arXiv subjects

M. Schmidt

Publications and source records attributed to M. Schmidt.

At least 73 records · Page 4Linked to original sources

arXiv2019

Pressure-tuning of the electrical-transport properties in the Weyl semimetal TaP

We investigated the pressure evolution of the electrical transport in the almost compensated Weyl semimetal TaP. In addition, we obtained information on the modifications of the Fermi-surface topology with pressure from the analysis of pronounced Shubnikov-de Haas (SdH) quantum oscillations present in the Hall-effect and magnetoresistance data. The simultaneous analysis of the Hall and longitudinal conductivity data in a two-band model revealed an only weak decrease in the electron- and hole charge-carrier densities up to 1.2 GPa, while the mobilities are essentially pressure independent along the a-direction of the tetragonal crystal structure. Only weak changes in the SdH frequencies for B||a and B||c point at a robust Fermi-surface topology. In contrast to the stability of the Fermi-surface topology and of the density of charge carriers, our results evidence a strong pressure variation of the magnitude of transverse magnetoresistance for B||a contrary to the results for B||c. We can relate the former to an increase in the charge-carrier mobilities along the crystallographic c-direction.

cond-mat.mtrl-sci↗

arXiv2019

The Innovative Design of the PANDA Barrel DIRC

The Barrel DIRC of the PANDA experiment at FAIR will cleanly separate pions from kaons for the physics program of PANDA. Innovative solutions for key components of the detector sitting in the strong magnetic field of the compact PANDA target spectrometer as well as two reconstruction methods were developed in an extensive prototype program. The technical design and present results from the test beam campaigns at the CERN PS in 2017 and 2018 are discussed.

physics.ins-det↗

arXiv2019

Quantum Critical Dynamics of a Heisenberg-Ising Chain in a Longitudinal Field: Many-Body Strings versus Fractional Excitations

We report a high-resolution terahertz spectroscopic study of quantum spin dynamics in the antiferromagnetic Heisenberg-Ising spin-chain compound BaCo$_2$V$_2$O$_8$ as a function of temperature and longitudinal magnetic field. Confined spinon excitations are observed in an antiferromagnetic phase below $T_N\simeq 5.5$ K. In a field-induced gapless phase above $B_c=3.8$ T, we identify many-body string excitations as well as low-energy fractional psinon/antipsinon excitations by comparing to Bethe-Ansatz calculations. In the vicinity of $B_c$, the high-energy string excitations are found to be dynamically dominant over the fractional excitations.

cond-mat.str-el↗

arXiv2019

The Barrel DIRC detector of PANDA

The PANDA experiment is one of the four large experiments being built at FAIR in Darmstadt. It will use a cooled antiproton beam on a fixed target within the momentum range of 1.5 to 15 GeV/c to address questions of strong QCD, where the coupling constant $α_s \gtrsim 0.3$. The luminosity of up to $2 \cdot 10^{32} cm^{-2}s^{-1}$ and the momentum resolution of the antiproton beam down to \mbox{$Δ$p/p = 4$\cdot10^{-5}$} allows for high precision spectroscopy, especially for rare reaction processes. Above the production threshold for open charm mesons the production of kaons plays an important role for identifying the reaction. The DIRC principle allows for a compact particle identification for charged particles in a hermetic detector, limited in size by the electromagnetic lead tungstate calorimeter. The Barrel DIRC in the target spectrometer covers polar angles between $22^\circ$ and $140^\circ$ and will achieve a pion-kaon separation of 3 standard deviations up to 3.5 GeV/$c$. Here, results of a test beam are shown for a single radiator bar coupled to a prism with $33^\circ$ opening angle, both made from synthetic fused silica read out with a photon detector array with 768 pixels.

physics.ins-det↗

arXiv2019

Magnonic Weyl states in Cu2OSeO3

The multiferroic ferrimagnet Cu$_2$OSeO$_3$ with a chiral crystal structure attracted a lot of recent attention due to the emergence of magnetic skyrmion order in this material. Here, the topological properties of its magnon excitations are systematically investigated by linear spin-wave theory and inelastic neutron scattering. When considering Heisenberg exchange interactions only, two degenerate Weyl magnon nodes with topological charges $\pm$2 are observed at high-symmetry points. Each Weyl point splits into two as the symmetry of the system is further reduced by including into consideration the nearest-neighbor Dzyaloshinsky-Moriya interaction, crucial for obtaining an accurate fit to the experimental spin-wave spectrum. The predicted topological properties are verified by surface state and Chern number analysis. Additionally, we predict that a measurable thermal Hall conductivity can be associated with the emergence of the Weyl points, the position of which can be tuned by changing the crystal symmetry of the material.

cond-mat.str-el↗

arXiv2018

Quantum Ising model on the frustrated square lattice

We investigate the role of a transverse field on the Ising square antiferromagnet with first-($J_1$) and second-($J_2$) neighbor interactions. Using a cluster mean-field approach, we provide a telltale characterization of the frustration effects on the phase boundaries and entropy accumulation process emerging from the interplay between quantum and thermal fluctuations. We found that the paramagnetic (PM) and antiferromagnetic phases are separated by continuous phase transitions. On the other hand, continuous and discontinuous phase transitions, as well as tricriticality, are observed in the phase boundaries between PM and superantiferromagnetic phases. A rich scenario arises when a discontinuous phase transition occurs in the classical limit while quantum fluctuations recover criticality. We also find that the entropy accumulation process predicted to occur at temperatures close to the quantum critical point can be enhanced by frustration. Our results provide a description for the phase boundaries and entropy behavior that can help to identify the ratio $J_2/J_1$ in possible experimental realizations of the quantum $J_1$-$J_2$ Ising antiferromagnet.

cond-mat.stat-mech↗

arXiv2018

$^{39}$Ar dating with small samples resolves ocean ventilation

Ocean ventilation is the integrated effect of various processes that propagate surface properties to the ocean interior. Its precise understanding is the prerequisite for addressing essential questions such as oxygen supply, the storage of anthropogenic carbon and the heat budget of the ocean. Currently employed observational methods to infer ventilation utilise transient tracers, i.e. tritium, SF$_6$, CFCs and the radioisotope $^{14}$C. However, their dating ranges are not suitable to resolve the dynamics of the deep ocean. The noble gas isotope $^{39}$Ar with a half-life of 269 years fills this gap. Its broad application has previously been hindered by its very low abundance, requiring at least 1000 litres of water for dating. Here we report on successful $^{39}$Ar dating with only 5 litres of water based on the atom-optical technique Atom Trap Trace Analysis. Our data reveal previously not quantifiable ventilation patterns in the Eastern Tropical North Atlantic, where we find that advection is more important for the ventilation of the intermediate depth range than previously assumed. This result implies faster ventilation than estimated in other studies and thus a significantly higher anthropogenic CO$_2$-storage. The demonstrated analytical capabilities now allow for a global collection of $^{39}$Ar data, which will have significant impact on our understanding of ocean ventilation.

physics.ao-ph↗

arXiv2018

Threshold Effects and the Line Shape of the X(3872) in Effective Field Theory

Latest measurements suggest that the X(3872) mass lies less than 200 keV away from the D0-Dbar0* threshold, reenforcing its interpretation as a loosely-bound mesonic molecule. This observation implies that in processes like D0-Dbar0-pi0 production, threshold effects could disguise the actual pole position of the X(3872). We propose a new effective field theory with D0, Dbar0 and pi0 degrees of freedom for the X(3872), considering Galilean invariance to be an exact symmetry. The D0* enters as a D0-pi0 p-wave resonance, allowing for a comprehensive study of the influence of pion interactions on the X(3872) width. We calculate relations between the mass of the X(3872), its width, and its line shape in D0-Dbar0-pi0 production up to next-to-leading order. Our results provide a tool for the extraction of the X(3872) pole position from the experimental data near threshold.

hep-ph↗

arXiv2018

A deterministically fabricated spectrally-tunable quantum dot based single-photon source

Spectrally-tunable quantum-light sources are key elements for the realization of long-distance quantum communication. A deterministically fabricated single-photon source with a photon-extraction efficiency of η=(20 +- 2) % and a tuning range of ΔE=2.5 meV is presented here. The device consists of a single pre-selected quantum dot monolithically integrated into a microlens which is bonded onto a piezoelectric actuator via thermocompression goldbonding. The thin gold layer simultaneously acts as a backside mirror for the quantum dot emission, which is efficiently extracted from the device by an optimized lens structure patterned via 3D in-situ electron-beam lithography. The single-photon nature of the emission is proven by photon-autocorrelation measurements with $g^{(2)}$ (τ=0)=0.04 +- 0.02. The combination of deterministic fabrication, spectral-tunability and high broadband photon-extraction efficiency makes the microlens single-photon source an interesting building block towards the realization of quantum repeaters networks.

cond-mat.mes-hall↗

arXiv2018

Photon number-resolved measurement of an exciton-polariton condensate

We measure the full photon-number distribution emitted from a Bose condensate of microcavity exciton-polaritons confined in a micropillar cavity. The statistics are acquired by means of a photonnumber resolving transition edge sensor. We directly observe that the photon-number distribution evolves with the non-resonant optical excitation power from geometric to quasi-Poissonian statistics, which is canonical for a transition from a thermal to a coherent state. Moreover, the photon-number distribution allows evaluating the higher-order photon correlations, shedding further light on the coherence formation and phase transition of the polariton condensate. The experimental data is analyzed in terms of thermal coherent states which allows one to directly extract the thermal and coherent fraction from the measured distributions. These results pave the way for a full understanding of the contribution of interactions in light-matter condensates in the coherence buildup at threshold.

cond-mat.mes-hall↗

arXiv2018

Intrinsic charge dynamics in high-Tc AFeAs(O,F) superconductors

We report the first determination of the in-plane complex optical conductivity of 1111 high-Tc superconducting iron oxypnictide single crystals PrFeAsO0.6F0.35 and thin films SmFeAsO1-xFx by means of bulk-sensitive conventional and micro-focused infrared spectroscopy, ellipsometry, and time-domain THz transmission spectroscopy. A strong itinerant contribution is found to exhibit a dramatic difference in coherence between the crystal and the film. Using extensive temperature-dependent measurements of THz transmission we identify a previously undetected 2.5-meV collective mode in the optical conductivity of SmFeAs(O,F), which is strongly suppressed at Tc and experiences an anomalous T-linear softening and narrowing below T*=110K>>Tc. The suppression of the infrared absorption in the superconducting state reveals a large optical superconducting gap with a similar gap ratio 2D=kBTc=7 in both materials, indicating strong pairing.

cond-mat.supr-con↗

arXiv2017

Reorientations, relaxations, metastabilities and multidomains of skyrmion lattices

Magnetic skyrmions are nano-sized topologically protected spin textures with particle-like properties. They can form lattices perpendicular to the magnetic field and the orientation of these skyrmion lattices with respect to the crystallographic lattice is governed by spin-orbit coupling. By performing small angle neutron scattering measurements, we investigate the coupling between the crystallographic and skyrmion lattices in both Cu$_2$OSeO$_3$ and the archetype chiral magnet MnSi. The results reveal that the orientation of the skyrmion lattice is primarily determined by the magnetic field direction with respect to the crystallographic lattice. In addition, it is also influenced by the magnetic history of the sample which can induce metastable lattices. Kinetic measurements show that these metastable skyrmion lattices may or may not relax to their equilibrium positions under macroscopic relaxation times. Furthermore, multidomain lattices may form when two or more equivalent crystallographic directions are favored by spin-orbit coupling and oriented perpendicular to the magnetic field.

cond-mat.str-el↗

arXiv2017

The PANDA DIRC Detectors at FAIR

The PANDA detector at the international accelerator Facility for Antiproton and Ion Research in Europe (FAIR) addresses fundamental questions of hadron physics. An excellent hadronic particle identification (PID) will be accomplished by two DIRC (Detection of Internally Reflected Cherenkov light) counters in the target spectrometer. The design for the barrel region covering polar angles between 22 deg. to 140 deg. is based on the successful BABAR DIRC with several key improvements, such as fast photon timing and a compact imaging region. The novel Endcap Disc DIRC will cover the smaller forward angles between 5 deg. (10 deg.) to 22 deg. in the vertical (horizontal) direction. Both DIRC counters will use lifetime-enhanced microchannel plate PMTs for photon detection in combination with fast readout electronics. Geant4 simulations and tests with several prototypes at various beam facilities have been used to evaluate the designs and validate the expected PID performance of both PANDA DIRC counters.

physics.ins-det↗

arXiv2017

Thermopower and thermal conductivity in the Weyl semimetal NbP

The Weyl semimetal NbP exhibits an extremely large magnetoresistance (MR) and an ultra-high mobility. The large MR originates from a combination of the nearly perfect compensation between electron- and hole-type charge carriers and the high mobility, which is relevant to the topological band structure. In this work we report on temperature- and field-dependent thermopower and thermal conductivity experiments on NbP. Additionally, we carried out complementary heat capacity, magnetization, and electrical resistivity measurements. We found a giant adiabatic magnetothermopower with a maximum of 800 $μ$V/K at 50 K in a field of 9 T. Such large effects have been observed rarely in bulk materials. We suggest that the origin of this effect might be related to the high charge-carrier mobility. We further observe pronounced quantum oscillations in both thermal conductivity and thermopower. The obtained frequencies compare well with our heat capacity and magnetization data.

cond-mat.mtrl-sci↗

arXiv2017

The 67P/Churyumov-Gerasimenko observation campaign in support of the Rosetta mission

We present a summary of the campaign of remote observations that supported the European Space Agency's Rosetta mission. Telescopes across the globe (and in space) followed comet 67P/Churyumov-Gerasimenko from before Rosetta's arrival until nearly the end of mission in September 2016. These provided essential data for mission planning, large-scale context information for the coma and tails beyond the spacecraft, and a way to directly compare 67P with other comets. The observations revealed 67P to be a relatively `well behaved' comet, typical of Jupiter family comets and with activity patterns that repeat from orbit-to-orbit. Comparison between this large collection of telescopic observations and the in situ results from Rosetta will allow us to better understand comet coma chemistry and structure. This work is just beginning as the mission ends -- in this paper we present a summary of the ground-based observations and early results, and point to many questions that will be addressed in future studies.

astro-ph.EP↗

arXiv2017

Electronically highly cubic conditions for Ru in alpha-RuCl3

We studied the local Ru 4d electronic structure of alpha-RuCl3 by means of polarization dependent x-ray absorption spectroscopy at the Ru-L2,3 edges. We observed a vanishingly small linear dichroism indicating that electronically the Ru 4d local symmetry is highly cubic. Using full multiplet cluster calculations we were able to reproduce the spectra excellently and to extract that the trigonal splitting of the t2g orbitals is -12 $\pm10$ meV, i.e. negligible as compared to the Ru 4d spin-orbit coupling constant. Consistent with our magnetic circular dichroism measurements, we found that the ratio of the orbital and spin moments is 2.0, the value expected for a Jeff = 1/2 ground state. We have thus shown that as far as the Ru 4d local properties are concerned, alpha-RuCl3 is an ideal candidate for the realization of Kitaev physics.

cond-mat.str-el↗

arXiv2017

Spin liquid and infinitesimal-disorder-driven cluster spin glass in the kagome lattice

The interplay between geometric frustration (GF) and bond disorder is studied in the Ising kagome lattice within a cluster approach. The model considers antiferromagnetic (AF) short-range couplings and long-range intercluster disordered interactions. The replica formalism is used to obtain an effective single cluster model from where the thermodynamics is analyzed by exact diagonalization. We found that the presence of GF can introduce cluster freezing at very low levels of disorder. The system exhibits an entropy plateau followed by a large entropy drop close to the freezing temperature. In this scenario, a spin-liquid (SL) behavior prevents conventional long-range order, but an infinitesimal disorder picks out uncompensated cluster states from the multi degenerate SL regime, potentializing the intercluster disordered coupling and bringing the cluster spin-glass state. To summarize, our results suggest that the SL state combined with low levels of disorder can activate small clusters, providing hypersensitivity to the freezing process in geometrically frustrated materials and playing a key role in the glassy stabilization. We propose that this physical mechanism could be present in several geometrically frustrated materials. In particular, we discuss our results in connection to the recent experimental investigations of the Ising kagome compound Co$_3$Mg(OH)$_6$Cl$_2$.

cond-mat.str-el↗

arXiv2016

Emergent Weyl fermion excitations in TaP explored by 181-Ta quadrupole resonance

The $^{181}$Ta quadrupole resonance (NQR) technique has been utilized to investigate the microscopic magnetic properties of the Weyl semi-metal TaP. We found three zero-field NQR signals associated with the transition between the quadrupole split levels for Ta with $I$=7/2 nuclear spin. A quadrupole coupling constant, $ν_{\mathrm{Q}}$ =19.250 MHz, and an asymmetric parameter of the electric field gradient, $η$ = 0.423 were extracted, in good agreement with band structure calculations. In order to examine the magnetic excitations, the temperature dependence of the spin lattice relaxation rate (1/$T_{\mathrm{1}}T$) has been measured for the $f_{\mathrm{2}}$-line ($\pm$5/2 $\leftrightarrow$ $\pm$3/2 transition). We found that there exists two regimes with quite different relaxation processes. Above $T\text{*}$ $\approx$ 30 K, a pronounced (1/$T_{\mathrm{1}}T$) $\propto$ $T^{2}$ behavior was found, which is attributed to the magnetic excitations at the Weyl nodes with temperature dependent orbital hyperfine coupling. Below $T\text{*}$, the relaxation is mainly governed by Korringa process with 1/$T_{\mathrm{1}}T$ = constant, accompanied by an additional $T^{-1/2}$ type dependence to fit our experimental data. We show that Ta-NQR is a novel probe for the bulk Weyl fermions and their excitations.

cond-mat.mtrl-sci↗