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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

Context: Recently developed concepts for higher efficacy ESWL with low-pressure wide focus systems resulting in finer fragmentation of the calculi. Objective: To compare two different electromagnet...

The vibrational anomalies of pentacene molecules have been investigated in conjunction with the high-energy excitonic features. Self-trapped excitons have been distinguished from the others.

Geometrical frustration, quantum entanglement and disorder may prevent long-range order of localized spins with strong exchange interactions, resulting in a novel state of matter. $\kappa$-(BEDT-TTF)$_2$-Cu$_2$(CN)$_3$ is considered the best approximation of this elusive quantum-spin-liquid state, but its ground-state properties remain puzzling. Here we present a multi-frequency electron-spin resonance study down to millikelvin temperatures, revealing a rapid drop of the spin susceptibility at $T^*=6\,\mathrm{K}$. This opening of a spin gap, accompanied by structural modifications, suggests the enigmatic `$6\,\mathrm{K}$-anomaly' as the transition to a valence-bond-solid ground state. We identify an impurity contribution that becomes dominant when the intrinsic spins form singlets. Only probing the electrons directly manifests the pivotal role of defects for the low-energy properties of quantum-spin systems without magnetic order., Comment: 34 pages, 17 figures

The electronic properties of molecular conductors can be readily varied via physical or chemical pressure as it enlarges the bandwidth W. This enables them to cross the Mott insulator-to-metal phase transition by reducing electronic correlations U/W. Here we introduce an alternative path by spatially expanding the molecular orbitals when partially replacing sulfur by selenium in the constituting bis-(ethylenedithio)-tetrathiafulvalene (BEDT-TTF) molecules of the title compound. We characterize how the insulating quantum-spin-liquid state is tuned via a Mott transition to the metallic Fermi-liquid state crossing a narrow region of superconductivity. The transport, dielectric, and optical measurements reveal that at this first-order phase transition, metallic regions coexist in the insulating matrix leading to pronounced percolative effects, which are most obvious in the strong enhancement of the dielectric constant at low temperatures.

The n-type organic semiconductor, β-phase single crystalline dichloro naphthalene diimide, Cl2–NDI, is investigated in a broad frequency range via optical spectroscopy. The temperature-dependent ab...

Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexplored regions. Charge ordered phases present in many compounds of the $${\hbox {(TMTTF)}}_2X$$ (TMTTF) 2 X family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor $${\hbox {(TMTTF)}}_2 {\hbox {NO}}_3$$ (TMTTF) 2 NO 3 . We found that the $${\hbox {(TMTTF)}}_2 {\hbox {NO}}_3$$ (TMTTF) 2 NO 3 compound develops a peculiar anisotropy with a doubled periodicity ($$ab'$$ a b ′ -plane) of the ESR linewidth below about $$T_{\text {CO}}= ({250\pm 10})~\hbox {K}$$ T CO = ( 250 ± 10 ) K . This behavior is similar to observations in the related compounds $${\hbox {(TMTTF)}}_2X$$ (TMTTF) 2 X ($$X = {\hbox {PF}}_6$$ X = PF 6 , $${\hbox {SbF}}_6$$ SbF 6 and $${\hbox {AsF}}_6$$ AsF 6 ), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous $${\hbox {(TMTTF)}}_2 {\hbox {ClO}}_4$$ (TMTTF) 2 ClO 4 , known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below $$T_{\text {CO}} \approx 250~\hbox {K}$$ T CO ≈ 250 K .

Time-domain magneto-THz spectroscopy is utilized to study the frustrated magnet averievite Cu$_{5-x}$Zn$_x$V$_2$O$_{10}$(CsCl). Pronounced THz resonances are observed in unsubstituted samples ($x=0$) when cooling below the onset of short-range magnetic correlations. The influence of external magnetic effects confirms the magnetic origin of these resonances. Increasing Zn substitution suppresses the resonances, as frustration effects dominate, reflecting the non-magnetic phases for $x> 0.25$ compounds. The temperature evolution of the THz spectra is complemented with electron spin resonance spectroscopy. This comparison allows a direct probe of the different contributions from magnetic order, frustration, and structural properties in the phase diagram of averievite. Our results illustrate the effect of magnetic interactions in THz spectra of frustrated magnets., Comment: 7 pages, 5 figures, plus supplement

Due to the small photon momentum, optical spectroscopy commonly probes magnetic excitations only at the center of the Brillouin zone; however, there are ways to override this restriction. In the case of the distorted kagome quantum magnet Y-kapellasite, Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$, under scrutiny here, the magnon density of states can be accessed over the entire Brillouin zone through three-center magnon excitations. This mechanism is aided by the three different magnetic sublattices and strong short-range correlations in the distorted kagome lattice. The results of THz time-domain experiments agree remarkably well with linear spin-wave theory. Relaxing the conventional zone-center constraint of photons gives a new aspect to probe magnetism in matter., Comment: 22 pages, 5 figures, plus supplement, accepted in Adv. Quantum Technol

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.

In order to understand the physical properties of the series of organic conductors $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]$X$ with $X$ = Cl, Br, and I, not only electronic correlations but also the effect of disorder has to be taken into account. While for Cl- and Br-containing salts the influence of both parameters were investigated and a universal phase diagram was proposed, the position of $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I is still not settled. Here we have conducted transport, infrared, and dielectric measurements on single crystals of the title compound to clarify its electronic state at low temperatures. The correlation strength was determined as $U/W \approx 2.2$; thus this salt is placed deeper in an insulating state compare to the two sister compounds. We found that inherent disorder leads to a Coulomb localized insulating state similar to the moderately x-ray-irradiated $\kappa$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl.

Time-domain terahertz and Fourier-transform infrared spectroscopy techniques are employed for investigating of the low-frequency dynamics of H 2 O@C 60 endofullerene at temperatures 6–300 K. A number of absorption lines associated with rotational transitions of water molecules entrapped inside C 60 cage are observed and assigned. Fitting absorption lines with Lorentzians allowed us to obtain the temperature dependences of the lines’ parameters – frequency position, strength and damping.

A THz vortex beam describes light with a larger than ordinary total angular momentum. Increased total angular momentum modifies the optical selection rules, opening a new territory of material research. We discuss the basic concept of vortex beams and a possible approach for future THz studies. The experiments on Landau level transitions will be given as an example.

The authors find that short-range charge correlations in prototypical electronic ferroelectrics show ultrafast enhancement upon photoexcitation, in contrast to the photoinduced melting of the long-range charge order.

The two-dimensional organic superconductor $\ensuremath{\lambda}\ensuremath{-}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ exhibits pronounced charge fluctuations below $T\ensuremath{\approx}150$ K, in contrast to the sibling compound $\ensuremath{\kappa}\ensuremath{-}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ that remains metallic down to milli-Kelvin. Infrared spectroscopy reveals only minor splitting in the vibrational features of the latter compound, common to other strongly dimerized $\ensuremath{\kappa}$-type salts. When the organic molecules are arranged in the $\ensuremath{\lambda}$-type pattern, however, a strong vibrational ${\ensuremath{\nu}}_{27}({b}_{1u})$ mode is present that forms a narrow doublet. Most important, when cooling $\ensuremath{\lambda}\ensuremath{-}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ below 150 K, two weak side modes appear due to charge disproportionation that amounts to $2\ensuremath{\delta}=0.14e$. In analogy to the ${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}$-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in $\ensuremath{\lambda}\ensuremath{-}{(\mathrm{BETS})}_{2}{\mathrm{GaCl}}_{4}$ at ${T}_{c}=4.7$ K. We discuss the possibility of a charge-density wave that coexists with the proposed spin-density-wave state.

We have tested the conduction band electrons of lanthanum hexaboride that is among the most effective electron-beam sources with one of the highest brightness of thermionic emission. We performed infrared spectroscopic, DC (direct current) resistivity and Hall-effect studies of LaB6 single crystals with various 10B and 11B isotope contents. We find that only a small amount of conduction electrons behave as Drude-type charge carriers while about 70% of the electrons are involved in collective oscillations of electron density coupled to vibrations of both the Jahn-Teller unstable rigid boron cage and rattling modes of La-ions loosely bound to the lattice. We suggest that exactly these non-equilibrium conduction electrons determine the extraordinary low work function of thermoemission in LaB6.

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., Sb) and optimization of the growth parameters (i.e., Sb flux and substrate temperature). The increased electron conductivity can be attributed to the elevated carrier concentration (1.5 × 10

The quasi-one-dimensional organic conductors (${\mathrm{TMTTF})}_{2}X$ with noncentrosymmetric anions commonly undergo charge- and anion-order transitions upon cooling. While for compounds with tetrahedral anions ($X={\mathrm{BF}}_{4}^{\ensuremath{-}}, {\mathrm{ReO}}_{4}^{\ensuremath{-}}$, and ${\mathrm{ClO}}_{4}^{\ensuremath{-}}$) the charge-ordered phase is rather well understood, the situation is less clear in the case of planar triangular anions, such as (${\mathrm{TMTTF})}_{2}{\mathrm{NO}}_{3}$. Here we explore the electronic and structural transitions by transport experiments, optical, and magnetic spectroscopy. This way we analyze the temperature dependence of the charge imbalance $2\ensuremath{\delta}$ and an activated behavior of $\ensuremath{\rho}(T)$ with ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CO}}\ensuremath{\approx}530\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ below ${T}_{\mathrm{CO}}=250\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Since (${\mathrm{TMTTF})}_{2}{\mathrm{NO}}_{3}$ follows the universal relation between charge imbalance $2\ensuremath{\delta}$ and size of the gap ${\mathrm{\ensuremath{\Delta}}}_{\mathrm{CO}}$, our findings suggest that charge order is determined by TMTTF stacks with little influence of the anions. Clear signatures of anion ordering are detected at ${T}_{\mathrm{AO}}=50\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. The tetramerization affects the dc transport, the vibrational features of donors and acceptors, and leads to formation of spin singlets.

The dimer Mott insulator $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{CN})}_{3}$ can be tuned into metallic and superconducting states on applying pressure of 1.5 kbar and more. We have performed dielectric measurements (7.5 kHz to 5 MHz) on $\ensuremath{\kappa}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{Cu}}_{2}{(\mathrm{CN})}_{3}$ single crystals as a function of temperature (down to $T=8$ K) and pressure (up to $p=4.3$ kbar). In addition to the relaxor-like dielectric behavior seen below 50 K at $p=0$, that moves toward lower temperatures with pressure, a second peak emerges in ${\ensuremath{\varepsilon}}_{1}(T)$ around $T=15$ K. When approaching the insulator-metal boundary, this peak diverges rapidly reaching ${\ensuremath{\varepsilon}}_{1}\ensuremath{\approx}{10}^{5}$. Our dynamical mean-field theory calculations substantiate that the dielectric catastrophe at the Mott transition is not caused by closing the energy gap, but due to the spatial coexistence of correlated metallic and insulating regions. We discuss the percolative nature of the first-order Mott insulator-to-metal transition in all details.

We present an optical conductivity study of the multifold semimetal PdGa, performed in a broad spectral range (100 - 20000 cm-1; 12 meV - 2.5 eV) down to T = 10 K. The conductivity at frequencies below 4000 cm-1 is dominated by free carriers while at higher frequencies interband transitions provide the major contribution. The spectra do not demonstrate a significant temperature evolution: only the intraband part changes as a function of temperature with the plasma frequency remaining constant. The interband contribution to the conductivity exhibits a broad peak at around 5500 cm-1 and increases basically monotonously at frequencies above 9000 cm-1. The band-structure-based computations reproduce these features of the interband conductivity and predict its linear-in-frequency behavior as frequency diminishes., Comment: 7+ pages, published in PRB

Temperature-dependent reflectivity measurements on the kagome metal CsV$_3$Sb$_5$ in a broad frequency range of $50-20000$ cm$^{-1}$ down to $T$=10 K are reported. The charge-density wave (CDW) formed below $T_{\rm CDW}$ = 94 K manifests itself in a prominent spectral-weight transfer from low to higher energy regions. The CDW gap of 60-75 meV is observed at the lowest temperature and shows significant deviations from an isotropic BCS-type mean-field behavior. Absorption peaks appear at frequencies as low as 200 cm$^{-1}$ and can be identified with interband transitions according to density-functional calculations. The change in the interband absorption compared to KV$_3$Sb$_5$ reflects the inversion of band saddle points between the K and Cs compounds. Additionally, a broader and strongly temperature-dependent absorption feature is observed below 1000 cm$^{-1}$ and assigned to a displaced Drude peak. It reflects localization effects on charge carriers., Comment: 9 pages, 7 figures

We performed angular and temperature-dependent electron-spin-resonance measurements in the quasi-two-dimensional organic conductor $\ensuremath{\kappa}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{I}$. The interlayer spin diffusion is much weaker compared to the Cl and Br analogs, which are antiferromagnetic insulator and paramagnetic metal, respectively; $\ensuremath{\kappa}\text{\ensuremath{-}}{(\text{BEDT-TTF})}_{2}\mathrm{Cu}[\mathrm{N}{(\mathrm{CN})}_{2}]\mathrm{I}$ behaves insulating when cooled below $T=200\phantom{\rule{4pt}{0ex}}\mathrm{K}$. A spin gap ($\mathrm{\ensuremath{\Delta}}\ensuremath{\approx}18\phantom{\rule{4pt}{0ex}}\mathrm{K}$) opens at low temperatures leading to a spin-singlet state. Due to intrinsic disorder a substantial number of spins ($\ensuremath{\sim}1%$) remains unpaired. We observe additional signals below $T=4\phantom{\rule{4pt}{0ex}}\mathrm{K}$ with a pronounced anisotropy indicating the presence of local magnetic moments coupled to some fraction of those unpaired spins.

Temperature- and frequency-dependent infrared spectroscopy identifies two contributions to the electronic properties of the magnetic kagome metal Fe_{3}Sn_{2}: two-dimensional Dirac fermions and strongly correlated flat bands. The interband transitions within the linearly dispersing Dirac bands appear as a two-step feature along with a very narrow Drude component due to intraband contribution. Low-lying absorption features indicate flat bands with multiple van Hove singularities. Localized charge carriers are seen as a Drude peak shifted to finite frequencies. The spectral weight is redistributed when the spins are reoriented at low temperatures; a sharp mode appears suggesting the opening of a gap due to the spin reorientation as the sign of additional Weyl nodes in the system.

Superconductivity in organic conductors is often tuned by the application of chemical or external pressure. With this type of tuning, orbital overlaps and electronic bandwidths are manipulated, whilst the properties of the molecular building blocks remain virtually unperturbed.Here, we show that the excitation of local molecular vibrations in the charge-transfer salt $\kappa-(BEDT-TTF)_2Cu[N(CN)_2]Br$ induces a colossal increase in carrier mobility and the opening of a superconducting-like optical gap. Both features track the density of quasi-particles of the equilibrium metal, and can be achieved up to a characteristic coherence temperature $T^* \approxeq 50 K$, far higher than the equilibrium transition temperature $T_C = 12.5 K$. Notably, the large optical gap achieved by photo-excitation is not observed in the equilibrium superconductor, pointing to a light induced state that is different from that obtained by cooling. First-principle calculations and model Hamiltonian dynamics predict a transient state with long-range pairing correlations, providing a possible physical scenario for photo-molecular superconductivity., Comment: 11 pages, 7 figures, supplementary material

We resolve the real-time coherent rotational motion of isolated water molecules encapsulated in fullerene-C60 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, C60 lattice vibrational damping is mitigated and the quantum dynamics of confined water are resolved with a markedly long rotational coherence, extended beyond 10 ps. The observed rotational transitions agree well with low-frequency rotational dynamics of single water molecules in the gas phase. However, some additional spectral features with their major contribution at ~2.26 THz are also observed which may indicate interaction between water rotation and the C60 lattice phonons. We also resolve the real-time change of the emission pattern of water after a sudden cooling to 4 K, signifying the conversion of ortho-water to para-water over the course of 10s hours. The observed long coherent rotational dynamics of isolated water molecules confined in C60 makes this system an attractive candidate for future quantum technology.

The most common species in liquid water, next to neutral $$\hbox {H}_2\hbox {O}$$H2O molecules, are the $$\hbox {H}_3\hbox {O}^+$$H3O+ and $$\hbox {OH}^-$$OH- 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 $$\hbox {H}_3\hbox {O}^+$$H3O+, $$\hbox {DH}_2\hbox {O}^+$$DH2O+, $$\hbox {HD}_2\hbox {O}^+$$HD2O+, and $$\hbox {D}_3\hbox {O}^+$$D3O+ ions in the IR spectra of light ($$\hbox {H}_2\hbox {O}$$H2O), heavy ($$\hbox {D}_2\hbox {O}$$D2O), and semi-heavy (HDO) water. We find that short-living ions, with concentrations reaching $$\sim 2\%$$∼2% of the content of water molecules, coexist with long-living pH-active ions on the picosecond timescale, thus making liquid water an effective ionic liquid in femtochemistry.

Long-lived circular Rydberg atoms are picking up increasing interest for boosting coherence times in Rydberg-based quantum simulation. We elaborate a novel approach to stabilize circular Rydberg states against spontaneous and blackbody-induced decay using a suppression capacitor made from indium tin oxide (ITO) thin films, which combine reflection of microwaves with transparency in the visible spectral range. To this end, we perform detailed characterization of such films using complementary spectroscopic methods at GHz and THz frequencies and identify conditions that allow for reaching circular-state lifetimes up to tens of milliseconds in a room-temperature environment. We discuss prospects of our findings in view of the quest for quantum simulations with high-$n$ circular Rydberg states at room temperature., Comment: 7 pages, 4 figures

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)

Water at the solid-liquid interface exhibits an anomalous ionic conductivity and dielectric constant compared to bulk water. Both phenomena still lack a detailed understanding. Here, we report radiofrequency measurements and analyses of the electrodynamic properties of interfacial water confined in nano-porous matrices formed by diamond grains of various sizes, ranging from 5 nm to 0.5 μm in diameter. Contrary to bulk water, the charge-carrying protons/holes in interfacial water are not mutually screened allowing for higher mobility in the external electric field. Thus, the protonic conductivity reaches a maximum value, which can be five orders of magnitude higher than that of bulk water. Our results aid in the understanding of physical and chemical properties of water confined in porous materials, and pave the way to the development of new type of highly-efficient proton-conductive materials for applications in electrochemical energy systems, membrane separations science and nano-fluidics.

The quasi-one-dimensional spin ladder compounds, ${\mathrm{BaFe}}_{2}{\mathrm{S}}_{3}$ and ${\mathrm{BaFe}}_{2}{\mathrm{Se}}_{3}$, are investigated by infrared spectroscopy and density functional theory (DFT) calculations. We observe strong anisotropic electronic properties and an optical gap in the leg direction that is gradually filled above the antiferromagnetic (AFM) ordering temperature, turning the systems into a metallic phase. Combining the optical data with the DFT calculations we associate the optical gap feature with the $p\ensuremath{-}d$ transition that appears only in the AFM ordered state. Hence, the insulating ground state along the leg direction is attributed to Slater physics rather than Mott-type correlations.

We investigate the lattice dynamics in the spin-1/2 frustrated kagome compound herbertsmithite ZnCu$_3$(OH)$_6$Cl$_2$ by a combination of infrared spectroscopy measurements and ab initio density functional theory calculations, and provide an unambiguous assignment of infrared-active lattice vibrations involving in-plane and out-of-plane atom displacements in the kagome layers. Upon cooling, non-thermal red-shifts and broadening appear specifically for modes that deform the kagome layer or affect the Cu-O-Cu bond angles, thus creating pronounced modifications of the antiferromagnetic exchange coupling. Our results indicate the presence of a strong magnetoelastic coupling to the spin system. We discuss the effects of this coupling and its relation to recent experiments reporting a global symmetry reduction of the kagome lattice symmetry.

We present an instrument to perform frequency-domain electron spin resonance (ESR) 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. We performed experiments on DPPH (2,2-diphenyl-1-picrylhydrazyl) as a standard to calibrate the field modulation as well as on a carbon fibre sample to estimate the overall sensitivity of the instruments. Spectra of a ruby sample in a broad frequency- and field range at cryogenic temperatures are recorded with and without field modulation. The comparison reveals the improved signal-to-noise ratio achieved by field modulation., Comment: The following article has been accepted by Review of Scientific Instruments. After it is published, it will be found at https://doi.org/10.1063/1.5141461

Employing optical spectroscopy we have performed a comparative study of the dielectric response of extracellular matrix and filaments of electrogenic bacteria Shewanella oneidensis MR-1, cytochrome c, and bovine serum albumin. Combining infrared transmission measurements on thin layers with data of the terahertz spectra, we obtain the dielectric permittivity and AC conductivity spectra of the materials in a broad frequency band from a few cm(−1) up to 7000 cm(−1) in the temperature range from 5 to 300 K. Strong absorption bands are observed in the three materials that cover the range from 10 to 300 cm(−1) and mainly determine the terahertz absorption. When cooled down to liquid helium temperatures, the bands in Shewanella oneidensis MR-1 and cytochrome c reveal a distinct fine structure. In all three materials, we identify the presence of liquid bound water in the form of librational and translational absorption bands at ≈ 200 and ≈ 600 cm(−1), respectively. The sharp excitations seen above 1000 cm(−1) are assigned to intramolecular vibrations.

We present a new experimental approach to investigate the magnetic properties of the anisotropic heavy-fermion system YbRh 2 Si 2 as a function of crystallographic orientation. Angle-dependent electron spin resonance (ESR) measurements are performed at a low temperature of 1.6 K and at an ESR frequency of 4.4 GHz utilizing a superconducting planar microwave resonator in a 4He-cryostat in combination with in-situ sample rotation. The obtained ESR g-factor of YbRh 2 Si 2 as a function of the crystallographic angle is consistent with results of previous measurements using conventional ESR spectrometers at higher frequencies and fields. Perspectives to implement this experimental approach into a dilution refrigerator and to reach the magnetically ordered phase of YbRh 2 Si 2 are discussed.

Water is subject to intense investigations due to its importance in biological matter but keeps many of its secrets. Here, we unveil an even other aspect by confining H2O molecules to nanosize cages. Our THz and infrared spectra of water in the gemstone beryl evidence quantum tunneling of H2O molecules in the crystal lattice. The water molecules are spread out when confined in a nanocage. In combination with low-frequency dielectric measurements, we were also able to show that dipolar coupling among the H2O molecules leads towards a ferroelectric state at low temperatures. Upon cooling, a ferroelectric soft mode shifts through the THz range. Only quantum fluctuations prevent perfect macroscopic order to be fully achieved. Beside the significance to life science and possible application, nanoconfined water may become the prime example of a quantum electric dipolar lattice.

Weak vibrational signals in the infrared and terahertz spectral region can be enhanced by orders of magnitude when employing the electromagnetic near fields of plasmonic nanostructures. This approach is known as antenna-assisted surface-enhanced infrared absorption (SEIRA) and allows for a broad range of possible sensing applications. In the present work, we investigate the scaling of the SEIRA enhancement with wavelength, particularly toward the molecular fingerprint region (500–1500 cm–1). We apply the concept of SEIRA to perform resonant antenna-enhanced spectroscopy of molecules in a spectral range from 4.5 to 45 THz (6.7–67 μm wavelength, 150–1500 cm–1) using a standard Fourier transform infrared spectrometer. We fabricate arrays of rectangular gold antennas by electron-beam lithography and coat them with 30 nm thick layers of the fullerenes C60 and C70, respectively. For the single digit THz measurements, we utilize spin-coated amino acids, particularly threonine. The resonances of the structures ar...

High quality PbFe12O19 single crystals were grown by a solution-melt crystallization technique and characterized by synchrotron radiation diffraction structural studies, Mossbauer, Raman and infrared spectral measurements. In the terahertz and sub-terahertz range two absorption bands are observed, which are associated with the dynamics of structurally disordered lead and iron ions. We develop a structural model that comprises Fe2+ ions at the (1/2 × 4e) (0, 0, z) position with bipyramidal oxygen environment, and Pb ions at the (24l) (x, y, z) position with generic strongly distorted 12 vertex polyhedron of oxygen. The atomic displacement ellipsoids of the oxygen surrounding the lead ion are stretched towards the Pb ion within the plane perpendicular to the hexagonal axis. The suggested model is in full agreement with the local symmetry of the Fe2 and Pb ions and with the chemical composition of the sample determined by X-ray analysis.

We have performed broad-band spectroscopic investigations of vibrational and relaxational excitations of water molecules confined to nanocages within artificial beryl and mineral cordierite crystals. Signatures of quantum critical phenomena within the H2O molecular network are registered in beryl. In cordierite, a density functional analysis is applied to reconstruct the potential energy landscape experienced by H2O molecules, revealing a pronounced anisotropy with a potential well of about 10 meV for the molecular dipole moment aligned along the b-axis. This anisotropy leads to a strongly temperature dependent and anisotropic relaxational response of the dipoles at radiofrequencies with the activation energies corresponding to the barriers of the rotational potential. At T ≈ 3 K, we identify signatures of a transition into a glassy state composed by clusters of H2O dipoles. Rich set of anisotropic and temperature-dependent excitations are observed in the terahertz frequency range which we associate with rotational/translational vibrations.

Coherent-source THz spectroscopy was employed for measuring the Faraday rotation angle of Mn$_{\mathbf {2-x}}$PtSn films (x $=$ 0; 0.2). In the THz experiments, we observe clear signatures of an anomalous Hall state, while any indications of the so-called topological Hall effect recently observed in these magnetically disordered systems in dc measurements - are absent. This demonstrates that the charge scattering mechanisms, responsible for the two phenomena, possess very different time scales: the anomalous Hall state is governed by the scattering process with the rates above THz frequencies, while the electron interaction, providing the topological Hall, possesses very low characteristic time scales.

We have studied terahertz and radio-frequency dielectric response of single water molecules periodically arranged in nano-cages formed by the crystal lattice of beryl. Below ≈20 K, quantum effects start to dominate the properties of the H2O dipolar system as manifested by a crossover between the Curie-Weiss and the Barrett regimes in the temperature behavior of the dielectric permittivity e'. Analyzing the temperature evolution of the inverse permittivity (e')-1 down to T = 0.3 K, we discover signatures of quantum critical behavior of coupled water molecular dipoles: its quadratic variation with temperature at T=3-12 K and a shallow minimum in the (e')-1(T) dependence below 3 K.

To understand mechanisms of high thermionic emission of LaB 6 we use infrared spectroscopy, DC resistivity and Hall-effect techniques to study conduction electrons state in LaB6 single crystals with different contents of 10B and 11B isotopes. We find that up to 70% of conduction electrons are strongly non-equilibrium due to involvement in the collective oscillations of electron density coupled to vibrations of Jahn-Teller unstable boron cage and rattling modes of La-ions. We claim that these non-equilibrium electrons determine the extraordinary low work function of thermoemission in LaB 6 .

In the all-organic paramagnetic correlated metal ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\text{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{SF}}_{5}{\mathrm{CH}}_{2}{\mathrm{CF}}_{2}{\mathrm{SO}}_{3}$ spin fluctuations can be ruled out as the glue to superconductivity below ${T}_{c}=5.5$ K. Instead, we demonstrate the coupling of charge degrees of freedom to the superconducting state, which occurs adjacent to a charge-ordered insulating phase. A comparison to isostructural ${\ensuremath{\beta}}^{\ensuremath{''}}\text{\ensuremath{-}}{(\mathrm{BEDT}\text{\ensuremath{-}}\mathrm{TTF})}_{2}{\mathrm{SF}}_{5}R{\mathrm{SO}}_{3}$ compounds reveals that superconductivity is suppressed when the system is tuned away from the insulator towards the metal; an enhanced charge imbalance, on the other hand, leads to an increase in ${T}_{c}$, suggesting an involvement of charge fluctuations. By locally probing the charge disproportionation through the superconducting transition, we discover slight modifications of the Fano coupling constant and intensity at ${T}_{c}$, indicating the effect of the paired electrons on the molecular vibrations.

Experimentally, we establish the quantitative phase diagram of 1/4-filled ${\ensuremath{\beta}}^{\ensuremath{''}}$-(BEDT-${\mathrm{TTF})}_{2}{\mathrm{SF}}_{5}$R${\mathrm{SO}}_{3}$ with a tendency toward charge order. Comprehensive optical, transport, and susceptibility measurements reveal the insulating nature and magnetic properties of the charge-ordered ground state. Going from $R=\mathrm{CHF}$ via ${\mathrm{CH}}_{2}{\mathrm{CF}}_{2}$ and ${\mathrm{CHFCF}}_{2}$ toward ${\mathrm{CH}}_{2}$, a finite charge disproportionation appears and grows up to $2\ensuremath{\delta}=0.5e$, as evidenced by charge-sensitive molecular vibrations. This is accompanied by an increase of electronic correlation strength, i.e., the intersite Coulomb repulsion $V$ becomes more pronounced in relation to the bandwidth $W$. The broadband electronic excitations and their anisotropy unveil a distinct charge pattern for $R={\mathrm{CH}}_{2}$ (checkerboard type) as compared to the other compounds (stripelike). Our results validate theoretical predictions for 1/4-filled systems, substantiating the importance of charge fluctuations for unconventional superconductivity at the verge between metal and insulator.

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. We show that the non-linear sample response, which relates to sample heating, can lead to strong signatures of standing waves even in a well-calibrated Corbino spectrometer. We demonstrate our findings with microwave measurements as a function of frequency, power, and temperature and for different lengths of the microwave transmission line. Finally, we note such non-linear effects beyond the case of superconductors by probing a VO

We report a detailed structural and spectroscopic study of the 1D 2:1 cation radical salt (tTTF)

We investigate the dc resistivity and Hall effect of the quasi-two-dimensional organic materials κ–(ET)2X, where X=Ag2(CN)3 and B(CN)4 and compare them with the results for X=Cu2(CN)3. All three compounds are considered to be quantum-spin-disordered Mott insulators. Despite high similarities in chemical composition and crystal structure, large differences in the dc resistivity and Hall coefficient are found. While around room temperature the dc transport properties are dominantly determined by the strength of the electron correlations, upon reducing the temperature, dc transport happens by hopping due to inherent disorder. The most disordered compound with X=Cu2(CN)3 turns out to have the lowest dc resistivity and the highest charge carrier density, i.e., in the phase diagram it is located closest to the metal-insulator transition. The least disordered compound with X=B(CN)4 shows the highest resistivity and the lowest carrier density, i.e., lies farthest from the metal-insulator transition. We explain such counterintuitive behavior within the theory of Mott-Anderson localization as a consequence of disorder-induced localized states within the correlation gap.

We report optical transmission measurements through a single crystal of the Weyl semimetal TaAs at terahertz frequencies. The measurements were performed at T=1.6 K using intense coherent light from a free-electron laser in high magnetic fields. We detected small but measurable changes in the sample’s transparency upon applying dc voltage (current) to the sample. In a constant magnetic field, the sign of the transmission change depends on the current direction. The effect disappears in zero magnetic field. These observations are qualitatively consistent with theory predictions for optical signatures of the chiral anomaly in a Weyl semimetal. Keywords: Topological semimetals, Weyl fermions, Chiral anomaly