arXiv subjects

S. Haas

Publications and source records attributed to S. Haas.

At least 19 recordsLinked to original sources

arXiv2020

CentOS Linux for the ATLAS MUCTPI Upgrade

A new Muon-to-Central-Trigger Processor Interface (MUCTPI) was built as part of the upgrade of the ATLAS Level-1 trigger system for the next Run of the Large Hadron Collider at CERN. The MUCTPI has 208 high-speed optical serial links for receiving muon candidates from the muon trigger detectors. Three high-end FPGAs are used for real-time processing of the muon candidates, for sending trigger information to other parts of the trigger system, and for sending summary information to the data acquisition and monitoring system. A System-on-Chip (SoC) is used for the control, configuration and monitoring of the hardware and the operation of the MUCTPI. The SoC consists of an FPGA part and a processor system. The FPGA part provides communication with the processing FPGAs, while the processor system runs software for communication with the run-control system of the ATLAS experiment. In this paper we will describe our experience with running CentOS Linux on the SoC. Cross-compilation together with the existing framework for building of the ATLAS trigger and data acquisition (TDAQ) software is being used in order to allow the deployment of the TDAQ software directly on the SoC.

physics.ins-det↗

arXiv2019

Spatial BCS-BEC crossover in superconducting p-n junctions

We present a theory of superconducting p-n junctions. We consider a 2-band model of doped bulk semiconductors with attractive interactions between the charge carriers and derive the superconducting order parameter, the quasiparticle density of states and the chemical potential as a function of semiconductor gap $\Delta_0$ and the doping level $\varepsilon$. We verify previous results for the quantum phase diagram (QPD) for a system with constant density of states in the conduction and valence band, which show BCS-Superconductor to Bose-Einstein-Condensation (BEC) and BEC to Insulator transitions as function of doping level and band gap. Then, we extend it to a 3D density of states and derive the QPD, finding that a BEC phase can only exist for small band gaps $\Delta_0 < \Delta_0^*$. For larger band gaps, there is a direct transition from an insulator to a BCS phase. Next, we apply this theory to study the properties of superconducting p-n junctions, deriving the spatial variation of the superconducting order parameter along the p-n junction. We find a spatial crossover between a BCS and BEC condensate, as the density of charge carriers changes across the p-n junction. For the 2D system, we find two regimes, when the bulk is in a BCS phase, a BCS-BEC-BCS junction with a single BEC layer, and a BCS-BEC-I-BEC-BCS junction with two layers of BEC condensates separated by an insulating layer. In 3D there can also be a conventional BCS-I-BCS junction for semiconductors with band gaps exceeding $\Delta_0^*$. Thus, there can be BEC layers in the well controlled setting of doped semiconductors, where the doping level can be varied to change the thickness of BEC layers, making Bose Einstein Condensates possibly accessible to experimental transport and optical studies in solid state materials.

cond-mat.supr-con↗

arXiv2018

Plasmonic Superconductivity in Layered Materials

Plasmonic excitations behave fundamentally different in layered materials in comparison to bulk systems. They form gapless modes, which in turn couple at low energies to the electrons. Thereby they can strongly influence superconducting instabilities. Here, we show how these excitations can be controlled from the outside via changes in the dielectric environment or in the doping level, which allows for external tuning of the superconducting transition temperature. By solving the gap equation for an effective system, we find that the plasmonic influence can both strongly enhance or reduce the transition temperature, depending on the details of the plasmon-phonon interplay. We formulate simple experimental guidelines to find plasmon- induced elevated transition temperatures in layered materials.

cond-mat.supr-con↗

arXiv2017

Disordered Quantum Spin Chains with Long-Range Antiferromagnetic Interactions

We investigate the magnetic susceptibility $χ(T)$ of quantum spin chains of $N=1280$ spins with power-law long-range antiferromagnetic coupling as a function of their spatial decay exponent $α$ and cutoff length $ξ$. The calculations are based on the strong disorder renormalization method which is used to obtain the temperature dependence of $χ(T)$ and distribution functions of couplings at each renormalization step. For the case with only algebraic decay ($ ξ= \infty$) we find a crossover at $α^*=1.066$ between a phase with a divergent low-temperature susceptibility $χ(T\rightarrow 0) $ for $α> α^*$ to a phase with a vanishing $χ(T\rightarrow 0) $ for $α< α^*$. For finite cutoff lengths $ξ$, this crossover occurs at a smaller $α^*(ξ)$. Additionally we study the localization of spin excitations for $ ξ= \infty$ by evaluating the distribution function of excitation energies and we find a delocalization transition that coincides with the opening of the pseudo-gap at $α_c=α^*$.

cond-mat.dis-nn↗

arXiv2016

Testing conformal mapping with kitchen aluminum foil

We report an experimental verification of conformal mapping with kitchen aluminum foil. This experiment can be reproduced in any laboratory by undergraduate students and it is therefore an ideal experiment to introduce the concept of conformal mapping. The original problem was the distribution of the electric potential in a very long plate. The correct theoretical prediction was recently derived by A. Czarnecki (Can. J. Phys. 92, 1297 (2014)).

physics.ed-ph↗

arXiv2016

Interplay of screening and superconductivity in low-dimensional materials

A quantitative description of Coulomb interactions is developed for two-dimensional superconducting materials, enabling us to compare intrinsic with external screening effects, such as those due to substrates. Using the example of a doped monolayer of MoS2 embedded in a tunable dielectric environment, we demonstrate that the influence of external screening is limited to a length scale, bounded from below by the effective thickness of the quasi-two-dimensional material and from above by its intrinsic screening length. As a consequence, it is found that unconventional Coulomb-driven superconductivity cannot be induced in MoS2 by tuning the substrate properties alone. Our calculations of the retarded Morel-Anderson Coulomb potential {μ*} reveal that the Coulomb interactions, renormalized by the reduced layer thickness and the substrate properties, can shift the onset of the electron-phonon driven superconducting phase in monolayer MoS2 but do not significantly affect the critical temperature at optimal doping.

cond-mat.supr-con↗

arXiv2016

Valley Plasmonics in the Dichalcogenides

The rich phenomenology of plasmonic excitations in the dichalcogenides is analyzed as a function of doping. The many-body polarization, the dielectric response function and electron energy loss spectra are calculated using an ab initio based model involving material-realistic Coulomb interactions, band structure and spin-orbit coupling. Focusing on the representative case of MoS$_2$, a plethora of plasmon bands are observed, originating from scattering processes within and between the conduction or valence band valleys. We discuss the resulting square-root and linear collective modes, arising from long-range versus short-range screening of the Coulomb potential. We show that the multi-orbital nature of the bands and spin-orbit coupling strongly affects inter-valley scattering processes by gapping certain two-particle modes at large momentum transfer.

cond-mat.mes-hall↗

arXiv2015

Many-Body Localization Transition in Random Quantum Spin Chains with Long-Range Interactions

While there are well established methods to study delocalization transitions of single particles in random systems, it remains a challenging problem how to characterize many body delocalization transitions. Here, we use a generalized real-space renormalization group technique to study the anisotropic Heisenberg model with long-range interactions, decaying with a power $α$, which are generated by placing spins at random positions along the chain. This method permits a large-scale finite-size scaling analysis. We examine the full distribution function of the excitation energy gap from the ground state and observe a crossover with decreasing $α$. At $α_c$ the full distribution coincides with a critical function. Thereby, we find strong evidence for the existence of a many body localization transition in disordered antiferromagnetic spin chains with long range interactions.

cond-mat.dis-nn↗

arXiv2014

Phase Diagram of Electron Doped Dichalcogenides

Using first principle calculations, we examine the sequence of phases in electron doped dichalcogenides, such as recently realized in field-gated MoS$_2$. Upon increasing the electron doping level, we observe a succession of semiconducting, metallic, superconducting and charge density wave regimes, i.e. in different order compared to the phase diagram of (semi-)metallic dichalcogenides such as TiSe$_2$. Both instabilities trace back to a softening of phonons which couple the electron populated conduction band minima. The superconducting dome, calculated using Eliashberg theory, is found to fit the experimentally observed phase diagram, obtained from resistivity measurements. The charge density wave phase at higher electron doping concentrations as predicted from instabilities in the phonon modes is further corroborated by detecting the accompanying lattice deformation in density functional based supercell relaxations. Upon charge density wave formation, doped MoS$_2$ remains metallic but undergoes a Lifschitz transition, where the number of Fermi pockets is reduced.

cond-mat.mtrl-sci↗

arXiv2009

Topological Order Following a Quantum Quench

We determine the conditions under which topological order survives a rapid quantum quench. Specifically, we consider the case where a quantum spin system is prepared in the ground state of the Toric Code Model and, after the quench, it evolves with a Hamiltonian that does not support topological order. We provide analytical results supported by numerical evidence for a variety of quench Hamiltonians. The robustness of topological order under non-equilibrium situations is tested by studying the topological entropy and a novel dynamical measure, which makes use of the similarity between partial density matrices obtained from different topological sectors.

quant-ph↗

arXiv2009

Topological order in paired states of fermions in two-dimensions with breaking of parity and time-reversal symmetries

We numerically evaluate the entanglement spectrum (singular value decomposition of the wavefunction) of paired states of fermions in two dimensions that break parity and time-reversal symmetries, focusing on the spin-polarized $p_x+ip_y$ case. The entanglement spectrum of the weak-pairing (BCS) phase contains a Majorana zero mode, indicating non-Abelian topological order. In contrast, for the strong-pairing (BEC) phase, we find no such mode, consistent with Abelian topological order.

cond-mat.str-el↗

arXiv2008

Entanglement, fidelity and topological entropy in a quantum phase transition to topological order

We present a numerical study of a quantum phase transition from a spin-polarized to a topologically ordered phase in a system of spin-1/2 particles on a torus. We demonstrate that this non-symmetry-breaking topological quantum phase transition (TOQPT) is of second order. The transition is analyzed via the ground state energy and fidelity, block entanglement, Wilson loops, and the recently proposed topological entropy. Only the topological entropy distinguishes the TOQPT from a standard QPT, and remarkably, does so already for small system sizes. Thus the topological entropy serves as a proper order parameter. We demonstrate that our conclusions are robust under the addition of random perturbations, not only in the topological phase, but also in the spin polarized phase and even at the critical point.

quant-ph↗

arXiv2007

High charge-carrier mobility and low trap density in a rubrene derivative

We have synthesized, crystallized and studied the structural and electric transport properties of organic molecular crystals based on a rubrene derivative with {\em t}-butyl sidegroups at the 5,11 positions. Two crystalline modifications are observed: one (A) distinct from that of rubrene with larger spacings between the naphtacene backbones, the other (B) with a in-plane structure presumably very similar compared to rubrene. The electric transport properties reflect the different structures: in the latter phase (B) the in-plane hole mobility of 12 cm$^2$/Vs measured on single crystal FETs is just as high as in rubrene crystals, while in the A phase no field-effect could be measured. The high crystal quality, studied in detail for B, reflects itself in the density of gap states: The deep-level trap density as low as $10^{15}$ cm$^{-3}$ eV$^{-1}$ has been measured, and an exponential band tail with a characteristic energy of 22 meV is observed. The bulk mobility perpendicular to the molecular planes is estimated to be of order of $10^{-3}$ -- $10^{-1}$ cm$^2$/Vs.

cond-mat.mtrl-sci↗

arXiv2007

Large Negative Thermal Expansion in Pentacene due to Steric Hindrance

The uniaxial negative thermal expansion in pentacene crystals along $a$ is a particularity in the series of the oligoacenes, and exeptionally large for a crystalline solid. Full x-ray structure analysis from 120 K to 413 K reveals that the dominant thermal motion is a libration of the rigid molecules about their long axes, modifying the intermolecular angle which describes the herringbone packing within the layers. This herringbone angle increases with temperature (by 0.3 -- 0.6$^{\circ}$ per 100 K), and causes an anisotropic rearrangement of the molecules within the layers, i.e. an expansion in the $b$ direction, and a distinct contraction along $a$. Additionally, a larger herringbone angle improves the cofacial overlap between adjacent, parallel molecules, and thus enhances the attractive van der Waals forces.

cond-mat.mtrl-sci↗

arXiv2007

Density of bulk trap states in organic semiconductor crystals: discrete levels induced by oxygen in rubrene

The density of trap states in the bandgap of semiconducting organic single crystals has been measured quantitatively and with high energy resolution by means of the experimental method of temperature-dependent space-charge-limited-current spectroscopy (TD-SCLC). This spectroscopy has been applied to study bulk rubrene single crystals, which are shown by this technique to be of high chemical and structural quality. A density of deep trap states as low as ~ 10^{15} cm^{-3} is measured in the purest crystals, and the exponentially varying shallow trap density near the band edge could be identified (1 decade in the density of states per ~25 meV). Furthermore, we have induced and spectroscopically identified an oxygen related sharp hole bulk trap state at 0.27 eV above the valence band.

cond-mat.mtrl-sci↗

arXiv2005

New World of Gossamer Superconductivity

Since the discovery of the high-T$_{c}$ cuprate superconductor La$_{2-x}$BaCuO$_{4}$ in 1986 by Bednorz and Müller, controversy regarding the nature or origin of this remarkable superconductivity has continued. However, d-wave superconductivity in the hole-doped cuprates, arising due to the anti-paramagnon exchange, was established around 1994. More recently we have shown that the mean field theory, like the BCS theory of superconductivity and Landau's Fermi liquid theory are adequate to describe the cuprates. The keys for this development are the facts that a)the pseudogap phase is d-wave density wave (dDW) and that the high-T$_{c}$ cuprate superconductivity is gossamer (i.e. it exists in the presence of dDW).

cond-mat.supr-con↗

arXiv2005

Gossamer Superconductivity, New Paradigm?

We shall review our recent works on d-wave density wave (dDW) and gossamer superconductivity (i.e. d-wave superconductivity in the presence of dDW) in high-T$_{c}$ cuprates and CeCoIn$_{5}$. a) We show that both the giant Nernst effect and the angle dependent magnetoresistance (ADMR) in the pseudogap phases of the cuprates and CeCoIn$_{5}$ are manifestations of dDW. b) The phase diagram of high-T$_{c}$ cuprates is understood in terms of mean field theory, which includes two order parameters $Δ_{1}$ and $Δ_{2}$, where one order parameter is from dDW and the other from d-wave superconductivity. c) In the optimally to the overdoped region we find the spatially periodic dDW, an analogue of the Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state, becomes more stable. d) In the underdoped region where $Δ_{2}/Δ_{1} \ll 1$ the Uemera relation is obtained within the present model. We speculate that the gossamer superconductivity is at the heart of high-T$_{c}$ cuprate superconductors, the heavy-fermion superconductor CeCoIn$_{5}$ and the organic superconductors $κ$- (ET)$_{2}$Cu(NCS)$_{2}$ and (TMTSF)$_2$PF$_{6}$.

cond-mat.supr-con↗

arXiv2005

Topological defects in triplet superconductors UPt$_{3}$, Sr$_{2}$RuO$_{4}$, etc

After a brief introduction on nodal superconductors, we review the topological defects in triplet superconductors such as UPt$_{3}$, Sr$_{2}$RuO$_{4}$, etc. This is in part motivated by the surprising discovery of Ana Celia Mota and her colleagues that in some triplet superconductors the flux motion is completely impeded (the ideal pinning). Among topological defects the most prominent is Abrikosov's vortex with quantum flux $ϕ_{0}= \frac{hc}{2e}$. Abrikosov's vortex is universal and ubiquitous and seen in both conventional and unconventional superconductors by the Bitter decoration technique, small angle neutron scattering (SANS), scanning tunneling microscopy (STM), micromagnetometer and more recently by Lorentz electron micrograph. In order to interpret the experiment by Mota et al a variety of textures are proposed. In particular, in analogy to superfluid $^{3}$He-A the $\hat{\ell}$-soliton and $\hat{d}$-soliton play the prominent role. We review these notions and point out possible detection of these domain walls and half-quantum vortices in some triplet superconductors.

cond-mat.supr-con↗