arXiv subjects

Dirk Wulferding

Publications and source records attributed to Dirk Wulferding.

At least 37 records · Page 2Linked to original sources

arXiv2023

One-ninth magnetization plateau stabilized by spin entanglement in a kagome antiferromagnet

The spin-1/2 antiferromagnetic Heisenberg model on a Kagome lattice is geometrically frustrated, which is expected to promote the formation of many-body quantum entangled states. The most sought-after among these is the quantum spin liquid phase, but magnetic analogs of liquid, solid, and supersolid phases may also occur, producing fractional plateaus in the magnetization. Here, we investigate the experimental realization of these predicted phases in the Kagome material YCu3(OD)6+xBr3-x (x=0.5). By combining thermodynamic and Raman spectroscopic techniques, we provide evidence for fractionalized spinon excitations and observe the emergence of a 1/9 magnetization plateau. These observations establish YCu3(OD)6+xBr3-x as a model material for exploring the 1/9 plateau phase.

cond-mat.str-el↗

arXiv2023

Fingerprints for anisotropic Kondo lattice behavior in the quasiparticle dynamics of the kagome metal Ni$_3$In

We present a temperature- and polarization-resolved phononic and electronic Raman scattering study in combination with the first-principles calculations on the kagome metal Ni$_3$In with anisotropic transport properties and non-Fermi liquid behavior. At temperatures below 50 K and down to 2 K, several Raman phonon modes, including particularly an interlayer shear mode, exhibit appreciable frequency and linewidth renormalization, reminiscent of the onset of the Kondo screening without an accompanying structural or magnetic phase transition. In addition, a low-energy electronic continuum observed in polarization perpendicular to the kagome planes reveals strong temperature dependence below 50 K, implying thermal depletion of incoherent quasiparticles, while the in-plane continuum remains invariant. These concomitant electronic and phononic Raman signatures suggest that Ni$_3$In undergoes an anisotropic electronic crossover from an incoherent to a coherent Kondo lattice regime below 50 K. We discuss the origin of the anisotropic incoherent-coherent crossover in association with the possible anisotropic Kondo hybridization involving localized Ni-$3d_{xz}$ flat-band electrons.

cond-mat.str-el↗

arXiv2023

Linear scaling relationship of Néel temperature and dominant magnons in pyrochlore ruthenates

We present a systematic Raman spectroscopy study on a series of pyrochlore ruthenates, a system which is not yet clearly settled on its magnetic origin and structure. Apart from the Raman-active phonon modes, new peaks that appear in the energy range of 15 - 35 meV below the Néel temperature are assigned as one-magnon modes. The temperature evolution of one-magnon modes displays no significant thermal dependence in mode frequencies while the intensities decrease monotonically. Remarkably, one-magnons from all compounds show similar characteristics with a single dominant peak at lower energy and weaker side peaks at a couple of meV higher energy. Most importantly, we uncover a striking proportionality between the dominant magnon mode energies and the Néel temperature values. Our results suggest the Ru ions may have similar or the same magnetic phase in all pyrochlore ruthenates of our study. We have thus found an avenue for directly tuning the magnetic exchange interaction by the selection of the $A$-site ion.

cond-mat.str-el↗

arXiv2023

Collective magnetic Higgs excitation in a pyrochlore ruthenate

The emergence of scalar Higgs-type amplitude modes in systems where symmetry is spontaneously broken has been a highly successful, paradigmatic description of phase transitions, with implications ranging from high-energy particle physics to low-energy condensed matter systems. Here, we uncover two successive high temperature phase transitions in the pyrochlore magnet Nd$_2$Ru$_2$O$_7$ at $T_{\mathrm{N}} = 147$ K and $T^* = 97$ K, that lead to giant phonon instabilities and culminate in the emergence of a highly coherent excitation. This coherent excitation, distinct from other phonons and from conventional magnetic modes, stabilizes at a low energy of 3 meV. We assign it to a collective Higgs-type amplitude mode, that involves bond energy modulations of the Ru$_4$ tetrahedra. Its striking two-fold symmetry, incompatible with the underlying crystal structure, highlights the possibility of multiple entangled broken symmetries.

cond-mat.str-el↗

arXiv2022

Emergent nematicity and intrinsic vs. extrinsic electronic scattering processes in the kagome metal CsV$_3$Sb$_5$

Fermi surface fluctuations and lattice instabilities in the 2D metallic kagome superconductor CsV$_3$Sb$_5$ are elucidated via polarization-resolved Raman spectroscopy. The presence of a weak electronic continuum in high-quality samples marks the cross-over into the charge-density-wave (CDW) ordered phase, while impurity-rich samples promote strong defect-induced electronic scattering processes that affect the coherence of the CDW phase. CDW-induced phonon anomalies appear below $T_{\mathrm{CDW}}$, with emergent $C2$ symmetry for one of the CDW amplitude modes, alluding to nematicity. In conjunction with symmetry-breaking lattice distortions, a kink-like hardening of the A$_{1g}$ phonon energy at $T_{\mathrm{CDW}}$ signifies a concerted interplay of electronic correlations and electron-phonon coupling in the exotic CDW order.

cond-mat.str-el↗

arXiv2021

Thermally populated versus field-induced triplon bound states in the Shastry-Sutherland lattice SrCu$_2$(BO$_3$)$_2$

The Shastry-Sutherland compound SrCu$_2$(BO$_3$)$_2$ constituting orthogonally coupled dimers harbors a $S=0$ singlet ground state. The confluence of strong interdimer interaction and frustration engenders a spectrum of low-energy excitations including localized triplons as well as singlet and triplet bound states. Their dynamics are controlled by an external magnetic field and temperature. Here, we employ high-field Raman spectroscopy to map the field and temperature evolution of such bosonic composite quasiparticles on approaching the 1/8 magnetization plateau. Our study unveils that the magnetic field and thermal fluctuations show remarkably similar effects in melting the singlet bound states, but are disparate in their effects on the fine spectral shapes. This, together with the anti-crossing of two singlet bound states in the intermediate field $B=10-16$ T, is discussed in terms of the correlated dynamics of frustrated, interacting bosons.

cond-mat.str-el↗

arXiv2021

Dimer Crystallization Induced by Elemental Substitution in the Honeycomb Lattice of Ru1-xOsxCl3

Substitution effects of Os for Ru in α-RuCl3 are investigated in a wide composition range of 0 =< x =< 0.67 in Ru1-xOsxCl3 by X-ray and electron diffraction, magnetic susceptibility, heat capacity, and Raman spectroscopy measurements. Apart from the Kitaev physics with antiferromagnetic interactions increasing with x, a rich phase diagram is obtained, which includes an antiferromagnetic long-range order below 12 K for x =< 0.15, a dome-shaped spin-singlet dimer phase below 130 K for 0.15 =< x =< 0.40, and a magnetic short-range order for x > 0.40. A dimerization as similarly observed in α-RuCl3 under high pressure occurs in the spin-singlet phase. It is suggested that Ru-Os pairs in the solid solutions tend to form dimers with short bonds and trigger the first-order transition in the presence of pseudo-threefold rotational symmetry for dimerization around a substituted Os atom only at low substitutions. This is a rare example of molecular orbital crystallization induced by elemental substitution in a highly disordered system. The short-range order at high substitutions may be related to a random-singlet state stabilized by bond disorder in the honeycomb net.

cond-mat.mtrl-sci↗

arXiv2021

Raman scattering of plane-wave and twisted light off chiral molecular liquids

We present an experimental study of the quasi-elastic Raman scattering (QES) of plane-wave and twisted light by liquid crystals. Depending on their temperature, these crystals can exhibit isotropic, nematic and chiral nematic phases. The question is addressed of how the phase of a crystal and the state of incident light can affect the quasi-elastic energy spectra of the scattered radiation, whose shape is usually described by the combination of Lorentzian and Gaussian components. Special attention is paid to the \textit{chiral phase}, for which the Raman QES spectrum is dominated by a Lorentzian with reduced linewidth, pointing to diminished disorder and configurational entropy. Moreover, this phase is also known for a regime of iridescence (selective backscattering) which arises when the wavelength of incident light becomes comparable with the chiral pitch length. Detailed measurements, performed in this \textit{resonant} regime and by employing twisted light, carrying various projections of the orbital angular momentum (OAM), have indicated a low-energy scattering surplus depending on OAM. We argue that this observation might indicate a transfer of angular momentum between light and the liquid crystal.

physics.optics↗

arXiv2021

Non-Abelian statistics in light scattering processes across interacting Haldane chains

The $S=1$ Haldane state is constructed from a product of local singlet dimers in the bulk and topological states at the edges of a chain. It is a fundamental representative of topological quantum matter. Its well-known representative, the quasi-one-dimensional SrNi$_2$V$_2$O$_8$ shows both conventional as well as unconventional magnetic Raman scattering. The former is observed as one- and two-triplet excitations with small linewidths and energies corresponding to the Haldane gap $Δ_H$ and the exchange coupling $J_c$ along the chain, respectively. Well-defined magnetic quasiparticles are assumed to be stabilized by interchain interactions and uniaxial single-ion anisotropy. Unconventional scattering exists as broad continua of scattering with an intensity $I(T)$ that shows a mixed bosonic / fermionic statistic. Such a mixed statistic has also been observed in Kitaev spin liquids and could point to a non-Abelian symmetry. As the ground state in the bulk of SrNi$_2$V$_2$O$_8$ is topologically trivial, we suggest its fractionalization to be due to light-induced interchain exchange processes. These processes are supposed to be enhanced due to a proximity to an Ising ordered state with a quantum critical point. A comparison with SrCo$_2$V$_2$O$_8$, the $S=1/2$ analogue to our title compound, supports these statements.

cond-mat.str-el↗

arXiv2020

Local characterization of a heavy-fermion superconductor via sub-Kelvin magnetic force microscopy

Using magnetic force microscopy operating at sub-Kelvin temperatures we characterize the heavy-fermion superconductor CeCoIn$_5$. We pinpoint the absolute London penetration depth $λ(0) = 435 \pm 20$ nm and report its temperature dependence, which is closely linked to the symmetry of the superconducting gap. In addition, we directly measure the pinning force of individual Abrikosov vortices and estimate the critical current density $j_c = 9 \times 10^4$ A/cm$^2$. In contrast to the related, well-established tunnel diode oscillator technique, our method is capable of resolving inhomogeneities $locally$ on the micrometer-scale at ultra-low temperature.

cond-mat.supr-con↗

arXiv2020

Effect of topology on quasi-particle interactions in the Weyl semimetal WP$_2$

We compare two crystallographic phases of the low-dimensional WP$_2$ to better understand features of electron-electron and electron-phonon interactions in topological systems. The topological $β$-phase, a Weyl semimetal with a giant magneto-resistance, shows a larger intensity of electronic Raman scattering compared to the topologically trivial $α$-phase. This intensity sharply drops for $T < T^* = 20$ K which evidences a crossover in the topological phase from marginal quasiparticles to a coherent low temperature regime. In contrast, the non-topological $α$-phase shows more pronounced signatures of electron-phonon interaction. Here there exist generally enlarged phonon linewidths and deviations from conventional anharmonicity in an intermediate temperature regime. These effects provide evidence for an interesting interplay of electronic correlations and electron-phonon coupling. Both interband and intraband electronic fluctuations are involved in these effects. Their dependence on symmetry as well as momentum conservation are critical ingredients to understand this interplay.

cond-mat.str-el↗

arXiv2019

Magnon bound states vs. anyonic Majorana excitations in the Kitaev honeycomb magnet $α$-RuCl$_3$

The pure Kitaev honeycomb model harbors a quantum spin liquid in zero magnetic fields, while applying finite magnetic fields induces a topological spin liquid with non-Abelian anyonic excitations. This latter phase has been much sought after in Kitaev candidate materials, such as $α$-RuCl$_3$. Currently, two competing scenarios exist for the intermediate field phase of this compound ($B=7-10$ T), based on experimental as well as theoretical results: (i) conventional multiparticle magnetic excitations of integer quantum number vs. (ii) Majorana fermionic excitations of possibly non-Abelian nature with a fractional quantum number. To discriminate between these scenarios a detailed investigation of excitations over a wide field-temperature phase diagram is essential. Here we present Raman spectroscopic data revealing low-energy quasiparticles emerging out of a continuum of fractionalized excitations at intermediate fields, which are contrasted by conventional spin-wave excitations. The temperature evolution of these quasiparticles suggests the formation of bound states out of fractionalized excitations.

cond-mat.str-el↗

arXiv2019

Systematic Raman study of optical phonons in $R$Ba$_2$Cu$_3$O$_{6+δ}$ ($R$ = Y, Dy, Gd, Sm, Nd): Antiferromagnetic coupling strength versus lattice parameters

We present a systematic study of the interplay between lattice parameters and the energy of the optical phonons as well as the antiferromagnetic coupling strength, $J$, in the high-$T_{\text{c}}$ superconducting cuprate $R$Ba$_2$Cu$_3$O$_{6+δ}$ (\mbox{R-123}, $R =$ Y, Dy, Gd, Sm, Nd) with hole-doping $p$ ($0.00<p \lesssim 0.04$). The energy of the $B_{1g}$-mode at $ν_{B1g}\approx~335$ cm$^{-1}$ has been found to relate systematically to the inverse of the lattice parameter $a$. Our results confirm the temperature dependent phonon splitting for Nd-123 at low doping, which has been reported for optimally-doped Nd-123. Surprisingly, $J$ is independent of $a$ for the first four $R$ families, and a general consistency between $T_{\text{c}}^{\text{max}}$ and $J$, as suggested in a previous investigation, could not be confirmed.

cond-mat.str-el↗

arXiv2018

Tuning the orbital-lattice fluctuations in the mixed spin-dimer system Ba$_{3-x}$Sr$_{x}$Cr$_{2}$O$_{8}$

In $A_{3}$Cr$_{2}$O$_{8}$, where $A$ = Sr or Ba, the Cr$^{5+}$ ions surrounded by oxygen ions in a tetrahedral coordination are Jahn-Teller active. The Jahn-Teller distortion leads to a structural transition and a related emergence of three twinned monoclinic domains below the structural phase transition. This transition is highly dynamic over an extended temperature range for $A$ = Sr. We have investigated mixed compounds Ba$_{3-x}$Sr$_{x}$Cr$_{2}$O$_{8}$ with $x=2.9$ and $x=2.8$ by means of X-ray and neutron diffraction, Raman scattering and calorimetry. Based on the obtained evolution of the phonon frequencies, we find a distinct suppression of the orbital-lattice fluctuation regime with increasing Ba content. This stands in contrast to the linear behaviour exhibited by unit cell volumes, atomic positions and intradimer spin-spin exchange interactions.

cond-mat.str-el↗

arXiv2016

Direct observation of the M2 phase with its Mott transition in a VO$_2$ film

In VO$_2$, the explicit origin of the insulator-to-metal transition is still disputable between Peierls and Mott insulators. Along with the controversy, its second monoclinic (M2) phase has received considerable attention due to the presence of electron correlation in undimerized vanadium ions. However, the origin of the M2 phase is still obscure. Here, we study a granular VO$_2$ film using conductive atomic force microscopy and Raman scattering. Upon the structural transition from monoclinic to rutile, we observe directly an intermediate state showing the coexistence of monoclinic M1 and M2 phases. The conductivity near the grain boundary in this regime is six times larger than that of the grain core, producing a donut-like landscape. Our results reveal an intra-grain percolation process, indicating that VO$_2$ with the M2 phase is a Mott insulator.

cond-mat.str-el↗

arXiv2016

Construction of a $^3$He magnetic force microscope with a vector magnet

We constructed a $^3$He magnetic force microscope operating at the base temperature of 300 mK under a vector magnetic field of 2-2-9 T in the $x-y-z$ direction. Fiber optic interferometry as a detection scheme is employed in which two home-built fiber walkers are used for the alignment between the cantilever and the optical fiber. The noise level of the laser interferometer is close to its thermodynamic limit. The capabilities of the sub-Kelvin and vector field are demonstrated by imaging the coexistence of magnetism and superconductivity in a ferromagnetic superconductor (ErNi$_2$B$_2$C) at $T$=500 mK and by probing a dipole shape of a single Abrikosov vortex with an in-plane tip magnetization.

physics.ins-det↗

arXiv2015

Spatially resolved penetration depth measurements and vortex manipulation in the ferromagnetic superconductor ErNi2B2C

We present a local probe study of the magnetic superconductor, ErNi$_2$B$_2$C, using magnetic force microscopy at sub-Kelvin temperatures. ErNi$_2$B$_2$C is an ideal system to explore the effects of concomitant superconductivity and ferromagnetism. At 500 mK, far below the transition to a weakly ferromagnetic state, we directly observe a structured magnetic background on the micrometer scale. We determine spatially resolved absolute values of the magnetic penetration depth $λ$ and study its temperature dependence as the system undergoes magnetic phase transitions from paramagnetic to antiferromagnetic, and to weak ferromagnetic, all within the superconducting regime. In addition, we estimate the absolute pinning force of Abrikosov vortices, which shows a position- and temperature dependence as well, and discuss the possibility of the purported spontaneous vortex formation.

cond-mat.supr-con↗

arXiv2015

Magnetic domain tuning and the emergence of bubble domains in the bilayer manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (x=0.32)

We report a magnetic force microscopy study of the magnetic domain evolution in the layered manganite La$_{2-2x}$Sr$_{1+2x}$Mn$_2$O$_7$ (with $x=0.32$). This strongly correlated electron compound is known to exhibit a wide range of magnetic phases, including a recently uncovered biskyrmion phase. We observe a continuous transition from dendritic to stripe-like domains, followed by the formation of magnetic bubbles due to a field- and temperature dependent competition between in-plane and out-of-plane spin alignments. The magnetic bubble phase appears at comparable field- and temperature ranges as the biskyrmion phase, suggesting a close relation between both phases. Based on our real-space images we construct a temperature-field phase diagram for this composition.

cond-mat.str-el↗