arXiv subjects

Martin Dressel

Publications and source records attributed to Martin Dressel.

At least 37 records · Page 2Linked to original sources

arXiv2024

Interplay of coupling, residual, and quasiparticle losses for the frequency- and temperature-dependent quality factor of superconducting resonators

The overall, loaded quality factor $Q_\mathrm{L}$ quantifies the loss of energy stored in a resonator. Here we discuss on general grounds how $Q_\mathrm{L}$ of a planar microwave resonator made of a conventional superconductor should depend on temperature and frequency. We consider contributions to $Q_\mathrm{L}$ due to dissipation by thermal quasiparticles ($Q_\mathrm{QP}$), due to residual dissipation ($Q_\mathrm{Res}$), and due to coupling ($Q_\mathrm{C}$). We present experimental data obtained with superconducting stripline resonators fabricated from lead (Pb), with different center conductor widths and different coupling gaps. We probe the resonators at various harmonics between 0.7 GHz and 6 GHz and at temperatures between 1.5 K and 7 K. We find a strongly frequency- and temperature-dependent $Q_\mathrm{L}$, which we can describe by a lumped-element model. For certain resonators at lowest temperatures we observe a maximum in the frequency-dependent $Q_\mathrm{L}$ when $Q_\mathrm{Res}$ and $Q_\mathrm{C}$ match, and here the measured $Q_\mathrm{L}$ can exceed $2\times 10^5$.

cond-mat.supr-con↗

arXiv2024

Gapped magnetic ground state in the spin-liquid candidate $κ$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$ suggested by magnetic spectroscopy

The nature of the magnetic ground state of highly frustrated systems remained puzzling to this day. Here, we have performed multifrequency electron spin resonance (ESR) measurements on a putative quantum spin liquid compound $κ$-(BEDT-TTF)$_2$Ag$_2$(CN)$_3$, which is a rare example of $S = 1/2$ spins on a triangular lattice. At high temperatures, the spin susceptibility exhibits a weak temperature dependence which can be described by the Heisenberg model with an antiferromagnetic exchange interaction of strength $J/k_B \approx 175$ K. At low temperatures, however, the rapid drop of the static spin susceptibility, together with monotonic decrease of the ESR linewidth indicates that strong singlet correlations develop below a pairing energy scale $T^*$ accompanied by a spin gap. On the other hand, a weak Curie-like spin susceptibility and the angular dependence of the linewidth suggest additional contribution from impurity spins. We propose the gradual formation of spin singlets with an inhomogeneous spin gap at low temperatures.

cond-mat.str-el↗

arXiv2024

Electronic properties of the dimerized organic conductor $κ$-(BETS)$_2$Mn[N(CN)$_2$]$_3$

The two-dimensional molecular conductor $κ$-(BETS)$_2$Mn[N(CN)$_2$]$_3$ undergoes a sharp metal-to-insulator phase transition at $T_{\rm MI}\approx$ 21 K, which has been under scrutiny for many years. We have performed comprehensive infrared investigations along the three crystallographic directions as a function of temperature down to 10 K, complemented by electron spin resonance and dc-transport studies. The in-plane anisotropy of the optical conductivity is more pronounced than in any other $κ$-type BEDT-TTF or related compounds. The metal-insulator transitions affects the molecular vibrations due to the coupling to the electronic system; in addition we observe a clear splitting of the charge-sensitive vibrational modes below $T_{\rm MI}$ that evidences the presence of two distinct BETS dimers in this compound. The Mn[N(CN)$_2$]$_3^-$ layers are determined by the chain structure of the anions resulting in a rather anisotropic behavior and remarkable temperature dependence of the vibronic features. At low temperatures the ESR properties are affected by the Mn$^{2+}$ ions via $π$-$d$-coupling and antiferromagnetic ordering within the $π$-spins: The $g$-factor shifts enormously with a pronounced in-plane anisotropy that flips as the temperature decreases; the lines broaden significantly; and the spin susceptibility increases upon cooling with a kink at the phase transition.

cond-mat.str-el↗

arXiv2024

Temperature-dependent generalized ellipsometry of the metal-insulator phase transition in low-symmetry charge-transfer salts

Determining the optical and electronic properties of strongly anisotropic materials with symmetries below orthorhombic remains challenging; generalized ellipsometry is a powerful technique in this regard. Here, we employ Mueller matrix spectroscopic and temperature-dependent ellipsometry to determine the frequency dependence of six components of the dielectric-function tensor of the two-dimensional charge-transfer salt $α$-(BEDT-TTF)$_2$I$_3$ across its metal-insulator transition. Our results offer valuable insights into temperature-dependent changes of the components of the spectroscopic dielectric-function tensor across the metal-insulator transition. This advanced method allows extension to other electronic transitions.

cond-mat.str-el↗

arXiv2024

Tuning the superconducting dome in granular aluminum thin films

Granular aluminum, which consists of nanometer-sized aluminum grains separated by aluminum oxide, is a peculiar superconductor. Its phase diagram as function of normal-state resistivity features a superconducting dome with a maximum critical temperature Tc well above the Tc = 1.2 K of pure aluminum. Here we show how the maximum Tc of this superconducting dome grows if the substrate temperature during deposition is lowered from 300 K to cooling with liquid nitrogen (150 K and 100 K) and liquid helium (25 K). The highest Tc we observe is 3.27 K. These results highlight that granular aluminum is a model system for complex phase diagrams of superconductors and demonstrate its potential in the context of high kinetic inductance applications. This is augmented by our observation of comparably sharp superconducting transitions of high-resistivity samples grown at cryogenic temperatures and by a thickness dependence even for films substantially thicker than the grain size.

cond-mat.supr-con↗

arXiv2024

Fermi-liquid behavior of non-altermagnetic RuO$_2$

Presence of magnetism in potentially altermagnetic RuO$_2$ has been a subject of intense debate. Using broadband infrared spectroscopy combined with density-functional band-structure calculations, we show that optical conductivity of RuO$_2$, the bulk probe of its electronic structure, is well described by the nonmagnetic model of this material. The sharp Pauli edge demonstrates the presence of a Dirac nodal line lying 45 meV below the Fermi level. Good match between the experimental and ab initio plasma frequencies underpins weakness of electronic correlations. The intraband part of the optical conductivity indicates Fermi-liquid behavior with two distinct scattering rates below 150 K. Fermi-liquid theory also accounts for the temperature-dependent magnetic susceptibility of RuO$_2$ and allows a consistent description of this material as paramagnetic metal.

cond-mat.mtrl-sci↗

arXiv2024

SrCu(OH)$_3$Cl, an ideal isolated equilateral triangle spin $S$ = 1/2 model system

We have investigated the magnetic ground state properties of the quantum spin trimer compound strontium hydroxy copper chloride SrCu(OH)$_3$Cl using bulk magnetization, specific heat measurements, nuclear magnetic resonance (NMR), and electron spin resonance (ESR) spectroscopy. SrCu(OH)$_3$Cl consists of layers with isolated Cu$^{2+}$ triangles and hence provides an opportunity to understand the magnetic ground state of an isolated system of \textit{S} = 1/2 arranged on an equilateral triangle. Although magnetization measurements do not exhibit a phase transition to a long-range ordered state down to \textit{T} = 2 K, they reveal the characteristic behavior of isolated trimers with an exchange of $J = 154$~K. The Curie-Weiss behavior changes around 50--80~K, as is also seen in the NMR spin-lattice relaxation rate. In zero magnetic field, our specific heat data establish a second-order phase transition to an antiferromagnetic ground state below \textit{T}= 1.2 K. We have drawn a magnetic field-temperature ($H$-$T$) phase diagram based on the specific heat measurements. The ESR data show divergence of the linewidth at lower temperatures, which precedes the phase transition to an antiferromagnetic long-range ordered state with unconventional critical exponents. The temperature variation of the $g$-factor further confirms the antiferromagnetic phase transition and reflects the underlying magneto-crystalline anisotropy of the compound.

cond-mat.str-el↗

arXiv2023

Ultrafast gap dynamics upon photodoping the Mott-insulating phase of a two-dimensional organic charge-transfer salt

We investigate experimentally the ultrafast changes in the spectral response of the Mott insulator $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]Cl ($κ$-Cl) upon photodoping with intense excitation at 1.6 eV and probing with continuum pulses simultaneously covering both the terahertz and infrared (IR) ranges (from 0 to 0.6 eV). A quantitative analysis of the differential reflectivity using a multi-band Lorentzian model provide absolute changes in spectral weights and objective global time constants for the relaxation vs. temperature. The transient conductivity spectra deduced from the analysis suggest that the transient photoinduced spectral weight is dominated by a progressive closure of the Mott gap with increasing excitation density, i.e. due to changes in the inter-Hubbard-band absorption by the remaining singly occupied states. We critically examine this scenario compared to that proposed previously, whereby the low-energy spectral weight is attributed to a Drude-like response of photoexcited doublons/holons. We also consider the observed slowing down of the relaxation rate with increasing excitation density, and temperature dependence of the initial doublon/holon density in terms of the phonon-mediated gap recombination model.

cond-mat.str-el↗

arXiv2023

Pressure evolution of electron dynamics in the superconducting kagome metal CsV$_3$Sb$_5$

The coexistence of the charge-density wave (CDW) and superconducting phases and their tunability under external pressure remains one of the key points in understanding the electronic structure of $A$V$_3$Sb$_5$ ($A$ = K, Rb, Cs) kagome metals. Here, we employ synchrotron-based infrared spectroscopy assisted by density-functional calculations to study the pressure evolution of the electronic structure at room temperature up to 17 GPa experimentally. The optical spectrum of CsV$_3$Sb$_5$ is characterized by the presence of localized carriers seen as a broad peak at finite frequencies in addition to the conventional metallic Drude response. The pressure dependence of this low-energy peak reflects the re-entrant behavior of superconductivity and may be interpreted in terms of electron-phonon coupling, varying with the growth and shrinkage of the Fermi surface. Moreover, drastic modifications in the low-energy interband absorptions are observed upon the suppression of CDW. These changes are related to the upward shift of the Sb2 $p_x+p_y$ band that eliminates part of the Fermi surface around the $M$-point, whereas band saddle points do not move significantly. These observations shed new light on the mixed electronic and lattice origin of the CDW in CsV$_3$Sb$_5$.

cond-mat.str-el↗

arXiv2023

Critical-point anomalies in doped CeRhIn5

The heavy-fermion compound CeRhIn$_5$ can be tuned through a quantum critical point, when In is partially replaced by Sn. This way additional charge carriers are introduced and the antiferromagnetic order is gradually suppressed to zero temperature. Here we investigate the temperature-dependent optical properties of CeRh(In$_{1-x}$Sn$_x$)$_5$ single crystals for $x = 4.4\%$, $6.9\%$ and $9.8\%$. With increasing Sn concentration the infrared conductivity reveals a clear enhancement of the $c$-$f$ hybridization strength. At low temperatures we observed a non-Fermi-liquid behavior in the frequency dependence of the scattering rate and effective mass in all three compounds. In addition, below a characteristic temperature $T^* \approx 10$ K, the temperature dependent resistivity $ρ(T)$ follows a $\log T$ behavior, typical for a non-Fermi liquid. The temperature-dependent magnetization also exhibits anomalous behavior below $T^*$. Our investigation reveal that below $T^*$ the system shows a pronounced non-Fermi-liquid behavior and $T^*$ monotonically increases as the quantum critical point is approached.

cond-mat.str-el↗

arXiv2023

Effect of nonhydrostatic pressure on the superconducting kagome metal CsV$_3$Sb$_5$

High-pressure single-crystal x-ray diffraction experiments reveal that the superconducting kagome metal CsV$_3$Sb$_5$ transforms from hexagonal ($P6/mmm$) to monoclinic ($C2/m$) symmetry above 10 GPa if nonhydrostatic pressure conditions are created in a diamond anvil cell with silicon oil as pressure-transmitting medium. This is contrary to the behavior of CsV$_3$Sb$_5$ under quasi-hydrostatic conditions in neon, with the hexagonal symmetry retained up to at least 20 GPa. Monoclinic distortion leaves the kagome planes almost unchanged but deforms honeycomb nets of the Sb atoms. While the onset of the distortion almost coincides with the reentrance of superconductivity, our \textit{ab initio} density-functional calculations reveal only minor changes in the electronic structure compared to the quasi-hydrostatic case. In particular, Fermi surface reconstruction driven by the formation of interlayer Sb-Sb bonds is observed in both monoclinic and hexagonal CsV$_3$Sb$_5$ structures at high pressures and comes out as the likely cause for the reentrant behavior.

cond-mat.supr-con↗

arXiv2023

Characterization of harmonic modes and parasitic resonances in multi-mode superconducting coplanar resonators

Planar superconducting microwave transmission line resonators can be operated at multiple harmonic resonance frequencies. This allows covering wide spectral regimes with high sensitivity, as it is desired e.g. for cryogenic microwave spectroscopy. A common complication of such experiments is the presence of undesired 'spurious' additional resonances, which are due to standing waves within the resonator substrate or housing box. Identifying the nature of individual resonances ('designed' vs. 'spurious') can become challenging for higher frequencies or if elements with unknown material properties are included, as is common for microwave spectroscopy. Here we discuss various experimental strategies to distinguish designed and spurious modes in coplanar superconducting resonators that are operated in a broad frequency range up to 20 GHz. These strategies include tracking resonance evolution as a function of temperature, magnetic field, and microwave power. We also demonstrate that local modification of the resonator, by applying minute amounts of dielectric or ESR-active materials, lead to characteristic signatures in the various resonance modes, depending on the local strength of the electric or magnetic microwave fields.

cond-mat.supr-con↗

arXiv2022

Multi-Center Magnon Excitations Open the Entire Brillouin Zone to Terahertz Magnetometry of Quantum Magnets

Due to the small photon momentum, optical spectroscopy commonly probes magnetic excitations only at the center of the Brillouin zone; however, there are ways to override this restriction. In the case of the distorted kagome quantum magnet Y-kapellasite, Y$_3$Cu$_9$(OH)$_{19}$Cl$_8$, under scrutiny here, the magnon density of states can be accessed over the entire Brillouin zone through three-center magnon excitations. This mechanism is aided by the three different magnetic sublattices and strong short-range correlations in the distorted kagome lattice. The results of THz time-domain experiments agree remarkably well with linear spin-wave theory. Relaxing the conventional zone-center constraint of photons gives a new aspect to probe magnetism in matter.

cond-mat.str-el↗

arXiv2022

Magnetic terahertz resonances above the Néel temperature in the frustrated kagome antiferromagnet averievite

Time-domain magneto-THz spectroscopy is utilized to study the frustrated magnet averievite Cu$_{5-x}$Zn$_x$V$_2$O$_{10}$(CsCl). Pronounced THz resonances are observed in unsubstituted samples ($x=0$) when cooling below the onset of short-range magnetic correlations. The influence of external magnetic effects confirms the magnetic origin of these resonances. Increasing Zn substitution suppresses the resonances, as frustration effects dominate, reflecting the non-magnetic phases for $x> 0.25$ compounds. The temperature evolution of the THz spectra is complemented with electron spin resonance spectroscopy. This comparison allows a direct probe of the different contributions from magnetic order, frustration, and structural properties in the phase diagram of averievite. Our results illustrate the effect of magnetic interactions in THz spectra of frustrated magnets.

cond-mat.str-el↗

arXiv2022

Role of Sb in the superconducting kagome metal CsV$_3$Sb$_5$ revealed by its anisotropic compression

Pressure evolution of the superconducting kagome metal CsV$_3$Sb$_5$ is studied with single-crystal x-ray diffraction and density-functional band-structure calculations. A highly anisotropic compression observed up to 5 GPa is ascribed to the fast shrinkage of the Cs-Sb distances and suppression of Cs rattling motion. This prevents Sb displacements required to stabilize the three-dimensional charge-density-wave (CDW) order and elucidates the disappearance of the CDW already at 2 GPa despite only minor changes in the electronic structure of the normal state. At higher pressures, vanadium bands still change only marginally, whereas antimony bands undergo a major reconstruction caused by the gradual formation of the interlayer Sb-Sb bonds. Our results exclude pressure tuning of vanadium kagome bands as the main mechanism for the non-trivial evolution of superconductivity in real-world kagome metals. Concurrently, we establish the central role of Sb atoms in the stabilization of a three-dimensional CDW and Fermi surface reconstruction.

cond-mat.supr-con↗

arXiv2022

Charge imbalance in $λ$-(BETS)$_2$GaCl$_4$ and their interplay with superconductivity

The two-dimensional organic superconductor $λ$-(BETS)$_2$GaCl$_4$ exhibits pronounced charge fluctuations below $T \approx 150$~K, in contrast to the sibling compound $κ$-(BETS)$_2$GaCl$_4$ that remains metallic down to milli-Kelvin. Infrared spectroscopy reveals only minor splitting in the vibrational features of the latter compound, common to other strongly dimerized $κ$-salts. When the organic molecules are arranged in the $λ$-pattern, however, a strong vibrational $ν_{27}(b_{1u})$ mode is present, that forms a narrow doublet. %indicating static charge imbalance of about 2\%. Most important, when cooling $λ$-(BETS)$_2$GaCl$_4$ below 150~K, two weak side modes appear due to charge disproportionation that amounts to $2δ=0.14e$. In analogy to the $β^{\prime\prime}$-type organic conductors, we propose that charge fluctuations play an important role in emerging of unconventional superconductivity in ł at $T_c=4.7$~K. We discuss the possibility of a charge-density-wave that coexists with the proposed spin-density-wave state.

cond-mat.str-el↗

arXiv2021

Charge-order phase transition in the quasi one-dimensional organic conductor (TMTTF)$_2$NO$_3$

Low-dimensional organic conductors show a rich phase diagram, which has, despite all efforts, still some unexploed regions. Charge ordered phases present in many compounds of the (TMTTF)$_2 X$ family are typically studied with their unique electronic properties in mind. An influence on the spin arrangement is, however, not expected at first glance. Here, we report temperature and angle dependent electron spin resonance (ESR) measurements on the quasi one-dimensional organic conductor (TMTTF)$_2$NO$_3$. We found that the (TMTTF)$_2$NO$_3$ compound develops a peculiar anisotropy with a doubled periodicity ($ab'$-plane) of the ESR linewidth below about $T_\text{CO}=250\pm 10$ K. This behavior is similar to observations in the related compounds (TMTTF)$_2 X$ ($X$ = PF$_6$, SbF$_6$ and AsF$_6$), where it has been attributed to relaxation processes of magnetically inequivalent sites in the charge-ordered state. For the structural analogous (TMTTF)$_2$ClO$_4$, known for the absence of charge order, such angular dependence of the ESR signal is not observed. Therefore, our ESR measurements lead us to conclude that a charge-order phase is stabilized in the title compound below $T_\text{CO} \approx 250$ K.

cond-mat.mtrl-sci↗

arXiv2021

Interacting electron spins in $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I investigated by ESR spectroscopy

We performed angular and temperature-dependent electron-spin-resonance measurements in the quasi-two-dimensional organic conductor $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I. The interlayer spin-diffusion is much weaker compared to the Cl- and Br-analogues, which are antiferromagnetic insulator and paramagnetic metal, respectively; $κ$-(BEDT-TTF)$_2$Cu[N(CN)$_2$]I behaves insulating when cooled below $T$ = 200 K. A spin gap ($Δ\approx$ 18 K) opens at low temperatures leading to a spin-singlet state. Due to intrinsic disorder a substantial number of spins ($\sim$ 1 $\%$) remains unpaired. We observe additional signals below $T$ = 4 K with a pronounced anisotropy indicating the presence of local magnetic moments coupled to some fraction of those unpaired spins.

cond-mat.mtrl-sci↗