arXiv subjects

Manuel Weber

Publications and source records attributed to Manuel Weber.

29 records · Page 2Linked to original sources

arXiv2020

A Tetramethylsilane TPC with Cherenkov light readout and 3D reconstruction

We describe the construction and calibration of a multi-channel liquid time projection chamber filled with Tetramethylsilane (TMS). Its charge readout system consists of 8 wires each in the $X$ and $Y$ directions. The chamber is also equipped with a Cherenkov light readout system consisting of a 5-inch photomultiplier tube (PMT) coupled to the liquid volume through a viewport. The energy scale of the detector is calibrated using positron-electron pairs produced by 4.4 MeV gamma rays emitted from an AmBe source, using an external trigger on the positron annihilation gammas. The external trigger is then reconfigured to tag cosmic ray muons passing through the active TMS volume, which are used to measure the stopping power in TMS and the electron lifetime in the detector. We find a most-probable energy loss from minimum ionising particles (MIPs) of $Δ_p/ds$ = (0.60$\pm$0.01) MeV/cm. We also derive an electron lifetime of 43$^{+680}_{-21}~μ$s by measuring the most-probable energy loss as a function of drift time. For both fast electron and muon signals, the PMT detects prompt Cherenkov light, demonstrating the possibility of random triggering. The room-temperature organic target medium, together with the self-triggering capabilities and long electron lifetimes reported in this work, make this an attractive technology to further explore for rare event detectors or other applications in the area of radiation measurements.

physics.ins-det↗

arXiv2020

Competing Orders and Unconventional Criticality in the Su-Schrieffer-Heeger Model

The phase diagram of the one-dimensional Su-Schrieffer-Heeger model of spinless fermions coupled to quantum phonons is determined by quantum Monte Carlo simulations. It differs significantly from previous work. In addition to Luttinger liquid and bond-order-wave (BOW) phases, we find an extended charge-density-wave (CDW) phase. Because of different broken symmetries, BOW and CDW phases are connected by a retardation-driven phase transition. Our results are consistent with the theory of the frustrated $XXZ$ chain, including unconventional power-law exponents at criticality, and an interpretation in terms of deconfined quantum criticality via proliferation of solitons.

cond-mat.str-el↗

arXiv2018

Thermal and quantum lattice fluctuations in Peierls chains

The thermodynamic and spectral properties of electrons coupled to quantum phonons are studied within the spinless Holstein model. Using quantum Monte Carlo simulations, we obtain accurate results for the specific heat and the compressibility, covering the entire range of electron-phonon couplings and phonon frequencies. To this end, we derive an efficient estimator for the specific heat using the properties of the perturbation expansion. This allows us to quantitatively test the predictions of Tomonaga-Luttinger liquid theory as well as the widely used adiabatic approximation for low phonon frequencies. A comparison with the spectral functions of electrons and phonons reveals that the formation of polaron excitations as well as the renormalization of the phonon mode across the Peierls transition have a pronounced effect on the specific heat in the adiabatic regime.

cond-mat.str-el↗

arXiv2018

Two-dimensional Holstein-Hubbard model: Critical temperature, Ising universality, and bipolaron liquid

The two-dimensional Holstein-Hubbard model is studied by means of continuous-time quantum Monte Carlo simulations. Using renormalization-group-invariant correlation ratios and finite-size extrapolation, the critical temperature of the charge-density-wave transition is determined as a function of coupling strength, phonon frequency, and Hubbard repulsion. The phase transition is demonstrated to be in the universality class of the two-dimensional Ising model and detectable via the fidelity susceptibility. The structure of the ground-state phase diagram and the possibility of a bipolaronic metal with a single-particle gap above $T_c$ are explored.

cond-mat.str-el↗

arXiv2017

Directed-Loop Quantum Monte Carlo Method for Retarded Interactions

The directed-loop quantum Monte Carlo method is generalized to the case of retarded interactions. Using the path integral, fermion-boson or spin-boson models are mapped to actions with retarded interactions by analytically integrating out the bosons. This yields an exact algorithm that combines the highly-efficient loop updates available in the stochastic series expansion representation with the advantages of avoiding a direct sampling of the bosons. The application to electron-phonon models reveals that the method overcomes the previously detrimental issues of long autocorrelation times and exponentially decreasing acceptance rates. For example, the resulting dramatic speedup allows us to investigate the Peierls quantum phase transition on chains of up to $1282$ sites.

cond-mat.str-el↗

arXiv2017

An RF-only ion-funnel for extraction from high-pressure gases

An RF ion-funnel technique has been developed to extract ions from a high-pressure (10 bar) noble-gas environment into vacuum ($10^{-6}$ mbar). Detailed simulations have been performed and a prototype has been developed for the purpose of extracting $^{136}$Ba ions from Xe gas with high efficiency. With this prototype, ions have been extracted for the first time from high-pressure xenon gas and argon gas. Systematic studies have been carried out and compared to the simulations. This demonstration of extraction of ions with mass comparable to that of the gas generating the high-pressure into vacuum has applications to Ba tagging from a Xe-gas time-projection chamber (TPC) for double beta decay as well as to the general problem of recovering trace amounts of an ionized element in a heavy (m$>40$ u) carrier gas.

physics.ins-det↗

arXiv2017

Continuous-time quantum Monte Carlo for fermion-boson lattice models: Improved bosonic estimators and application to the Holstein model

We extend the continuous-time interaction-expansion quantum Monte Carlo method with respect to measuring observables for fermion-boson lattice models. Using generating functionals, we express expectation values involving boson operators, which are not directly accessible because simulations are done in terms of a purely fermionic action, as integrals over fermionic correlation functions. We also demonstrate that certain observables can be inferred directly from the vertex distribution, and present efficient estimators for the total energy and the phonon propagator of the Holstein model. Furthermore, we generalize the covariance estimator of the fidelity susceptibility, an unbiased diagnostic for phase transitions, to the case of retarded interactions. The new estimators are applied to half-filled spinless and spinful Holstein models in one dimension. The observed renormalization of the phonon mode across the Peierls transition in the spinless model suggests a soft-mode transition in the adiabatic regime. The critical point is associated with a minimum in the phonon kinetic energy and a maximum in the fidelity susceptibility.

cond-mat.str-el↗

arXiv2016

Thermodynamic and spectral properties of adiabatic Peierls chains

We present exact numerical results for the effects of thermal fluctuations on the experimentally relevant thermodynamic and spectral properties of Peierls chains. To this end, a combination of classical Monte Carlo sampling and exact diagonalization is used to study adiabatic half-filled Holstein and Su-Schrieffer-Heeger models. The classical nature of the lattice displacements in combination with parallel tempering permit simulations on large system sizes and a direct calculation of spectral functions in the frequency domain. Most notably, the long-range order and the associated Peierls gap give rise to a distinct low-temperature peak in the specific heat. The closing of the gap and suppression of order by thermal fluctuations involves in-gap excitations in the form of soliton-antisoliton pairs, and is also reflected in the dynamic density and bond structure factors as well as in the optical conductivity. We compare our data to the widely used mean-field approximation, and highlight relations to symmetry-protected topological phases and disorder problems.

cond-mat.str-el↗

arXiv2015

Phonon spectral function of the one-dimensional Holstein-Hubbard model

We use the continuous-time interaction expansion (CT-INT) quantum Monte Carlo method to calculate the phonon spectral function of the one-dimensional Holstein-Hubbard model at half-filling. Our results are consistent with a soft-mode Peierls transition in the adiabatic regime, and the existence of a central peak related to long-range order in the Peierls phase. We explain a previously observed feature at small momenta in terms of a hybridization of charge and phonon excitations. Tuning the system from a Peierls to a metallic phase with a nonzero Hubbard interaction suppresses the central peak, but a significant renormalization of the phonon dispersion remains. In contrast, the dispersion is only weakly modified in the Mott phase. We discuss finite-size effects, the relation to the dynamic charge structure factor, as well as additional sum rules and their implications. Finally, we reveal the existence of a discrete symmetry in a continuum field theory of the Holstein model, which is spontaneously broken in the Peierls phase.

cond-mat.str-el↗

arXiv2015

Excitation spectra and correlation functions of quantum Su-Schrieffer-Heeger models

We study one-dimensional Su-Schrieffer-Heeger (SSH) models with quantum phonons using a continuous-time quantum Monte Carlo method. Within statistical errors, we obtain identical results for the SSH model with acoustic phonons, and a related model with a coupling to an optical bond phonon mode. Based on this agreement, we first study the Peierls metal-insulator transition of the spinless SSH model, and relate it to the Kosterlitz-Thouless transition of a spinless Luttinger liquid. In the Peierls phase, the spectral functions reveal the single-particle and charge gap, and a central peak related to long-range order. For the spinful SSH model, which has a dimerized ground state for any nonzero coupling, we reveal a symmetry-related degeneracy of spin and charge excitations, and the expected spin and charge gaps as well as a central peak. Finally, we study the SSH-$UV$ model with electron-phonon and electron-electron interaction. We observe a Mott phase with critical spin and bond correlations at weak electron-phonon coupling, and a Peierls phase with gapped spin excitations at strong coupling. We relate our findings to the extended Hubbard model, and discuss the physical origin of the agreement between optical and acoustic phonons.

cond-mat.str-el↗

arXiv2014

Kondo screening of spin-charge separated fluxons by a helical liquid

The insertion of a magnetic $π$ flux into a quantum spin Hall insulator creates four localized, spin-charge separated states: the charge and spin fluxons with either charge $Q=\pm1$ or spin $S_z=\pm1/2$, respectively. In the presence of repulsive Coulomb interactions, the charged states are gapped out and a local moment is formed. We consider the Kane-Mele-Hubbard model on a ribbon with zigzag edges to construct an impurity model where the spin fluxon is screened by the helical edge liquid. In the noninteracting model, the hybridization between fluxon and edge states is dominated by the extent of the latter. It becomes larger with increasing spin-orbit coupling $λ$ but only has nonzero values for even distances between the $π$ flux and the edge. For the interacting system, we use the continuous-time quantum Monte Carlo method, which we have extended by global susceptibility measurements to reproduce the characteristic Curie law of the spin fluxon. However, due to the finite extent of the fluxons, the local moment is formed at rather low energies. The screening of the spin fluxon leads to deviations from the Curie law that follow the universal behavior obtained from a data collapse. Additionally, the Kondo resonance arises in the local spectral function between the two low-lying Hubbard peaks.

cond-mat.str-el↗