Robert Biele

TiS3 Transistors with Tailored Morphology and Electrical Properties

Control over the morphology of TiS3 is demonstrated by synthesizing 1D nanoribbons and 2D nanosheets. The nanosheets can be exfoliated down to a single layer. Through extensive characterization of the two morphologies, differences in the electronic properties are found and attributed to a higher density of sulphur vacancies in nanosheets, which, according to density functional theory calculations, leads to an n-type doping.

Titanium trisulfide (TiS3) : A 2D semiconductor with quasi-1D optical and electronic properties

We present characterizations of few-layer titanium trisulfide (TiS3) flakes which, due to their reduced in-plane structural symmetry, display strong anisotropy in their electrical and optical properties. Exfoliated few-layer flakes show marked anisotropy of their in-plane mobilities reaching ratios as high as 7.6 at low temperatures. Based on the preferential growth axis of TiS3 nanoribbons, we develop a simple method to identify the in-plane crystalline axes of exfoliated few-layer flakes through angle resolved polarization Raman spectroscopy. Optical transmission measurements show that TiS3 flakes display strong linear dichroism with a magnitude (transmission ratios up to 30) much greater than that observed for other anisotropic two-dimensional (2D) materials. Finally, we calculate the absorption and transmittance spectra of TiS3 in the random-phase-approximation (RPA) and find that the calculations are in qualitative agreement with the observed experimental optical transmittance.

Electronics and optoelectronics of quasi-1D layered transition metal trichalcogenides

This work was supported by the Netherlands Organization for Scientific Research (NWO/FOM). AJM-M acknowledges the financial support of MINISTERIO DE CIENCIA E INNOVACION (MICINN) (Spain) through the scholarship BES2012–057346. R.D’A and RB acknowledge financial support by the DYN-XC-TRANS (Grant No. FIS2013-43130-P), NanoTHERM (Grant No. CSD2010- 00044), and SElecT-DFT (FIS2016-79464-P) of the Ministerio de Economia y Competitividad (MINECO), and Grupo Consolidado UPV/EHU del Gobierno Basco (Grant No. IT578-13). RB acknowledges the financial support of the Ministerio de Educacion, Cultura y Deporte (Grant No. FPU12/01576). AC-G acknowledges financial support from the European Commission under the Graphene Flagship, contract CNECTICT-604391, from the MINECO (Ramon y Cajal 2014 program, RYC-2014-01406) and from the MICINN (MAT2014-58399-JIN). MIRE Group thanks MINECO (MAT2015-65203R) for financial support. E Flores also acknowledges the Mexican National Council for Science and Technology (CONACyT).

Electronic Bandgap and Exciton Binding Energy of Layered Semiconductor TiS3

A study of the electronic and optical bandgap is presented in layered TiS3, an almost unexplored semiconductor that has attracted recent attention because of its large carrier mobility and inplane anisotropic properties, to determine its exciton binding energy. Scanning tunneling spectroscopy and photoelectrochemical measurements are combined with random phase approximation and Bethe–Salpeter equation calculations to obtain the electronic and optical bandgaps and thus the exciton binding energy. Experimental values are found for the electronic bandgap, optical bandgap, and exciton binding energy of 1.2 eV, 1.07 eV, and 130 meV, respectively, and 1.15 eV, 1.05 eV, and 100 meV for the corresponding theoretical results. The exciton binding energy is orders of magnitude larger than that of common semiconductors and comparable to bulk transition metal dichalcogenides, making TiS3 ribbons a highly interesting material for optoelectronic applications and for studying excitonic phenomena even at room temperature.

A stochastic approach to open quantum systems

Stochastic methods are ubiquitous to a variety of fields, ranging from physics to economics and mathematics. In many cases, in the investigation of natural processes, stochasticity arises every time one considers the dynamics of a system in contact with a somewhat bigger system, an environment with which it is considered in thermal equilibrium. Any small fluctuation of the environment has some random effect on the system. In physics, stochastic methods have been applied to the investigation of phase transitions, thermal and electrical noise, thermal relaxation, quantum information, Brownian motion and so on. In this review, we will focus on the so-called stochastic Schrödinger equation. This is useful as a starting point to investigate the dynamics of open quantum systems capable of exchanging energy and momentum with an external environment. We discuss in some detail the general derivation of a stochastic Schrödinger equation and some of its recent applications to spin thermal transport, thermal relaxation, and Bose-Einstein condensation. We thoroughly discuss the advantages of this formalism with respect to the more common approach in terms of the reduced density matrix. The applications discussed here constitute only a few examples of a much wider range of applicability.

Controlling heat and particle currents in nanodevices by quantum observation

We demonstrate that in a standard thermo-electric nanodevice the current and heat flows are not only dictated by the temperature and potential gradient, but also by the external action of a local quantum observer that controls the coherence of the device. Depending on how and where the observation takes place, the direction of heat and particle currents can be independently controlled. In fact, we show that the current and heat flow in a quantum material can go against the natural temperature and voltage gradients. Dynamical quantum observation offers new possibilities for the control of quantum transport far beyond classical thermal reservoirs. Through the concept of local projections, we illustrate how we can create and directionality control the injection of currents (electronic and heat) in nanodevices. This scheme provides novel strategies to construct quantum devices with application in thermoelectrics, spintronic injection, phononics, and sensing among others. In particular, highly efficient and selective spin injection might be achieved by local spin projection techniques.THERMOELECTRICS: Observe and controlThe presence of a quantum observer, acting locally in a thermoelectric nanodevice, is shown to be enough to change its heat and electronic transport path. In quantum mechanics the observation of a phenomenon can affect its outcome. Local quantum observation of a system can induce continuous and dynamic changes in its quantum coherence, thus providing another level of control of its properties. Angel Rubio and colleagues study theoretically this idea in a nanodevice whose left and right side are connected to hot and thermal baths, respectively; this configuration forces the energy and particles to flow clockwise. Introduction of a quantum observer however, induces a counter-clockwise particle ring-current due to the localized electronic state and symmetry breaking of the system. Such heat flow control might prove useful for spintronics, quantum phononics, and sensing devices.

Time-Dependent Thermal Transport Theory

Understanding thermal transport in nanoscale systems presents important challenges to both theory and experiment. In particular, the concept of local temperature at the nanoscale appears difficult to justify. Here, we propose a theoretical approach where we replace the temperature gradient with controllable external blackbody radiations. The theory recovers known physical results, for example, the linear relation between the thermal current and the temperature difference of two blackbodies. Furthermore, our theory is not limited to the linear regime and goes beyond accounting for nonlinear effects and transient phenomena. Since the present theory is general and can be adapted to describe both electron and phonon dynamics, it provides a first step toward a unified formalism for investigating thermal and electronic transport.

Strain-induced band gap engineering in layered TiS3

By combining ab initio calculations and experiments, we demonstrate how the band gap of the transition metal trichalcogenide TiS3 can be modified by inducing tensile or compressive strain. In addition, using our calculations, we predicted that the material would exhibit a transition from a direct to an indirect band gap upon application of a compressive strain in the direction of easy electrical transport. The ability to control the band gap and its nature could have a significant impact on the use of TiS3 for optical applications. We go on to verify our prediction via optical absorption experiments that demonstrate a band gap increase of up to 9% (from 0.99 to 1.08 eV) upon application of tensile stress along the easy transport direction.

The temperature dependent anisotropy constants of epitaxially grown PrCo5+x

The temperature dependent anisotropy of a highly textured epitaxial Pr–Co film with a single orientation of the crystallographic c-axis along MgO[001] is investigated by measuring angle dependent hysteresis loops at various temperatures. The measured magnetization curves are compared with calculated magnetization curves, which allows for a full analysis of the temperature dependent anisotropy constants of first and second order, K1 and K2 and the determination of the saturation polarization. The analysis reveals that the Pr15.4Co84.6 film undergoes a spin reorientation transition from an easy axis anisotropy to an easy cone anisotropy at 108 K. The room temperature values of K1 and K2 measured for this Pr15.4Co84.6 film are 5.0 MJ m−3 and 0.5 MJ m−3, respectively. The difference to the bulk PrCo5 single crystal values is ascribed to the high Co content of the Pr15.4Co84.6 film.

Application of a time-convolutionless stochastic Schrödinger equation to energy transport and thermal relaxation

Quantum stochastic methods based on effective wave functions form a framework for investigating the generally non-Markovian dynamics of a quantum-mechanical system coupled to a bath. They promise to be computationally superior to the master-equation approach, which is numerically expensive for large dimensions of the Hilbert space. Here, we numerically investigate the suitability of a known stochastic Schrödinger equation that is local in time to give a description of thermal relaxation and energy transport. This stochastic Schrödinger equation can be solved with a moderate numerical cost, indeed comparable to that of a Markovian system, and reproduces the dynamics of a system evolving according to a general non-Markovian master equation. After verifying that it describes thermal relaxation correctly, we apply it for the first time to the energy transport in a spin chain. We also discuss a portable algorithm for the generation of the coloured noise associated with the numerical solution of the non-Markovian dynamics.