Staff

Abstract:
Nanotubes and nanorods have been recently established as very good materials to work as electron sources in a field emission (FE) process. These are one-dimensional materials and electron-electron interactions are expected to play a crucial role in their physics. Here we study the influence of electron-electron interactions on the field emission. We study the problem in the low energy regime; thus we need to abandon the antiadiabatic approximation and derive tunneling amplitude for a finite duration of the tunneling process. In this work we identified the parameters when exact analytic expression for tunneling current can be given. We obtained formalism that enables one to capture at the same time the collective effects due to electron-electron interactions and thermionic emission. Our results reveal that different types of nanotubes, and their minigap/compressibility parameters, can be easily distinguished based on FE measurements on these materials.

Abstract:
We calculated the structural, electronic, and magnetic properties of FeSe within density-functional theory at
the generalized gradient approximation level. First, we studied how the bandwidth of the d-bands at the Fermi
energy is renormalized by adding simple corrections: Hubbard U , Hund’s J, and by introducing long-range
magnetic orders. We found that introducing either a striped or a staggered dimer antiferromagnetic order brings
the bandwidths—which are starkly overestimated at the generalized gradient approximation level—closer to
those experimentally observed. Second, for the ferromagnetic, the striped, checkerboard, and staggered dimer
antiferromagnetic order, we investigate the change in magnetic formation energy with local magnetic moment
of Fe at a pressure up to 6 GPa. The bilinear and biquadratic exchange energies are derived from the Heisenberg
model and noncollinear first-principles calculations, respectively. We found a nontrivial behavior of the spin-
exchange parameters on the magnetization, and we put forward a field-theory model that rationalizes these results
in terms of two-dimensional spin and orbital fluctuations. The character of these fluctuations can be either that
of a standard density wave or a topological vortex. Topological vortices can result in mesoscopic magnetization
structures.

Abstract:
Upon cooling, condensed-matter systems typically transition into states of lower symmetry. The converse—i.e., the emergence of higher symmetry at lower temperatures—is extremely rare. In this work, we show how an unusually isotropic magnetoresistance in the highly anisotropic, one-dimensional conductor Li0.9Mo6O17 and its temperature dependence can be interpreted as a renormalization group (RG) flow toward a so-called separatrix. This approach is equivalent to an emergent symmetry in the system. The existence of two distinct ground states, Mott insulator and superconductor, can then be traced back to two opposing RG trajectories. By establishing a direct link between quantum field theory and an experimentally measurable quantity, we uncover a path through which emergent symmetry might be identified in other candidate materials.

Abstract:
We conducted a thorough investigation into the temperature dependence of the metallic-like bands of Si(553)-Au using angular-resolved photoemission spectroscopy (ARPES). Our study addresses the challenges posed by the short-term stability of the surface and photo-voltage effects, which we overcame to extract changes in the band-filling and Fermi-velocity. Our findings shed light on the low-temperature phase of the step edge in Si(553)-Au, which has been a topic of ongoing debate regarding its structural or electronic nature. Through comparison with theoretical predictions of a structural-related low-temperature to high-temperature phase transition, we discovered that the band-filling and Fermi-velocity do not change accordingly, thereby ruling out this scenario. Our study contributes to a better understanding of this material system and provides an important reference for future research.

Abstract:
Herein, the canonical Fowler–Nordheim theory is extended by computing the zero-temperature transmission probability for the more general case of a barrier described by a fractional power law. An exact analytical formula is derived, written in terms of Gauss hypergeometric functions, that fully capture the transmission probability for this generalized problem, including screened interaction with the image potential. First, the quality of approximation against the so far most advanced formulation of Fowler–Nordheim, where the transmission is given in terms of elliptic integrals, is benchmarked. In the following, as the barrier is given by a power law, in detail, the dependence of the transmission probability on the exponent of the power law is analyzed. The formalism is compared with results of numerical calculations and its possible experimental relevance is discussed. Finally, it is discussed how the presented solution can be linked in some specific cases with an exact quantum-mechanical solution of the quantum well problem.

Abstract:
One of the key issues in the physics of topological insulators is whether the topologically non trivial properties survive at finite temperatures and, if so, whether they disappear only at the temperature of topological gap closing. Here, we study this problem, using quantum fidelity as a measure, by means of ab-initio methods supplemented by an effective dissipative theory built on the top of the ab-initio electron and phonon band structures. In the case of SnTe, the prototypical crystal topological insulator, we reveal the presence of a characteristic temperature, much lower than the gap-closing one, that marks a loss of coherence of the topological state. The transition is not present in a purely electronic system but it appears once we invoke coupling with a dissipative bosonic bath. Features in the dependence with temperature of the fidelity susceptibility can be related to changes in the band curvature, but signatures of a topological phase transition appear in the fidelity only though the non-adiabatic coupling with soft phonons. Our argument is valid for valley topological insulators, but in principle can be generalized to the broader class of topological insulators which host any symmetry-breaking boson.

Abstract:
In the present article, a unifying approach to description of different Heisenberg models (i.e., XY model, isotropic case, easy-plane, and easy-axis regimes) for spin distributions on curved surfaces is discussed. After introduction of isothermal (conformal) metrics on surfaces of revolution, the axially symmetric solution of the Euler–Lagrange equation for the quasi-one-dimensional magnetic Hamiltonian has been obtained for all four discussed cases in the form of an incomplete elliptic integral of the third kind. It has been shown that the limiting case of XY model can be achieved from an easy-plane regime when the parameter of anisotropy of interaction amongst spins ? is approaching −1, while the isotropic case can be achieved from both easy-plane and easy-axis regimes when ? is approaching 0. After the application of the corresponding boundary conditions, the soliton-like solutions have been obtained that either cover the whole surface (in a one-twist or multi-twist manner) or can be arranged in the form of a periodic lattice structure on the curved surface. The general unifying scheme has been illustrated using the exemplary surfaces of revolution with zero (cylinder), negative (catenoid), and positive (sphere) Gaussian curvatures.

Keywords:
elliptic integrals and elliptic functions, Heisenberg models for spin distributions, isothermal (conformal) metrics, soliton-like solutions, surfaces of revolution

Abstract:
The worldwide energy crisis and environmental deterioration are probably humanity’s greatest challenges. Thermoelectricity, which allows for the mutual conversion between thermal and electrical energy, has become a promising technology to alleviate this challenge. Increasingly more research focuses on how to fabricate and apply thermoelectric materials for harvesting energy and regulating the indoor thermal environment. However, only a few studies have focused on cementitious materials with thermoelectric potential. Thermoelectric cement is a composite material in which particular additives can enhance the thermoelectric performance of ordinary cement. By potentially replacing traditional construction materials with thermoelectric cement in building ap-plications, electricity could be generated from waste heat, reducing the use of fossil fuels, and supplementing other renewable energy sources like solar and wind. This article presents a review of fundamentals, fabrication, characterization, composition, and performance, as well as modeling methods and opportunities for thermo-electric cement composites. The literature reviewed covers the period from 1998 to 2020 related to thermo-electric cement. It also presents the challenges and problems to overcome for further development and provide future research directions of thermoelectric cement.

Keywords:
thermoelectric cement composites, additives, thermoelectric generator, thermoelectric cooler, seebeck coefficient, figure of merit

Abstract:
Time-periodic driving facilitates a wealth of novel quantum states and quantum engineering. The interplay of loquet states and strong interactions is particularly intriguing, which we study using timeperiodic fields in a one-dimensional quantum gas, modeled by a Luttinger liquid with periodically changing interactions. By developing a time-periodic operator algebra, we are able to solve and analyze the complete set of nonequilibrium steady states in terms of a Floquet-Bogoliubov ansatz and known analytic functions. Complex valued Floquet eigenenergies occur when integer multiples of the driving frequency approximately match twice the dispersion energy, which correspond to resonant states. In experimental systems of Lieb-Liniger bosons we predict a change from power-law correlations to dominant collective density wave excitations at the corresponding wave numbers as the frequency is lowered below a characteristic cutoff.

Abstract:
We study the effects of Rashba splitting on two-leg ladders with weakly screened Coulomb interactions. Past research has shown that in this class of systems the two backscattering channels with the largest amplitude favor ordering of canonically conjugated collective fields which effectively renders the system gapless. Here we show that the band-dependent Rashba spin-orbit interaction breaks this symmetry of scattering channels, leading to a new fixed point with yet unexplored instabilities. Exotic properties can be found, for instance, the mixing of the magnetism with the charge-density wave. We then investigate the physical consequences of this partial spectral gap opening. We find a striking difference in signatures of order observed in single-particle (spectral-function) and two-body (susceptibilities) experimental probes. We conclude this paper by comparing theoretical and experimental results obtained on the Au-Si(553) platform. In STM measurements, we identify the lowest-lying soliton excitation as a hallmark of the gapped sine-Gordon model in an apparently metallic system. This implies the presence of partial spectral gaps opening. Furthermore, by ARPES measurements we confirm the expected temperature dependence of the outer bands backfolding. These two findings constitute the experimental evidence of the many-body physics proposed here.

Abstract:
We derive a scheme to calculate the Tomonaga-Luttinger liquid’s (TLL) parameters and the holon velocity in a quasi-one-dimensional (quasi-1D) material that consists of two-leg ladders coupled through Coulomb interactions. First, we obtain an analytic formula for electron-electron interaction potential along the conducting axis for a generalized charge distribution in a plane perpendicular to it. Then, we introduce many-body screening that is present in a quasi-1D material by proposing an approximation for the charge susceptibility. Based on this we are able to find the TLL’s parameters and velocities. We then show how to use these to validate the experimental angle-resolved photoemission spectroscopy data measured recently in p polarization in NbSe3. Although we focus our study on this specific material, it is applicable for any quasi-1D system that consists of two-leg ladders as its basic units.

Abstract:
PbTe is a leading thermoelectric material at intermediate temperatures, largely thanks to its low lattice thermal conductivity. However, its efficiency is too low to compete with other forms of power generation. This efficiency can be effectively enhanced by designing nanostructures capable of scattering phonons over a wide range of length scales to reduce the lattice thermal conductivity. The presence of grain boundaries can reduce the thermal conductivity to ∼0.5 W m −1 K−1 for small acancy concentrations and grain sizes. However, grains anneal at finite temperature, and equilibrium and metastable grain size distributions determine the extent of the reduction in thermal conductivity. In the present work, we propose a phase-field model informed by molecular dynamics simulations to study the annealing process in PbTe and how it is affected by the presence of grain boundaries and voids. We find that the thermal conductivity of PbTe is reduced by up to 35% in the porous material at low temperatures. We observe that a phase transition at a finite density of voids governs the kinetics of impeding grain growth by Zener pinning.

Abstract:
The temperature renormalization of the bulk band structure of a topological crystalline insulator, SnTe, is calculated using first-principles methods. We explicitly include the effect of thermal-expansion-induced modification of electronic states and their band inversion on electron-phonon interaction.We show that the direct gap decreases with temperature, as both thermal expansion and electron-phonon interaction drive SnTe towards the phase transition to a topologically trivial phase as temperature increases. The band gap renormalization due to electron-phonon interaction exhibits a nonlinear dependence on temperature as the material approaches the phase transition, while the lifetimes of the conduction band states near the band edge show a nonmonotonic behawior with temperature. These effects should have important implications on bulk electronic and thermoelectric transport in SnTe and other topological insulators.

Abstract:
Topological insulators are frequently also one of the best-known thermoelectric materials. It has been recently discovered that in three-dimensional (3D) topological insulators each skew dislocation can host a pair of one-dimensional (1D) topological states—a helical Tomonaga–Luttinger liquid (TLL). We derive exact analytical formulae for thermoelectric Seebeck coefficient in TLL and investigate up to what extent one can ascribe the outstanding thermoelectric properties of Bi2Te3 to these 1D topological states. To this end we take a model of a dense dislocation network and find an analytic formula for an overlap between 1D (the TLL) and 3D electronic states. Our study is applicable to a weakly n-doped Bi2Te3 but also to a broader class of nano-structured materials with artificially created 1D systems. Furthermore, our results can be used at finite frequency settings, e.g. to capture transport activated by photo-excitations.

Abstract:
We explore a mechanism that allows to couple electrons with the transverse-optical (TO) phonon branch in a regime when the TO mode becomes highly anharmonic and drives the ferroelectric phase transition. We show that this anharmonicity, which leads to a collective motion of ions, is able to couple electronic and lattice displacement fields. An effective correlated electron-ion dynamics method is required to capture the effect of the onset of the local electric polarization due to this collective behavior close to the quantum critical point. We identify an intermediate temperature range where an emergent phonon drag may contribute substantially to thermoelectric conductivity in this regime. We find that, under optimal conditions, this extra contribution may be larger than values achieved so far in the benchmark material, PbTe. In the last part we make a case for the importance of our results in the generic problem of anharmonic electron-lattice dynamics.

Abstract:
We report on an emerging symmetry axis in the magnetoresistance of bulk single crystals of quasi–one-dimensional Li0.9Mo6O17 below Tmin = 25 K, the temperature at which the electrical resistivity experiences a minimum. Detailed angle-dependent magnetoresistance sweeps reveal that this symmetry axis is induced by the development of a negative magnetoresistance, which is suppressed only for magnetic fields oriented along the poles of the MoO6 octahedra that form the conducting chains. We show that this unusual negative magnetoresistance is consistent with the melting of dark excitons, composed of previously omitted orbitals within the t2g manifold that order below Tmin. The unveiled symmetry axis in directional magnetic fields not only provides evidence for the crystallization of these dark excitons but also sheds new light on the long-standing mystery of the metal-insulator transition in Li0.9Mo6O17.

Abstract:
Because of the thermoelectric (TE) effect (or Seebeck effect), a difference of potential is generated as a consequence of a temperature gradient across a sample. The TE effect has been mostly studied and engineered in semiconducting materials and it already finds several commercial applications. Only recently the TE effect in cement-based materials has been demonstrated and there is a growing interest in its potential. For instance, a temperature gradient across the external walls of a building can be used to generate electricity. By the inverse of the TE effect (or Peltier effect), one can also seek to control the indoor temperature of a building by biasing TE elements embedded in its external walls. In designing possible applications, the TE properties of cement-based materials must be determined as a function of their chemical composition. For instance, the TE properties of cement paste can be enhanced by the addition of metal oxide (e.g., Fe2O3) powder. In this paper, a single thermoelectric leg is studied using the finite element method. Metal oxide additives in the cement paste are modelled as spherical inhomogeneities. The thermoelectric properties of the single components are based on experimental data, while the overall thermoelectric properties of the composites are obtained from the numerical model. The results of this numerical study are interpreted according to the effective medium theory (EMT) and its generalisation (GEMT). KEY WORDS: Cement composites; Thermoelectrics; Seebeck C

Keywords:
Cement composites; Thermoelectrics; Seebeck Coefficient; Electrical Conductivity; Thermal Conductivity

Abstract:
Nanotubes and nanorods have been recently established as very good materials to build electron sources in the cold emission process. These are 1D materials where electron-electron interactions are known to play a crucial role in their physics. The interactions in 1D systems lead to a collective modes’ physics that is usually described using Tomonaga-Luttinger liquid (TLL) formalism. The advantage is that within thismethod all correlation functions are known and can be expressed interms of power laws with non-universal, interaction dependent, ex-ponents. To capture this situation we generalize a canonical Fowler-Northeim theory of field emission to solve the case of a barrier described by any power-law potential. With this generalization, expressed in terms of a confluent hypergeometric function, we are able to compute currents from arrays of carbon nanotubes. We shall present results showing an influence of various interaction terms, as encoded in varying TLL parameters, as well as effects of a finite temperature.