arXiv subjects

Jörg Sichelschmidt

Publications and source records attributed to Jörg Sichelschmidt.

At least 19 recordsLinked to original sources

arXiv2026

Evolution of electron spin resonance through a metallic quantum critical phase diagram

In the heavy-fermion metal YbRh2Si2, quantum criticality at a suppressed antiferromagnetic order is governed by the interplay of local magnetic moments and itinerant conduction electrons. We demonstrate how this can be investigated by a new experimental approach that enables the observation of electron spin resonance (ESR) across a broad range of frequencies and fields at very low temperatures. This allowed us to cover a large part of the phase diagram from the paramagnetic Fermi-liquid phase to the phase with antiferromagnetic order and including the quantum-critical regime. Both the ESR g-factor and the linewidth present distinct behaviors in these three regimes, providing further insight into the physics across a quantum critical point. Notably, when cooling down at a field directly towards the quantum critical point, both g-factor and linewidth continuously decrease. Furthermore, we observe a very good matching of the g-factor behavior upon field-tuning and temperature-tuning towards the quantum-critical point. We analyze and discuss the results in the context of present theories on ESR in strongly correlated electron systems.

cond-mat.str-el↗

arXiv2026

Novel Quantum Spin Liquid States in the $S = {\frac{1}{2}}$ Three-Dimensional Compound Y$_{3}$Cu$_{2}$Sb$_{3}$O$_{14}$

The three-dimensional $S = {\frac{1}{2}}$ system Y$_{3}$Cu$_{2}$Sb$_{3}$O$_{14}$ consists of two inequivalent Cu$^{2+}$ sites, each forming an edge shared triangular lattice. Our magnetic susceptibility $χ(T)$, specific heat $C_p(T)$, $^{89}$Y nuclear magnetic resonance (NMR), muon spin relaxation ($\upmu\mathrm{SR}$), and electron spin resonance (ESR) measurements on this system confirm the absence of any long-range magnetic ordering and the persistence of spin dynamics down to 0.077 K. In $^{89}$Y NMR we find an anomaly at about 120 K which we suggest arises from a fraction of the spins condensing into a singlet (a valence bond solid VBS) state. A plateau in the muon relaxation rate is observed between 60 K and 10 K (signifying the VBS state from a fraction of the spins) followed by an increase and another plateau below about 1 K (presumably signifying the quantum spin liquid state from all the spins). Our density functional theory calculations find a dominant antiferromagnetic interaction along the body diagonal with inequivalent Cu(1) and Cu(2) ions alternately occupying the corners of the cube. All other near-neighbour interactions between the Cu ions are also found to be antiferromagnetic and are thought to drive the frustration.

cond-mat.str-el↗

arXiv2026

Observation of a gapped phase in the one-dimensional $S = {\frac{1}{2}}$ Heisenberg antiferromagnetic chain Cu(Ampy)ClBr

Spin-1/2 Heisenberg antiferromagnetic frustrated spin chain systems display exotic ground states with unconventional excitations and distinct quantum phase transitions as the ratio of next-nearest-neighbor to nearest-neighbor coupling is tuned. We present a comprehensive investigation of the structural, magnetic, and thermodynamics properties of the spin-1/2 compound, Cu(Ampy)ClBr (Ampy= C$_6$H$_8$N$_2$ = 2-(Aminomethyl)pyridine) via x-ray diffraction, magnetization, specific heat, $^1$H nuclear magnetic resonance (NMR), electron spin resonance (ESR), and muon spin relaxation ($μ$SR) techniques. The crystal structure features an anisotropic triangular chain lattice of magnetic Cu$^{2+}$ ions. Our bulk and local probe experiments detect neither long-range magnetic ordering nor spin freezing down to 0.06 K despite the presence of moderate antiferromagnetic interaction between Cu$^{2+}$ spins as reflected by a Curie-Weiss temperature of about $-9$ K from the bulk susceptibility data. A broad maximum is observed at about 9 K in magnetic susceptibility and specific heat data, indicating the onset of short-range spin correlations. At low temperatures, the zero-field magnetic specific heat and the $^1$H NMR spin-lattice relaxation rate follow an exponential temperature dependence, indicating the presence of gapped magnetic excitations. Furthermore, persistent spin dynamics down to 0.088 K observed by zero-field $μ$SR evidences lack of any static magnetism.

cond-mat.str-el↗

arXiv2025

Unusual magnetic order in Eu$_{10}$Hg$_{55}$

In solid-state compounds, the valence of europium can sometimes be mixed -- which is especially favored in structures with several positions for the europium atoms. In this work, we study the Eu-based intermetallic noncentrosymmetric system Eu$_{10}$Hg$_{55}$ which has 65 atoms per unit cell and 4 distinct crystallographic positions for europium and 17 positions for mercury. Our detailed analysis of magnetism of large single crystals suggests that europium in Eu$_{10}$Hg$_{55}$ might be present in two valence states, resulting in a fragile magnetic ground state. Due to the cage-like structure with a large distance between the Eu atoms, those atoms are weakly ferromagnetically coupled and Eu$_{10}$Hg$_{55}$ orders at low temperatures, below $T_{1} = 5.5$ K, with a subsequent spin re-orientation at $T_{2} = 4.3$ K. There is no sign of magnetic frustration. Interestingly, the magnetic ordering of europium sub-lattices results in a magnetization pole reversal with a weak ferrimagnetic ground state. Additional magnetic phases can be induced by application of a modest external magnetic field.

cond-mat.mtrl-sci↗

arXiv2025

Electron Spin Resonance of Eu on triangular layers in EuT2P2 (T =Mn, Zn, Cd)

We report on the electron spin resonance (ESR) of Eu$^{2+}$ in Eu$T_{2}$P$_{2}$ ($T$=Mn, Zn, Cd) single crystals. The temperature dependencies of ESR linewidth and resonance shift show a similar behaviour when approaching the Eu-ordered state -- a divergence towards $T_{\rm N}$, indicating the growing importance of magnetic correlations and the build-up of internal magnetic fields. For $T$=Mn an additional temperature scale of $\approx 47$~K has considerable impact on linewidth, resonance field and intensity. This points to the presence of Mn magnetic correlations which yet were not reported.

cond-mat.str-el↗

arXiv2025

Magnetic frustration and weak Mn magnetic ordering in EuMn$_2$P$_2$

We report on the electron spin resonance (ESR), heat capacity, magnetization, nuclear magnetic resonance (NMR), magnetic circular and linear dichroism (XMCD, XMLD), as well as the electrical resistivity of EuMn$_{2}$P$_{2}$ single crystals. Antiferromagnetic order of Eu was observed in several quantities at $T^{\rm Eu}_{\rm N}\,=\,18\,\rm K$. The temperature dependencies of ESR linewidth and resonance shift show, when approaching the Eu-ordered state, a divergence towards $T^{\rm Eu}_{\rm N}$, indicating the growing importance of magnetic correlations and the build-up of internal magnetic fields. An additional temperature scale of $\approx 47\,\rm K$ has considerable impact on linewidth, resonance field and intensity. This points to the presence of weak Mn-based ordering. The observed ESR line is interpreted as an Eu$^{2+}$ resonance, which probes the weak magnetic background of the Mn subsystem. Such picture is suggested by the lineshape which keeps to be Lorentzian across the $47\,\rm K$ scale and by the ESR intensity which can be described by the same Curie-Weiss temperature above and below $47\,\rm K$. In the same temperature range anomalies were observed at $48.5\,\rm K$ and $51\,\rm K$ in the heat capacity data as well as a pronounced broadening of the NMR signal of the EuMn$_{2}$P$_{2}$ samples. In XMCD and XMLD measurements, this weak magnetic order could not be detected in the same temperature range which might be due to the small magnetic moment, with a potential $c$-component or frustration.

cond-mat.str-el↗

arXiv2024

Optical study on electronic structure of the locally non-centrosymmetric CeRh$_2$As$_2$

The electronic structures of the heavy-fermion superconductor CeRh$_2$As$_2$ with the local inversion symmetry breaking and the reference material LaRh$_2$As$_2$ have been investigated by using experimental optical conductivity ($σ_1(ω)$) spectra and first-principal DFT calculations. In the low-temperature $σ_1(ω)$ spectra of CeRh$_2$As$_2$, a $4f$-conduction electron hybridization and heavy quasiparticles are clearly indicated by a mid-infrared peak and a narrow Drude peak. In LaRh$_2$As$_2$, these features are absent in the $σ_1(ω)$ spectrum, however, it can nicely be reproduced by DFT calculations. For both compounds, the combination between a local inversion symmetry breaking and a large spin-orbit (SO) interaction plays an important role for the electronic structure, however, the SO splitting bands could not be resolved in the $σ_1(ω)$ spectra due to the small SO splitting size.

cond-mat.str-el↗

arXiv2023

Colossal magnetoresistance in EuZn$_2$P$_2$ and its electronic and magnetic structure

We investigate single crystals of the trigonal antiferromagnet EuZn$_2$P$_2$ ($P\overline{3}m1$) by means of electrical transport, magnetization measurements, X-ray magnetic scattering, optical reflectivity, angle-resolved photoemission spectroscopy (ARPES) and ab-initio band structure calculations (DFT+U). We find that the electrical resistivity of EuZn$_2$P$_2$ increases strongly upon cooling and can be suppressed in magnetic fields by several orders of magnitude (CMR effect). Resonant magnetic scattering reveals a magnetic ordering vector of $q = (0\, 0\, \frac{1}{2})$, corresponding to an $A$-type antiferromagnetic (AFM) order, below $T_{\rm N} = 23.7\,\rm K$. We find that the moments are canted out of the $a-a$ plane by an angle of about $40^{\circ}\pm 10^{\circ}$ degrees and aligned along the [100] in the $a-a$ plane. We observe nearly isotropic magnetization behavior for low fields and low temperatures which is consistent with the magnetic scattering results. The magnetization measurements show a deviation from the Curie-Weiss behavior below $\approx 150\,\rm K$, the temperature below which also the field dependence of the material's resistivity starts to increase. An analysis of the infrared reflectivity spectrum at $T=295\,\rm K$ allows us to resolve the main phonon bands and intra-/interband transitions, and estimate indirect and direct band gaps of $E_i^{\mathrm{opt}}=0.09\,\rm{eV}$ and $E_d^{\mathrm{opt}}=0.33\,\rm{eV}$, respectively, which are in good agreement with the theoretically predicted ones. The experimental band structure obtained by ARPES is nearly $T$-independent above and below $T_{\rm N}$. The comparison of the theoretical and experimental data shows a weak intermixing of the Eu 4$f$ states close to the $Γ$ point with the bands formed by the phosphorous 3$p$ orbitals leading to an induction of a small magnetic moment at the P sites.

cond-mat.str-el↗

arXiv2023

Electron spin resonance study on the 4f honeycomb quantum magnet YbCl3

The local magnetic properties of Yb$^{3+}$ in the layered honeycomb material YbCl$_{3}$ were investigated by electron spin resonance on single crystals. For in-plane and out-of-plane field orientations the $g$-factor shows a clear anisotropy ($g_\|=2.97(8)$ and $g_\bot =1.53(4)$), whereas the low temperature exchange coupling and the spin relaxation display a rather isotropic character. At elevated temperatures the contribution of the first excited crystal field level ($21\pm2$~meV) dominates the spin relaxation.

cond-mat.str-el↗

arXiv2022

Diluting a triangular-lattice spin liquid: Synthesis and characterization of NaYb$_{1-x}$Lu$_{x}$S$_2$ single crystals

Yb-based magnets, with a perfect triangular lattice of pseudospin-$1/2$ Yb$^{3+}$ ions, have emerged as candidates for realizing a quantum spin-liquid state, with NaYbS$_2$ being a prominent example. Here we present the solid-solution series NaYb$_{1-x}$Lu$_{x}$S$_2$ with well-defined single crystals over the entire substitution range $0\le x\le 1$. Chemical and structural analysis indicate a statistically homogeneous replacement of Yb$^{3+}$ by Lu$^{3+}$ ions. We magnetically characterize the relatively small single crystals using electron spin resonance (ESR). Below $30$K the ESR intensity can be well described by a Curie-Weiss function for all \textit{x}, with a decreasing Weiss temperature with increasing Lu content. This reduction of the average magnetic interaction upon Lu substitution is also supported by magnetization measurements. Importantly, no signs of magnetic or spin-glass order are detected down to $2$K for any $x$. For $x>0.5$ the ESR linewidth strongly increases, indicating the breakup of the magnetic system into disconnected clusters as expected from percolation physics. The experimental magnetization data are found to be in good agreement for all $x$ with results of classical Monte-Carlo simulations for a triangular-lattice Heisenberg model, amended with a small second-neighbor interaction. Taken together, our results establish NaYb$_{1-x}$Lu$_{x}$S$_2$ as a family of diluted triangular-lattice spin liquids.

cond-mat.str-el↗

arXiv2021

Electron Spin Resonance of the itinerant ferromagnets LaCrGe3, CeCrGe3 and PrCrGe3

We report Electron Spin Resonance of the itinerant ferromagnets LaCrGe3, CeCrGe3, and PrCrGe3. These compounds show well defined and very similar spectra of itinerant Cr 3d spins in the paramagnetic temperature region. Upon cooling and crossing the Cr-ferromagnetic ordering (below around 90 K) strong spectral structures start to dominate the resonance spectra in a quite different manner in the three compounds. In the Ce- and Pr-compounds the resonance is only visible in the paramagnetic region whereas in the La-compound the resonance can be followed far below the ferromagnetic ordering temperature. This behavior will be discussed in terms of the specific interplay between the 4f and 3d magnetism which appears quite remarkable since CeCrGe3 displays heavy fermion behavior even in the magnetically ordered state.

cond-mat.str-el↗

arXiv2021

Optical evidence of local and itinerant states in Ce- and Yb-heavy-fermion compounds

The electronic properties of Cerium (Ce) and ytterbium (Yb) intermetallic compounds may display a more local or more itinerant character depending on the interplay of the exchange interactions among the $4f$ electrons and the Kondo coupling between $4f$ and conduction electrons. For the more itinerant case, the materials form heavy-fermions once the Kondo effect is developed at low temperatures. Hence, a temperature variation occurs in the electronic structure that can be traced by investigating the optical conductivity ($σ(ω)$) spectra. Remarkably, the temperature variation in the $σ(ω)$ spectrum is still present in the more localized case, even though the Kondo effect is strongly suppressed. Here, we clarify the local and itinerant character in the electronic structure by investigating the temperature dependence in the $σ(ω)$ spectra of various Ce and Yb compounds with a tetragonal ThCr$_2$Si$_2$-type crystal structure. We explain the temperature change in a unified manner. Above temperatures of about 100 K, the temperature dependence of the $σ(ω)$ spectra is mainly due to the electron-phonon interaction, while the temperature dependence below is due to the Kondo effect.

cond-mat.str-el↗

arXiv2020

Gapless quantum spin liquid in the triangular system Sr$_{3}$CuSb$_{2}$O$_{9}$

We report gapless quantum spin liquid behavior in the layered triangular Sr$_{3}$CuSb$_{2}$O$_{9}$ (SCSO) system. X-ray diffraction shows superlattice reflections associated with atomic site ordering into triangular Cu planes well-separated by Sb planes. Muon spin relaxation ($μ$SR) measurements show that the $S = \frac{1}{2}$ moments at the magnetically active Cu sites remain dynamic down to 65 mK in spite of a large antiferromagnetic exchange scale evidenced by a large Curie-Weiss temperature $θ_{\mathrm{cw}} \simeq $ -143 K as extracted from the bulk susceptibility. Specific heat measurements also show no sign of long-range order down to 0.35 K. The magnetic specific heat ($\mathit{C}$$_{\mathrm{m}}$) below 5 K reveals a $\mathit{C}$$_{\mathrm{m}}$ $=$ $γT$ + $αT$$^{2}$ behavior. The significant $T$$^{2}$ contribution to the magnetic specific heat invites a phenomenology in terms of the so-called Dirac spinon excitations with a linear dispersion. From the low-$T$ specific heat data, we estimate the dominant exchange scale to be $\sim $ 36 K using a Dirac spin liquid ansatz which is not far from the values inferred from microscopic density functional theory calculations ($\sim $ 45 K) as well as high-temperature susceptibility analysis ($\sim$ 70 K). The linear specific heat coefficient is about 18 mJ/mol-K$^2$ which is somewhat larger than for typical Fermi liquids.

cond-mat.str-el↗

arXiv2019

Effective Spin-1/2 Moments on a Yb$^{3+}$ Triangular Lattice: an ESR Study

We investigated the spin dynamics by electron spin resonance (ESR) of the Yb-based, effective spin-1/2 delafossites NaYbO$_{2}$, AgYbO$_{2}$, LiYbS$_{2}$, NaYbS$_{2}$, and NaYbSe$_{2}$ which all show an absence of magnetic order down to lowest reachable temperatures and thus are prime candidates to host a quantum spin-liquid ground state in the vicinity of long range magnetic order. Clearly resolved ESR spectra allow to obtain well-defined $g$ values which are determined by the crystal field of the distorted octahedral surrounding of the Yb-ions in trigonal symmetry. This local crystal field information provides important input to characterize the effective $S = 1/2$ Kramers doublet as well as the anisotropic exchange coupling between the Yb ions which is crucial for the nature of the groundstate. The ESR linewidth $ΔB$ is characterised by the spin dynamics and is mainly determined by the anisotropic exchange coupling. We discuss and compare $ΔB$ of the above mentioned delafossites focussing on the low temperature behaviour which is dominated by the growing influence of spin correlations.

cond-mat.str-el↗

arXiv2019

Magneto-Optics of the Weyl Semimetal TaAs in the THz and IR Regions

The magnetic-field dependence of optical reflectivity [$R(ω)$] and optical conductivity [$σ(ω)$] spectra of the ideal type-I Weyl semimetal TaAs has been investigated at the temperature of 10 K in the terahertz (THz) and infrared (IR) regions. The obtained $σ(ω)$ spectrum in the THz region of $\hbarω\leq15$ meV is strongly affected by the applied magnetic field ($B$): The Drude spectral weight is rapidly suppressed and an energy gap originating from the optical transition in the lowest Landau levels appears with a gap size that increases in proportion to $\sqrt{B}$, which suggests linear band dispersions. The obtained THz $σ(ω)$ spectra could be scaled not only in the energy scale by $\sqrt{B}$ but also in the intensity by $1/\sqrt{B}$ as predicted theoretically. In the IR region for $\hbarω\geq17$ meV, on the other hand, the observed $R(ω)$ peaks originating from the optical transitions in higher Landau levels are proportional to linear-$B$ suggesting parabolic bands. The different band dispersions originate from the crossover from the Dirac to the free-electron bands.

cond-mat.str-el↗

arXiv2019

Kondo-lattice ferromagnets and their peculiar order along the magnetically hard axis

We show that Ce- and Yb-based Kondo-lattice ferromagnets order mainly along the magnetically hard direction of the ground state Kramers doublet determined by crystalline electric field (CEF). Here we argue that this peculiar phenomenon, that was believed to be rare, is instead the standard case. Moreover, it seems to be independent on the Curie temperature $T_\mathrm{C}$, crystalline structure, size of the ordered moment and type of ground state wave function. On the other hand, all these systems show the Kondo coherence maximum in the temperature dependence of the resistivity just above $T_\mathrm{C}$ which indicates a Kondo temperature of a few Kelvin. An important role of fluctuations is indicated by the non-mean-field like transition in specific heat measurements as well as by the suppression of this effect by a strong Ising-like anisotropy. We discuss possible theoretical scenarios.

cond-mat.str-el↗

arXiv2018

Electron Spin Resonance on the spin-1/2 triangular magnet NaYbS2

The delafossite structure of NaYbS2 contains a planar spin-1/2 triangular lattice of Yb3+ ions and features a possible realisation of a quantum spin-liquid state. We investigated the Yb3+ spin dynamics by Electron Spin Resonance (ESR) in single-crystalline samples of NaYbS2. Very clear spectra with a well-resolved and large anisotropy could be observed down to the lowest accessible temperature of 2.7 K. In contrast to the ESR properties of other known spin-liquid candidate systems, the resonance seen in NaYbS2 is accessible at low fields (< 1T) and is narrow enough for accurate characterisation of the relaxation rate as well as the g factor of the Yb3+ spins.

cond-mat.str-el↗

arXiv2017

Angle-dependent electron spin resonance of YbRh$_2$Si$_2$ measured with planar microwave resonators and in-situ rotation

We present a new experimental approach to investigate the magnetic properties of the anisotropic heavy-fermion system YbRh$_2$Si$_2$ as a function of crystallographic orientation. Angle-dependent electron spin resonance (ESR) measurements are performed at a low temperature of 1.6 K and at an ESR frequency of 4.4 GHz utilizing a superconducting planar microwave resonator in a $^4$He-cryostat in combination with in-situ sample rotation. The obtained ESR g-factor of YbRh$_2$Si$_2$ as a function of the crystallographic angle is consistent with results of previous measurements using conventional ESR spectrometers at higher frequencies and fields. Perspectives to implement this experimental approach into a dilution refrigerator and to reach the magnetically ordered phase of YbRh$_2$Si$_2$ are discussed.

cond-mat.str-el↗