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Using THz spectroscopy, we show that the spin-wave spectrum of multiferroic BiFeO$_3$ in its high-field canted antiferromagnetic state is well described by a spin model that violates rhombohedral symmetry. We demonstrate that the monoclinic distortion of the canted antiferromagnetic state is induced by the single-ion magnetoelastic coupling between the lattice and the two nearly anti-parallel spins. The revised spin model for BiFeO$_3$ contains two new single-ion anisotropy terms that violate rhombohedral symmetry and depend on the direction of the magnetic field., Comment: 28 pages (main & supplementary), 2 figures (main article), 15 figures (supplementary material)

The optical properties of colloidal cesium lead halide perovskite (CsPbBr3) nanocrystals are examined by time-resolved and polarization-resolved spectroscopy in high magnetic fields up to 30 T. We unambiguously show that at cryogenic temperatures the emission is dominated by recombination of negatively charged excitons with radiative decay time of 300 ps. The additional long-lived emission, which decay time shortens from 40 down to 8 ns and in which the decay time shortens and relative amplitude increases in high magnetic fields, evidences the presence of a dark exciton. We evaluate g-factors of the bright exciton gX = +2.4, the electron ge = +2.18, and the hole gh = −0.22.

Spin excitations of magnetoelectric LiNiPO4 are studied by infrared absorption spectroscopy in the THz spectral range as a function of magnetic field through various commensurate and incommensurate magnetically ordered phases up to 33 T. Six spin resonances and a strong two-magnon continuum are observed in zero magnetic field. Our systematic polarization study reveals that some of the excitations are usual magnetic-dipole active magnon modes, while others are either electromagnons, being only electric-dipole active, or magnetoelectric, that is both electric- and magnetic-dipole active spin excitations. Field-induced shifts of the modes for all three orientations of the field along the orthorhombic axes allow us to refine the values of the relevant exchange couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction on the level of a four-sublattice mean-field spin model. This model also reproduces the spectral shape of the two-magnon absorption continuum, found to be electric-dipole active in the experiment.

The continued development of novel drugs, proteins, and advanced materials strongly rely on our ability to self-assemble molecules in solids with the most suitable structure (polymorph) in order to exhibit desired functionalities. The search for new polymorphs remains a scientific challenge, that is at the core of crystal engineering and there has been a lack of effective solutions to this problem. Here we show that by crystallizing the polyaromatic hydrocarbon coronene in the presence of a magnetic field, a polymorph is formed in a β-herringbone structure instead of the ubiquitous γ-herringbone structure, with a decrease of 35° in the herringbone nearest neighbour angle. The β-herringbone polymorph is stable, preserves its structure under ambient conditions and as a result of the altered molecular packing of the crystals, exhibits significant changes to the optical and mechanical properties of the crystal., Polymorphism, the presence of different crystal structures of the same molecular system, provides an opportunity to discover new phenomena and properties. Here, the authors crystallize coronene in the presence of a magnetic field, forming a different polymorph, which remains stable under ambient conditions.

We control the linear polarization of emission from the coherently emitting ${K}^{+}$ and ${K}^{\ensuremath{-}}$ valleys (valley coherence) in monolayer ${\mathrm{WS}}_{2}$ with an out-of-plane magnetic field of up to 25 T. The magnetic-field-induced valley Zeeman splitting causes a rotation of the emission polarization with respect to the excitation by up to 35\ifmmode^\circ\else\textdegree\fi{} and reduces the polarization degree by up to 16%. We explain both of these phenomena with a model based on two noninteracting coherent two-level systems. We deduce that the coherent light emission from the valleys decays with a time constant of ${\ensuremath{\tau}}_{c}=260\text{ }\text{ }\mathrm{fs}$.

Wave function engineering has become a powerful tool to tailor the optical properties of semiconductor colloidal nanocrystals. Core–shell systems allow to design the spatial extent of the electron (e) and hole (h) wave functions in the conduction- and valence bands, respectively. However, tuning the overlap between the e- and h-wave functions not only affects the oscillator strength of the coupled e–h pairs (excitons) that are responsible for the light emission, but also modifies the e–h exchange interaction, leading to an altered excitonic energy spectrum. Here, we present exciton lifetime measurements in a strong magnetic field to determine the strength of the e–h exchange interaction, independently of the e–h overlap that is deduced from lifetime measurements at room temperature. We use a set of CdTe/CdSe core/shell heteronanocrystals in which the electron–hole separation is systematically varied. We are able to unravel the separate effects of e–h overlap and e–h exchange on the exciton lifetimes, and we present a simple model that fully describes the recombination lifetimes of heteronanostructures (HNCs) as a function of core volume, shell volume, temperature, and magnetic fields.

An alignment technique based on the combination of magnetic fields and a liquid crystal (LC) template uses the advantages of both approaches: the magnetic fields offer non-contact methods that apply to all sample sizes and shapes, whilst the LC templates offer high susceptibilities. The combination introduces a route to control the spatial organization of materials with low intrinsic susceptibilities. We demonstrate that we can unidirectionally align one such material, peptide amphiphiles in water, on a centimeter scale at a tenfold lower magnetic field by using a lyotropic chromonic liquid crystal as a template. We can transform the aligned supramolecular assemblies into optically active π-conjugated polymers after photopolymerization. Lastly, by reducing the magnetic field strength needed for addressing these assemblies, we are able to create more complex structures by initiating self-assembly of our supramolecular materials under competing alignment forces between the magnetically induced alignment of the assemblies (with a positive diamagnetic anisotropy) and the elastic force dominated alignment of the template (with a negative diamagnetic anisotropy), which is directed orthogonally. Although the approach is still in its infancy and many critical parameters need optimization, we believe that it is a very promising technique to create tailor-made complex structures of (aqueous) functional soft matter.

We show that applying magnetic fields up to 30 T has a dramatic effect on the ultrafast spin dynamics in ferrimagnetic GdFeCo. Upon increasing the field beyond a critical value, the dynamics induced by a femtosecond laser excitation strongly increases in amplitude and slows down significantly. Such a change in spin response is explained by different dynamics of the Gd and FeCo magnetic sublattices following a spin-flop phase transition from a collinear to a noncollinear spin state.

Low-temperature polarized and time-resolved photoluminescence spectroscopy in high magnetic fields (up to 30 T) has been used to study the spin-polarization, spin-relaxation, and radiative lifetimes of excitons in wurtzite semiconductor (e.g., CdSe) colloidal nanocrystals. The applied magnetic field leads to a significant degree of circular polarization of the exciton photoluminescence, accompanied by a reduction in the photoluminescence decay time. The circular polarization arises from the Zeeman splitting of exciton levels, whereas lifetime reduction results from a polarization-preserving field-induced mixing of exciton levels. We analyze these experimental findings in terms of a simple model that combines both Zeeman effect and exciton-level mixing, as a function of the relative orientation of the nanocrystal c-axis and the magnetic field. This model is able to simultaneously describe the degree of circular polarization and lifetime reduction of the exciton photoluminescence, permitting us to quantify the exciton, electron, and hole g-factors.

We report on a new effect caused by the electron-phonon coupling in a stoichiometric rare-earth antiferromagnetic crystal subjected to an external magnetic field, namely, the appearance of a nonzero gap in the spectrum of electronic excitations in an arbitrarily small field. The effect was registered in the low-temperature far-infrared (terahertz) reflection spectra of an easy-axis antiferromagnet PrFe_{3}(BO_{3})_{4} in magnetic fields B_{ext}∥c. Both paramagnetic and magnetically ordered phases (including a spin-flop one) were studied in magnetic fields up to 30 T, and two bifurcation points were observed. We show that the field behavior of the coupled modes can be successfully explained and modeled on the basis of the equation derived in the framework of the theory of coupled electron-phonon modes, with the same field-independent electron-phonon interaction constant |W|=14.8 cm^{-1}.

The ac magnetoconductance of bulk InSb at THz frequencies in high magnetic fields, as measured by the transmission of THz radiation, shows a field-induced transmission, which at high temperatures ($\ensuremath{\approx}100\text{ }\text{ }\mathrm{K}$) is well explained with classical magnetoplasma effects (helicon waves). However, at low temperatures (4 K), the transmitted radiation intensity shows magnetoquantum oscillations that represent the Shubnikov--de Haas effect at THz frequencies. At frequencies above 0.9 THz, when the radiation period is shorter than the Drude scattering time, an anomalously high transmission is observed in the magnetic quantum limit that can be interpreted as carrier localization at high frequencies.

We report spectroscopic measurements of the THz radiation generated by the free electron laser (FEL) FLARE using electron spin resonance at high magnetic fields, which show that the 150 micropulses circulating in the resonator of the FEL have a large degree of coherence. This feature can be exploited for highresolution ESR and spin echo with subμs time resolution.

Through application of an external magnetic field upon jellification of poly(3-hydroxybutyric acid-co-3-hydroxyvaleric) acid (PHBV) polymer in different solvents, an anisotropic organogel is obtained. This material presents two alignment steps in an external magnetic field, in the liquid phase and during the jellification, both phenomena measured by magnetic field induced linear birefringence. Remarkably, the organogel developed in this study presents a strong level of birefringence, 80% of its maximum, in an external magnetic field as low as 2 T resulting from the magnetic alignment of the fibers of the material. This anisotropic material shows changes of absorbance upon rotation of a polarizer switching from transparent to opaque. In addition, its suprastructure does not influence the luminescent properties of encapsulated chromophores, allowing the formation of colored anisotropic materials.

Transition-metal dichalcogenides can be easily produced as atomically thin sheets, exhibiting the possibility to optically polarize and read out the valley pseudospin of extremely stable excitonic quasiparticles present in these 2D semiconductors. Here, we investigate a monolayer of tungsten disulphide in high magnetic fields up to 30\,T via photoluminescence spectroscopy at low temperatures. The valley degeneracy is lifted for all optical features, particularly for excitons, singlet and triplet trions, for which we determine the g factor separately. While the observation of a diamagnetic shift of the exciton and trion resonances gives us insight into the real-space extension of these quasiparticles, magnetic field induced valley polarization effects shed light onto the exciton and trion dispersion relations in reciprocal space. The field dependence of the trion valley polarizations is in line with the predicted trion splitting into singlet and triplet configurations.

Magnetic fields have only recently been included in theoretical simulations of high-mass star formation. The simulations show that magnetic fields can play a crucial role not only in the formation and dynamics of molecular outflows, but also in the evolution of circumstellar disks. Therefore, new measurements of magnetic fields at milliarcsecond resolution close to massive young stellar objects (YSOs) are fundamental for providing new input for numerical simulations and for understanding the formation process of massive stars. The polarized emission of 6.7 GHz CH3OH masers allows us to investigate the magnetic field close to the massive YSO where the outflows and disks are formed. Recently, we have detected with the EVN CH3OH maser polarized emission towards 10 massive YSOs. From a first statistical analysis we have found evidence that magnetic fields are primarily oriented along the molecular outflows. To improve our statistics we are carrying on a large observational EVN campaign for a total of 19 sources, the preliminary results of the first seven sources are presented in this contribution. Furthermore, we also describe our efforts to estimate the Lande' g-factors of the CH3OH maser transition to determine the magnetic field strength from our Zeeman-splitting measurements., Accepted for publication in the proceeding of the "12th European VLBI Network Symposium and Users Meeting", eds Tarchi et al. PoS(EVN 2014)041

Both the collective (plasmon) and the single particle (Drude) excitations of an electron gas can be controlled and modified by an external magnetic field B. At finite B, plasmon gives rise to a magnetoplasmon mode and the Drude term to a cyclotron resonance. These magnetic effects are expected to be extremely strong for Dirac electrons with a linear energy-momentum dispersion, like those present in graphene and topological insulators (TIs). Here, we investigate both the plasmon and the Drude response versus B in Bi2Se3 topological insulator. At low B, the cyclotron resonance is still well separated in energy from the magnetoplasmon mode; meanwhile, both excitations asymptotically converge at the same energy for increasing B, consistently with a dynamical mass for Dirac carriers of mD* = 0.18 ± 0.01 me. In TIs, one then achieves an excellent magnetic control of plasmonic excitations and this could open the way toward plasmon controlled terahertz magneto-optics.

Optical spectroscopy in high magnetic fields $B\leq65$ T is used to reveal the very different nature of carriers in monolayer and bulk transition metal dichalcogenides. In monolayer WSe$_{2}$, the exciton emission shifts linearly with the magnetic field and exhibits a splitting which originates from the magnetic field induced valley splitting. The monolayer data can be described using a single particle picture with a Dirac-like Hamiltonian for massive Dirac fermions, with an additional term to phenomenologically include the valley splitting. In contrast, in bulk WSe$_{2}$ where the inversion symmetry is restored, transmission measurements show a distinctly excitonic behavior with absorption to the 1s and 2s states. Magnetic field induces a spin splitting together with a small diamagnetic shift and cyclotron like behavior at high fields, which is best described within the hydrogen model., just accepted in Nano Letters http://pubs.acs.org/doi/abs/10.1021/acs.nanolett.5b00626 (equations corrected)

Polymer-stabilized liquid crystals (PSLCs) are materials composed of a polymer mesh in a continuous phase of liquid crystal. The polymer mesh provides an anchor point for alignment of the liquid crystalline bulk. The macroscopic extent of order in such systems depends on the order parameter of the liquid crystal (given by the temperature) and the domain order parameter, induced by external stimuli, such as (rubbed) substrates or magnetic or electric fields. We studied thick PSLCs where substrate interactions cannot be employed and used magnetic fields instead. We show how the polymerization conditions, i.e., the temperature and the magnetic field, influence the overall order parameter in 4-octyl-4′-cyanobiphenyl (8CB)-based PSLCs. Optimal macroscopic alignment was obtained in samples polymerized at room temperature and at magnetic fields in excess of 5 T. The effect of mesh network can be quantified by introducing a phenomenological constant, which is correlated to the order parameter at the polymerizatio...

The fanned out, noncollinear spin structure of the Fe(Co) sublattice in amorphous NdFeCo and PrFeCo alloys is shown to strongly affect its ultrafast laser-induced magnetization dynamics. An overshooting effect is discovered at low applied magnetic fields, where the magnetization temporarily increases above its equilibrium value. We explain this phenomenon by considering the dynamics of the noncollinear spin structure. After femtosecond laser excitation the system first reconstructs magnetic order on a time scale of 50--150 ps arriving at a state with a smaller opening angle of the fan. Subsequently, the original opening angle is restored on a time scale of nanoseconds. Increasing the field up to 0.6 T we can fully close the fan and therewith suppress the overshooting behavior.

A micrometric photoluminescence imaging technique is used to study the in-plane motion of excitons in single GaAs quantum wells, as a function of well width, temperature and magnetic field. Contrary to previous investigations, it is found that the motion of resonantly excited excitons can be resolved by far-field optics. For relatively wide wells anomalies in the exciton motion are observed, which suggests a contribution of exciton polaritons to the in-plane motion.

We report an optical study of Side Chain Polymer Liquid Crystals (SCPLCs) in magnetic fields up to 20T. A novel sensitive set-up, which allows simultaneous birefringence and static light scattering measurements, was used to study polynorborene and polyacrylate based SCPLCs. We find that only magnetic fields bigger than a threshold value B min are able to induce orientation effects and this only at the crossing of the Isotropic-Nematic (IN) transition when cooling from the melt. The existence of a minimum field B min proves to be general, while its value is strongly dependent on the polymeric backbone elasticity, polymerization degree and spacer length. We have produced bulk and thin film aligned samples, stable at room temperature, characterized by an optical uniaxial anisotropy, with a director order parameter S D ∼0.9999.

We report a systematic study of low-energy lattice vibrations in the layered systems KY(MoO4)2, KDy(MoO4)2, KEr(MoO4)2, and KTm(MoO4)2. A layered crystal structure and low symmetry of the local environment of the rare-earth ion cause the appearance of vibrational and electronic excitations in Terahertz frequencies. The interaction between these excitations leads to sophisticated dynamical properties, including non-linear effects in paramagnetic resonance spectra. The THz study in magnetic field allows for the clear distinction between lattice vibrations and electronic excitations. We measured the THz transmission spectra and show that the low energy lattice vibrations in binary molybdates can be well described within the quasi-one-dimensional model. The developed model describes the measured far-infrared spectra, and results of our calculations agree with previous Raman and ultrasound studies.

The two dimensional diffusion of single excitons in GaAs/AlGaAs quantum wells has been measured, at 4.2 K and in magnetic fields B up to 12 T, using spatially-resolved photoluminescence spectroscopy. At zero field an exceptionally high diffusion constant D (600 cm 2 /s) has been found, which reduces quadratically with the application of a small B , due to the field induced increase of the excitonic mass. For higher magnetic fields the diffusion constant decreases as D ∼ B −2 , in agreement with theoretical predictions which include quantum corrections in the exciton motion.

The in-plane motion of excitons (X) and negatively charged excitons (X−) in a doped CdTe/CdMgTe quantum well has been measured at 4.2 K in magnetic fields up to 12 T, using spatially resolved photoluminescence (PL) spectroscopy. The populations of X and X− have been found to be in local dynamical equilibrium. The measured PL patterns in real space could therefore be described by considering free and mobile excitons in equilibrium with localised charged excitons.

We have measured the near-infrared photoluminescence spectrum of phosphorus doped silicon (Si: P) and extracted the donor-bound exciton (D0X) energy at magnetic fields up to 28 T. At high field the Zeeman effect is strongly nonlinear because of the diamagnetic shift, also known as the quadratic Zeeman effect (QZE). The magnitude of the QZE is determined by the spatial extent of the wave-function. High field data allows us to extract values for the radius of the neutral donor (D0) ground state, and the light and heavy hole D0X states, all with more than an order of magnitude better precision than previous work. Good agreement was found between the experimental state radius and an effective mass model for D0. The D0X results are much more surprising, and the radius of the mJ=±3/2 heavy hole is found to be larger than that of the mJ=±1/2 light hole.

We have developed a sensitive, multipurpose torsional magnetometer with optical detection of the torque. The use of a feedback system with a current coil mounted with the sample allows direct, quantitative determination of the magnetization with a sensitivity of 10−12 J/T in a Bitter-magnet and 2×10−13 J/T at 15 T in a superconducting magnet. The system can be used over a wide range of temperatures and up to high magnetic fields. To demonstrate the sensitivity and versatility of our magnetometer, we present magnetization measurements of a multisubband two-dimensional electron gas (3×1011 spins) and of a 0.13 mg crystal of the organic conductor κ-(BEDT-TTF)2Cu(NCS)2.

We show that cooling side chain polymer liquid crystals in a magnetic field, from the isotropic to nematic and subsequently to the glass phase, results in a macroscopically ordered, transparent, and strongly birefringent material. The aligned samples retain their properties after the field is removed and can be dealigned only by heating them to the isotropic phase. To induce alignment, a threshold field B t h is necessary, which strongly depends on the chemical structure details. B t h reflects the strength of mesogene-polymer backbone coupling, and we will show that this interaction is responsible for the stability of the induced alignment at zero field.

Optical reflectivity studies of the ferromagnetic metal (FMM) to antiferromagnetic insulator (AFI) phase transition are performed on Nd0.5Sr0.5MnO3 manganite in a wide temperature and magnetic field range. The formation of a domain structure in the AFI state during the FMM−AFI phase transition is observed. It is shown that the two types of domains observed are energetically equivalent states. On the basis of the experimental results and symmetry analysis we conclude that these domains are crystal twins. The twin domain structure of the AFI state in the Nd0.5Sr0.5MnO3 is visible in reflected unpolarized light due to a different tilting of the surface in the domains. The two-phase domain structure FMM+AFI formed in the vicinity of the phase transition is also studied. It is found that a thermodynamically equilibrium two-phase stripe domain structure does not develop. The absence of the magnetic intermediate state is due to the large energy of the interphase wall, which results in the stripe structure period...