Magnetic structure and properties of the honeycomb antiferromagnet [Na(OH)]Mn(NCS).

We report the magnetic structure and properties of a thiocyanate-based honeycomb magnet [Na(OH)]Mn(NCS) which crystallises in the unusual low-symmetry trigonal space group 3̄. Magnetic measurements on powder samples show this material is an antiferromagnet (ordering temperature = 18.1(6) K) and can be described by nearest neighbour antiferromagnetic interactions = -11.

2D Time-Domain Spectroscopy for Determination of Energy and Momentum Relaxation Rates of Hydrogen-Like Donor States in Germanium.

We present measurements of the coherence times of excited states of hydrogen-like arsenic impurities in germanium (Ge:As) using a table-top two-dimensional time-domain spectroscopy (2D-TDS) system. We show that this laboratory system is capable of resolving the coherence lifetimes of atomic-like excited levels of impurity centers in semiconductors, such as those used in solid-state quantum information technologies, on a subpicosecond time scale. By fitting the coherent nonlinear response of the system with the known intracenter transition frequencies, we are able to monitor coherent population transfer and decay of the transitions from the 2p and 2p states for different low excitation pulse fields.

Visible-Light Driven Control Over Triply and Quadruply Hydrogen-Bonded Supramolecular Assemblies.

Supramolecular polymers offer tremendous potential to produce new "smart" materials, however, there remains a need to develop systems that are responsive to external stimuli. In this work, visible-light responsive hydrogen-bonded supramolecular polymers comprising photoresponsive supramolecular synthons (I-III) consisting of two hydrogen bonding motifs (HBMs) connected by a central ortho-tetrafluorinated azobenzene have been characterized by DOSY NMR and viscometry. Comparison of different hydrogen-bonding motifs reveals that assembly in the low and high concentration regimes is strongly influenced by the strength of association between the HBMs.

Exploring the Stability and Disorder in the Polymorphs of L-Cysteine through Density Functional Theory and Vibrational Spectroscopy.

Static and dynamic density functional calculations are reported for the four known polymorphs of l-cysteine. Static calculations are used to explore the relative free energies (within the harmonic approximation) of the polymorphs as a function of pressure. An important feature of the structural differences between the polymorphs is shown to be the dihedral angle of the C-C-S-H bond.

Direct Detection of Hydrogen Bonds in Supramolecular Systems Using H-N Heteronuclear Multiple Quantum Coherence Spectroscopy.

Hydrogen-bonded supramolecular systems are usually characterized in solution through analysis of NMR data such as complexation-induced shifts and nuclear Overhauser effects (nOe). Routine direct detection of hydrogen bonding particularly in multicomponent mixtures, even with the aid of 2D NMR experiments for full assignment, is more challenging. We describe an elementary rapid H-N HMQC NMR experiment which addresses these challenges without the need for complex pulse sequences.

Photoconductive arrays on insulating substrates for high-field terahertz generation.

We report on the design, fabrication and characterisation of large-area photoconductive THz array structures, consisting of a thin LT-GaAs active region transferred to an insulating substrate using a wafer-scale bonding process. The electrically insulating, transparent substrate reduces the parasitic currents in the devices, allowing peak THz-fields as high as 120 kV cm to be generated over a bandwidth >5 THz. These results are achieved using lower pulse energies than demanded by conventional photoconductive arrays and other popular methods of generating high-field THz radiation.

On-Chip Terahertz-Frequency Measurements of Liquids.

Terahertz-frequency-range measurements can offer potential insight into the picosecond dynamics, and therefore function, of many chemical systems. There is a need to develop technologies capable of performing such measurements in aqueous and polar environments, particularly when it is necessary to maintain the full functionality of biological samples. In this study, we present a proof-of-concept technology comprising an on-chip planar Goubau line, integrated with a microfluidic channel, which is capable of low-loss, terahertz-frequency-range spectroscopic measurements of liquids.

Free-space terahertz radiation from a LT-GaAs-on-quartz large-area photoconductive emitter.

We report on large-area photoconductive terahertz (THz) emitters with a low-temperature-grown GaAs (LT-GaAs) active layer fabricated on quartz substrates using a lift-off transfer process. These devices are compared to the same LT-GaAs emitters when fabricated on the growth substrate. We find that the transferred devices show higher optical-to-THz conversion efficiencies and significantly larger breakdown fields, which we attribute to reduced parasitic current in the substrate.

PDielec: The calculation of infrared and terahertz absorption for powdered crystals.

The Python package PDielec is described, which calculates the infrared absorption characteristics of a crystalline material supported in a non-absorbing medium. PDielec post processes solid-state quantum mechanical and molecular mechanical calculations of the phonons and dielectric response of the crystalline material. Using an effective medium method, the package calculates the internal electric field arising from different particle morphologies and calculates the resulting shift in absorption frequency and intensity arising from the coupling between a phonon and the internal field.

Laser Feedback Interferometry as a Tool for Analysis of Granular Materials at Terahertz Frequencies: Towards Imaging and Identification of Plastic Explosives.

We propose a self-consistent method for the analysis of granular materials at terahertz (THz) frequencies using a quantum cascade laser. The method is designed for signals acquired from a laser feedback interferometer, and applied to non-contact reflection-mode sensing. Our technique is demonstrated using three plastic explosives, achieving good agreement with reference measurements obtained by THz time-domain spectroscopy in transmission geometry.

Spectroscopy of polycrystalline materials using thinned-substrate planar Goubau line at cryogenic temperatures.

We investigate the effect of substrate thickness on the transmission bandwidth of on-chip terahertz-frequency-range planar Goubau lines both experimentally and theoretically. The bandwidth and frequency resolution are improved through substrate thinning and geometry modifications (reducing reflections arising from the THz photoconductive generators and detectors). We demonstrate that the enhanced bandwidth (2 THz) and resolution (3.

Effect of molecular size and particle shape on the terahertz absorption of a homologous series of tetraalkylammonium salts.

The absorption coefficient and refractive index have been measured for a homologous series of tetraalkylammonium bromides over the frequency range 0.3-5.5 THz.

Broadband terahertz time-domain spectroscopy of drugs-of-abuse and the use of principal component analysis.

Terahertz frequency time-domain spectroscopy has been used to analyse a wide range of samples containing cocaine hydrochloride, heroin and ecstasy--common drugs-of-abuse. We investigated real-world samples seized by law enforcement agencies, together with pure drugs-of-abuse, and pure drugs-of-abuse systematically adulterated in the laboratory to emulate real-world samples. In order to investigate the feasibility of automatic spectral recognition of such illicit materials by terahertz spectroscopy, principal component analysis was employed to cluster spectra of similar compounds.

Calculation and measurement of terahertz active normal modes in crystalline PETN.

The terahertz frequency spectrum of pentaerythritol tetranitrate (PETN) is calculated using Discover with the COMPASS force field, CASTEP and PWscf. The calculations are compared to each other and to terahertz spectra (0.3-3 THz) of crystalline PETN recorded at 4 K.