Here we show, for the first time, evidence of the primary molecular structures in humic substances (HS), the most abundant naturally occurring organic molecules on Earth, and their associations as mixtures in terrestrial systems. Multi-dimensional nuclear magnetic resonance (NMR) experiments show us that the major molecular structural components in the mixtures operationally defined as HS are aliphatic acids, ethers, esters and alcohols; aromatic lignin derived fragments; polysaccharides and polypeptides. By means of diffusion ordered spectroscopy, distinct diffusion coefficients consistent with relatively low molecular weight molecules were observed for all the components in the mixtures, and saccharides were the largest single class of component present. Liquid chromatography NMR confirmed that HS components can be easily separated and nuclear Overhauser effect (NOE) enhancements support the finding that the components are of relatively low molecular weight

The appeal of lasers can be attributed to both their ubiquitous applications and their role as model systems for elucidating nonequilibrium and cooperative phenomena [1]. Introducing novel concepts in lasers thus has a potential for both applied and fundamental implications [2]. Here we experimentally demonstrate that the coupling between carrier spin and light polarization in common semiconductor lasers can enable room-temperature modulation frequencies above 200 GHz, exceeding by nearly an order of magnitude the best conventional semiconductor lasers. Surprisingly, this ultrafast operation relies on a short carrier spin relaxation time and a large anisotropy of the refractive index, both commonly viewed as detrimental in spintronics [3] and conventional lasers [4]. Our results overcome the key speed limitations of conventional directly modulated lasers and offer a prospect for the next generation of low-energy ultrafast optical communication.The global internet traffic will continue its dramatic increase in the near future [5]. Short-range and energy-efficient optical communication networks provide most of the communication bandwidth to secure the digital revolution. Key devices for high-speed optical interconnects, in particular in server farms, are current-driven intensity-modulated vertical-cavity surface-emitting lasers (VCSELs) [4]. Analogous to a driven damped harmonic oscillator, modulated lasers have a resonance frequency f R for the relaxation oscillations of the light intensity [6]. For higher frequencies the response decays and reaches half of its low-frequency value at f 3dB ≈ 1 + √ 2f R , which quantifies the usable frequency range [4]. In conventional VCSELs the modulation bandwidth is limited by the dynamics of the coupled carrier-photon system and parasitic as well as thermal effects. The current record is f 3dB = 34 GHz [7]. Common approaches to enhance the bandwidth rely on the expression f R = v g aS/τ p /(2π), where v g is the group velocity, a the differential gain, S the photon density, and τ p the * markus.lindemann@rub.de † nils.gerhardt@rub.de arXiv:1807.02820v1 [cond-mat.mes-hall]

We report spin-induced polarization oscillations in vertical-cavity surface-emitting lasers above threshold and at room temperature. The oscillation frequency is 11.6 GHz, which is significantly higher than the modulation bandwidth of less than 4 GHz in the device. The oscillation frequency is determined by an additional resonance frequency in birefringence containing microcavities, which is potentially much higher than the conventional relaxation oscillation frequency. The damping of the oscillations can be controlled by the current, allowing for oscillation lifetimes much longer than the spin lifetime in the device as well as for short bursts potentially interesting for information transmission.

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Measurements of the thermal conductivity as a function of temperature and magnetic field in the 2D dimer spin system SrCu2(BO3)2 are presented. In zero magnetic field the thermal conductivity along and perpendicular to the magnetic planes shows a pronounced double-peak structure as a function of temperature. The low-temperature maximum is drastically suppressed with increasing magnetic field. Our quantitative analysis reveals that the heat current is due to phonons and that the double-peak structure arises from pronounced resonant scattering of phonons by magnetic excitations.During the last few years the thermal conductivity (κ) of low dimensional spin systems has attracted considerable interest [1][2][3][4][5][6]. One reason is that in these materials a large magnetic contribution κ mag to the heat current may be present as observed e.g. for the spin ladder material Sr 14−x Ca x Cu 24 O 41 [6]. Another reason is that the phononic heat current κ ph probes the spectrum of magnetic excitations as well as the spin-phonon coupling [3][4][5]. The latter is very important in some of the low dimensional spin systems, e.g. in the spin-Peierls compound CuGeO 3 [4,5]. Both issues, the magnetic contribution to the heat current as well as the interaction of the phonons with magnetic excitations, are to a large extent unexplored and not well understood.A material of particular interest in this context is SrCu 2 (BO 3 ) 2 (SCBO) [7]. The Cu 2+ ions form a quasi-2D spin system which is by virtue of the crystal geometry an experimental realization of the Shastry-Sutherland model [8]. In SCBO the intra-dimer and inter-dimer couplings are of magnitude J 1 ≃ 72 K and J 2 ≃ 43 K, i.e. J 2 /J 1 ≃ 0.6 [9]. As expected for this ratio SCBO has a dimerized singlet ground-state separated from the excited triplet states by a finite gap ∆ ≃ 35 K as seen in the magnetic susceptibility [10] or in inelastic neutron scattering [11]. From the latter it is also known that the triplet excitations are almost dispersionless, i.e the group velocity is very small, in agreement with theoretical calculations [12][13][14][15]. Thus, a sizeable κ mag is not expected for SCBO making this material a natural candidate to study the influence of magnetic excitations on κ ph . In this letter we present measurements of the thermal conductivity of SCBO along (κ a ) and perpendicular (κ c ) to the 2D spin planes in a large temperature (2.5-275 K) and magnetic field range (0-17 T). In zero field, both κ a and κ c show pronounced double-peak structures as a function of T . For both directions the low-T maximum is drastically suppressed by a magnetic field. We present a model based on a purely phononic thermal conductivity and explain the double-peak structure and its field dependence by strong damping of the phononic heat current due to resonant scattering of phonons by magnetic excitations. With the same parameters our model considerably improves the description of the magnetic field dependence of the specific heat of SCBO [16].For our study two samples of rectan...

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We present a THz investigation of histo-pathological samples including the larynx of a pig and a human liver with metastasis. Our measurements show that different types of tissue can be clearly distinguished in THz transmission images, either within a single image or by a comparison of images obtained for different frequency windows. This leads to the problem that images obtained for different frequencies inherently have a different spatial resolution. An image obtained from two such images by a simple mathematical operation may contain artefacts. We discuss measures to deal with this problem. Furthermore, we investigate the possibility of improving the spatial resolution of THz images. Finally, we present a cw THz imaging system based on a photomixer and an external cavity semiconductor laser that allows for simultaneous two-mode operation. The cw system is less expensive and more compact than conventional time-domain imaging systems.

We analyze the potential for the spin manipulation of vertical-cavity surface-emitting lasers (VCSELs) by operating them electrically and injecting additional spin-polarized carriers by polarized optical excitation. The output polarization of the VCSELs can be easily controlled by the spin orientation of the optically injected carriers when the injection current does not exceed the threshold current