectric measurements on insulating materials at cryogenic temperatures can be challenging, depending on the frequency and temperature ranges of interest. We present a technique to study the dielectric properties of bulk dielectrics at GHz frequencies. A superconducting coplanar Nb resonator is deposited directly on the material of interest, and this resonator is then probed in distant-flip-chip geometry with a microwave feedline on a separate chip.

Two Linear Regimes in Optical Conductivity of a Type-I Weyl Semimetal: The Case of Elemental Tellurium.

Employing high-pressure infrared spectroscopy we unveil the Weyl semimetal phase of elemental Te and its topological properties. The linear frequency dependence of the optical conductivity provides clear evidence for metallization of trigonal tellurium (Te-I) and the linear band dispersion above 3.0 GPa.

Cryogenic frequency-domain electron spin resonance spectrometer based on coplanar waveguides and field modulation.

We present an instrument to perform frequency-domain electron spin resonance experiments that is based on coplanar waveguides and field modulation. A large parameter space in frequency (up to 25 GHz), magnetic field (up to 8 T), and temperature (down to 1.6 K) is accessible.

Revealing the Intrinsic Electronic Structure of 3D Half-Heusler Thermoelectric Materials by Angle-Resolved Photoemission Spectroscopy.

Accurate determination of the intrinsic electronic structure of thermoelectric materials is a prerequisite for utilizing an electronic band engineering strategy to improve their thermoelectric performance. Herein, with high-resolution angle-resolved photoemission spectroscopy (ARPES), the intrinsic electronic structure of the 3D half-Heusler thermoelectric material ZrNiSn is revealed. An unexpectedly large intrinsic bandgap is directly observed by ARPES and is further confirmed by electrical and optical measurements and first-principles calculations.

Characterizing dielectric properties of ultra-thin films using superconducting coplanar microwave resonators.

We present an experimental approach for cryogenic dielectric measurements on ultrathin insulating films. Based on a coplanar microwave waveguide design, we implement superconducting quarter-wave resonators with inductive coupling, which allows us to determine the real part ε of the dielectric function at gigahertz frequencies and sample thicknesses down to a few nanometers. We perform simulations to optimize resonator coupling and sensitivity, and we demonstrate the possibility to quantify ε with a conformal mapping technique in a wide sample-thickness and ε-regime.

CMOS-Compatible Antimony-Doped Germanium Epilayers for Mid-Infrared Low-Loss High-Plasma-Frequency Plasmonics.

Antimony (Sb) heavily-doped germanium (Ge)-on-silicon (Si) epitaxial films are investigated as mid-infrared (MIR) plasmonic materials. Structural, electrical, and optical properties have been improved by proper choice of dopant species (i.e.

Role of non-linear effects and standing waves in microwave spectroscopy: Corbino measurements on superconductors and VO.

Broadband microwave spectroscopy can probe material properties in wide spectral and temperature ranges. The quality of such measurements crucially depends on the calibration, which also removes from the obtained spectra signatures of standing waves. Here we consider cryogenic Corbino-type reflection measurements on superconductors close to the critical temperature.

Charge localization in 1D tetramerized organic conductors: the special case of (tTTF)ClO.

We report a detailed structural and spectroscopic study of the 1D 2:1 cation radical salt (tTTF)ClO, where tTTF  =  trimethylenetetrathiafulvalene, which exhibits a semiconductor-semiconductor phase transition at ca. T  =  137 K. Crystal structures are determined above and below the transition; the tTTF molecules in stacks are grouped into weakly interacting tetramers.

Single-Gap Superconductivity and Dome of Superfluid Density in Nb-Doped SrTiO_{3}.

SrTiO_{3} exhibits a superconducting dome upon doping with Nb, with a maximum critical temperature T_{c}≈0.4  K. Using microwave stripline resonators at frequencies from 2 to 23 GHz and temperatures down to 0.

Electrodynamics of quantum spin liquids.

Quantum spin liquids attract great interest due to their exceptional magnetic properties characterized by the absence of long-range order down to low temperatures despite the strong magnetic interaction. Commonly, these compounds are strongly correlated electron systems, and their electrodynamic response is governed by the Mott gap in the excitation spectrum. Here we summarize and discuss the optical properties of several two-dimensional quantum spin liquid candidates.

On the depolarization in granular thin films: a Mueller-matrix approach.

We describe a general method to disclose the information hidden in Mueller matrices experimentally obtained from depolarizing samples. Although spectroscopic Mueller-matrix ellipsometry allows for a model-free characterization of inhomogeneous samples, i.e.

Large-Area Two-Dimensional Plasmonic Meta-Glasses and Meta-Crystals: a Comparative Study.

The geometrical arrangement of metallic nanoparticles plays a crucial role on the optical response of nanoplasmonic samples due to particle-particle interactions. In this work, large-area, two-dimensional meta-glasses (random arrangements) and meta-crystals (periodic arrangements) made of identical metallic nanoparticles are investigated for three different particle densities of 5, 10, and 15 discs/μm. A direct comparison between random and periodically ordered arrays is presented.

The metal-insulator transition in the organic conductor β-(BEDT-TTF)Hg(SCN)Cl.

We explore the nature of the metal-insulator transition in the two-dimensional organic compound β-(BEDT-TTF)Hg(SCN)Cl by x-ray, electrical transport, ESR, Raman, and infrared investigations. Magnetic and vibrational spectroscopy concurrently reveal a gradual dimerization along the stacking direction (a-b), setting in already at the crossover temperature of 150 K from the metallic to the insulating state. A spin gap of Δ=47 meV is extracted.

Physical interpretation of Mueller matrix spectra: a versatile method applied to gold gratings.

The interaction of nanostructures, periodic or random, with polarized light creates very rich physics where scattering, diffraction and absorbance are linked to a variety of dispersive modes and coupling effects. Each of these excitations depends strongly on polarization, angle of incidence, azimuthal orientation of the sample and wavelength. The entire optical response can be obtained, independently from any model, by measuring the Mueller matrices at various k-vectors over a broad frequency range.

Pressure-dependent optical investigations of Fabre salts in the charge-ordered state.

In a comprehensive infrared study, the molecular vibrational features of (TMTTF)SbF, (TMTTF)AsF and (TMTTF)PF single crystals have been measured down to temperatures as low as 7 K by applying hydrostatic pressure up to 11 kbar. We follow the charge disproportionation below the critical temperatures T as pressure increases, and determine the critical pressure values p at which the charge-ordered phase is suppressed. The coexistence of the spin-Peierls phase with charge order is explored at low temperatures, and the competition of these two phases is observed.

Depolarization of a randomly distributed plasmonic meander metasurface characterized by Mueller matrix spectroscopic ellipsometry.

Metallic nanostructures offer efficient solutions in polarization control with a very low thickness. In this report, we investigate the optical properties of a nano-fabricated plasmonic pseudo-depolarizer using Mueller matrix spectroscopic ellipsometry in transmission configuration. The depolarizer is composed of 256 square cells, each containing a periodically corrugated metallic film with random orientation.