arXiv subjects

Christian Rüegg

Publications and source records attributed to Christian Rüegg.

At least 19 recordsLinked to original sources

arXiv2025

Fingerprints of supersymmetric spin and charge dynamics observed by inelastic neutron scattering

Supersymmetry is an algebraic property of a quantum Hamiltonian that, by giving every boson a fermionic superpartner and vice versa, may underpin physics beyond the Standard Model. Fractional bosonic and fermionic quasiparticles are familiar in condensed matter, as in the spin and charge excitations of the $t$-$J$ model describing electron dynamics in one-dimensional materials, but this type of symmetry is almost unknown. However, the triplet excitations of a quantum spin ladder in an applied magnetic field provide a supersymmetric analogue of the $t$-$J$ chain. Here we perform neutron spectroscopy on the spin-ladder compounds (C$_5$D$_{12}$N)$_2$CuBr$_4$ and (C$_5$D$_{12}$N)$_2$CuCl$_4$ over a range of applied fields and temperatures, and apply matrix-product-state methods to the ladder and equivalent chain models. From the momentum-resolved dynamics of a single charge-like excitation in a bath of fractional spins, we find essential differences in thermal broadening between the supersymmetric and non-supersymmetric sectors. The persistence of a strict zone-centre pole at all temperatures constitutes an observable consequence of supersymmetry that marks the beginning of supersymmetric studies in experimental condensed matter.

cond-mat.str-el↗

arXiv2023

Structural and magnetic properties of $β$-Li$_2$IrO$_3$ after grazing-angle focused ion beam thinning

Manipulating the size and orientation of quantum materials is often used to tune emergent phenomena, but precise control of these parameters is also necessary from an experimental point of view. Various synthesis techniques already exist, such as epitaxial thin film growth and chemical etching, that are capable of producing specific sample dimensions with high precision. However, certain materials exist as single crystals that are often difficult to manipulate, thereby limiting their studies to a certain subset of experimental techniques. One particular class of these materials are the lithium and sodium iridates that are promising candidates for hosting a Kitaev quantum spin liquid state. Here we present a controlled method of using a focused ion beam at grazing incidence to reduce the size of a $β$-Li$_2$IrO$_3$ single crystal to a thickness of 1 $μm$. Subsequent x-ray diffraction measurements show the lattice remains intact, albeit with a larger mosaic spread. The integrity of the magnetic order is also preserved as the temperature dependent magnetic diffraction peak follows the same trend as its bulk counterpart with a transition temperature at TN = 37.5 K. Our study demonstrates a technique that opens up the possibility of nonequilibrium experiments where submicron thin samples are often essential.

cond-mat.mtrl-sci↗

arXiv2023

Ultrafast Frustration-Breaking and Magnetophononic Driving of Singlet Excitations in a Quantum Magnet

Ideal magnetic frustration forms the basis for the emergence of exotic quantum spin states that are entirely nonmagnetic. Such singlet spin states are the defining feature of the Shastry-Sutherland model, and of its faithful materials realization in the quantum antiferromagnet SrCu$_2$(BO$_3$)$_2$. To address these states on ultrafast timescales, despite their lack of any microscopic order parameter, we introduce a nonlinear magnetophononic mechanism to alter the quantum spin dynamics by driving multiple optical phonon modes coherently and simultaneously. We apply intense terahertz pulses to create a nonequilibrium modulation of the magnetic interactions that breaks the ideal frustration of SrCu$_2$(BO$_3$)$_2$, such that previously forbidden physics can be driven in a coherent manner. Specifically, this driving populates a purely magnetic excitation, the singlet branch of the two-triplon bound state, by resonance with the difference frequency of two pumped phonons. Our results demonstrate how light-driven phonons can be used for the ultrafast and selective manipulation of interactions in condensed matter, even at frequencies far from those of the pump spectrum, offering valuable additional capabilities for the dynamical control of quantum many-body phenomena.

cond-mat.str-el↗

arXiv2021

Revealing three-dimensional quantum criticality by Sr-substitution in Han Purple

Classical and quantum phase transitions (QPTs), with their accompanying concepts of criticality and universality, are a cornerstone of statistical thermodynamics. An exemplary controlled QPT is the field-induced magnetic ordering of a gapped quantum magnet. Although numerous "quasi-one-dimensional" coupled spin-chain and -ladder materials are known whose ordering transition is three-dimensional (3D), quasi-2D systems are special for several physical reasons. Motivated by the ancient pigment Han Purple (BaCuSi$_{2}$O$_{6}$), a quasi-2D material displaying anomalous critical properties, we present a complete analysis of Ba$_{0.9}$Sr$_{0.1}$CuSi$_{2}$O$_{6}$. We measure the zero-field magnetic excitations by neutron spectroscopy and deduce the magnetic Hamiltonian. We probe the field-induced transition by combining magnetization, specific-heat, torque and magnetocalorimetric measurements with low-temperature nuclear magnetic resonance studies near the QPT. By a Bayesian statistical analysis and large-scale Quantum Monte Carlo simulations, we demonstrate unambiguously that observable 3D quantum critical scaling is restored by the structural simplification arising from light Sr-substitution in Han Purple.

cond-mat.str-el↗

arXiv2020

Fractional antiferromagnetic skyrmion lattice induced by anisotropic couplings

Magnetic skyrmions are topological solitons with a nanoscale winding spin texture that hold promise for spintronics applications. Until now, skyrmions have been observed in a variety of magnets that exhibit nearly parallel alignment for the neighbouring spins, but theoretically, skyrmions with anti-parallel neighbouring spins are also possible. The latter, antiferromagnetic skyrmions, may allow more flexible control compared to the conventional ferromagnetic skyrmions. Here, by combining neutron scattering and Monte Carlo simulations, we show that a fractional antiferromagnetic skyrmion lattice with an incipient meron character is stabilized in MnSc$_2$S$_4$ through anisotropic couplings. Our work demonstrates that the theoretically proposed antiferromagnetic skyrmions can be stabilized in real materials and represents an important step towards implementing the antiferromagnetic-skyrmion based spintronic devices.

cond-mat.str-el↗

arXiv2020

Crystal-electric-field excitations in a quantum-spin-liquid candidate NaErS$_2$

The delafossite family of compounds with a triangular lattice of rare earth ions has been recently proposed as a candidate host for quantum spin liquid (QSL) states. To realize QSLs, the crystal-electric-field (CEF) ground state of the rare earth ions should be composed of a doublet that allows sizable quantum tunneling, but till now the knowledge on CEF states in the delafossite compounds is still limited. Here we employ inelastic neutron scattering (INS) to study the CEF transitions in a powder sample of the delafossite NaErS$_2$, where the large total angular momentum $J = 15/2$ of the Er$^{3+} $ ions and the resulting plethora of CEF transitions enable an accurate fit of the CEF parameters. Our study reveals nearly isotropic spins with large $J_z = \pm 1/2$ components for the Er$^{3+}$ CEF ground states, which might facilitate the development of a QSL state. The scaling of the obtained CEF Hamiltonian to different rare earth ions suggests that sizable $J_z = \pm 1/2$ components are generally present in the CEF ground states, supporting the ternary sulfide delafossites as potential QSL hosts.

cond-mat.str-el↗

arXiv2019

Magnetic phase diagram of the triangular antiferromagnetic $Cs_2CuCl_{4-x}Br_x$ mixed system

The novel magnetic phase diagram of the $Cs_2CuCl_{4-x}Br_x$ mixed system is established by means of single crystal neutron diffraction in the lowest temperature region and zero magnetic field. Two long-range ordered magnetic phases exist in this mixed system depending on the Cl/Br concentration. In the rich Cl concentration range, the ordered magnetic state occurs below the ordering temperature $T_N = 0.51(1)K$ for $Cs_2CuCl_3Br_1$ and at $Cs_2CuCl_{2.6}Br_{1.4}$ below $T_N = 0.24(2)K$. Magnetic order with a temperature-independent position $(0, 0.573(1), 0)$ below the ordering temperature $T_N = 0.63(1)K$ appears in the rich Br concentration for $Cs_2CuCl_{0.6}Br_{3.4}$. Between the rich Cl and rich Br concentration ranges (two magnetic phases), there is a range of x without magnetic order down to $50mK$. A suggestion about the magnetic exchange paths in the $bc$-layer for different regimes is presented, which can be controlled depending on the preferred Br-occupation in the [CuX4] tetrahedra. The density functional theory (DFT) calculations of the exchange coupling constants J, J^' for some ordered compositions of the mixed system $Cs_2CuCl_{4-x}Br_x$ indicate that these are not frustrated.

cond-mat.mtrl-sci↗

arXiv2019

Floating Zone Growth of Sr Substituted Han Purple: Ba$_{0.9}$Sr$_{0.1}$CuSi$_{2}$O$_{6}$

We present a route to grow single crystals of Ba$_{0.9}$Sr$_{0.1}$CuSi$_{2}$O$_{6}$ suitable for inelastic neutron studies via the floating zone technique. Neutron single crystal diffraction was utilized to check their bulk quality and orientation. Finally, the high quality of the grown crystals was proven by X-ray diffraction and magnetic susceptibility.

cond-mat.mtrl-sci↗

arXiv2019

Tomonaga-Luttinger liquid spin dynamics in the quasi-one dimensional Ising-like antiferromagnet BaCo$_2$V$_2$O$_8$

Combining inelastic neutron scattering and numerical simulations, we study the quasi-one dimensional Ising anisotropic quantum antiferromagnet \bacovo\ in a longitudinal magnetic field. This material shows a quantum phase transition from a Néel ordered phase at zero field to a longitudinal incommensurate spin density wave at a critical magnetic field of 3.8 T. Concomitantly the excitation gap almost closes and a fundamental reconfiguration of the spin dynamics occurs. These experimental results are well described by the universal Tomonaga-Luttinger liquid theory developed for interacting spinless fermions in one dimension. We especially observe the rise of mainly longitudinal excitations, a hallmark of the unconventional low-field regime in Ising-like quantum antiferromagnet chains.

cond-mat.str-el↗

arXiv2018

Interplay between structure and magnetism in the low-dimensional spin system $K(C_8H_{16}O_4)_2CuCl_3{*}H_2O$

Materials based on a crown ether complex together with magnetic ions, especially Cu(II), can be used to synthesize new low dimesional quantum spin systems. We have prepared the new crown ether complex Di-μ-chloro-bis(12-crown-4)-aquqdichloro-copper(II)-potassium, $K(C_8H_{16}O_4)_2CuCl_3{*}H_2O$ (1), determined its structure, and analyzed its magnetic properties. Complex (1) has a monoclinic structure and crystallizes in space group $P2_1/n$ with the lattice parameters of $a=9.5976(5)Å$, $b=11.9814Å, c=21.8713Å$ and $β=100.945(4)°$. The magnetic properties of this compound have been investigated in the temperature range 1.8 K - 300 K. The magnetic susceptibility shows a maximum at 23 K, but no 3-D long range magnetic order down to 1.8 K. The S=1/2 Cu(II) ions form antiferromagnetically coupled dimers with Cu-Cl distances of $2.2554(8)Å$ and $4.683(6)Å$, and a Cu-Cl-Cu angle of $115.12(2)°$ with $2J_{dimer}=-2.96meV (-23.78 cm^{-1})$. The influence of $H_2O$ on the Cl-Cu-Cl exchange path is analyzed. Our results show that the values of the singlet-triplet splitting are increasing considering $H_2O$ molecules in the bridging interaction. This is supported by Density functional theory (DFT) calculations of coupling constants with Perdew and Wang (PWC), Perdew, Burke and Ernzenrhof (PBE) and strongly constrained and appropriately normed (SCAN) exchange-correlation function show excellent agreement for the studied compound.

cond-mat.mtrl-sci↗

arXiv2018

Dipolar spin ice states with fast monopole hopping rate in CdEr$_2$X$_4$ (X = Se, S)

Excitations in a spin ice behave as magnetic monopoles, and their population and mobility control the dynamics of a spin ice at low temperature. CdEr$_2$Se$_4$ is reported to have the Pauling entropy characteristic of a spin ice, but its dynamics are three-orders of magnitude faster than the canonical spin ice Dy$_2$Ti$_2$O$_7$. In this letter we use diffuse neutron scattering to show that both CdEr$_2$Se$_4$ and CdEr$_2$S$_4$ support a dipolar spin ice state -- the host phase for a Coulomb gas of emergent magnetic monopoles. These Coulomb gases have similar parameters to that in Dy$_2$Ti$_2$O$_7$, i.e. dilute and uncorrelated, so cannot provide three-orders faster dynamics through a larger monopole population alone. We investigate the monopole dynamics using ac susceptometry and neutron spin echo spectroscopy, and verify the crystal electric field Hamiltonian of the Er$^{3+}$ ions using inelastic neutron scattering. A quantitative calculation of the monopole hopping rate using our Coulomb gas and crystal electric field parameters shows that the fast dynamics in CdEr$_2$X$_4$ (X = Se, S) are primarily due to much faster monopole hopping. Our work suggests that CdEr$_2$X$_4$ offer the possibility to study alternative spin ice ground states and dynamics, with equilibration possible at much lower temperatures than the rare earth pyrochlore examples.

cond-mat.str-el↗

arXiv2018

Giant pressure dependence and dimensionality switching in a metal-organic quantum antiferromagnet

We report an extraordinary pressure dependence of the magnetic interactions in the metal-organic system [(CuF$_2$(H$_2$O)$_2$)$_2$pyrazine]. At zero pressure, this material realizes a quasi-two-dimensional (Q2D) spin-1/2 square-lattice Heisenberg antiferromagnet. By high-pressure, high-field susceptibility measurements we show that the dominant exchange parameter is reduced continuously by a factor of 2 upon compression. Above 18 kbar, a phase transition occurs, inducing an orbital re-ordering that switches the dimensionality, transforming the Q2D lattice into weakly coupled chains (Q1D). We explain the microscopic mechanisms for both phenomena by combining detailed x-ray and neutron diffraction results with quantitative modeling using spin-polarized density functional theory.

cond-mat.mtrl-sci↗

arXiv2017

Crystal growth with oxygen partial pressure of the $BaCuSi_2O_6$ and $Ba_{1-x}Sr_xCuSi_2O_6$ spin dimer compounds

$BaCuSi_2O_6$ is a quasi-two dimensional spin dimer system and a model material for studying Bose-Einstein condensation (BEC) of magnons in high magnetic fields. The new $Ba_{1-x}Sr_xCuSi_2O_6$ mixed system, which can be grown with x < 0.3, and $BaCuSi_2O_6$, both grown by using a crystal growth method with enhanced oxygen partial pressure, have the same tetragonal structure ($I4_1/acd$) at room temperature. The mixed system shows no structural phase transition, so that the tetragonal structure is stable down to low temperatures. The oxygen partial pressure acts as control parameter for the growth process. A detailed understanding of the crystal structure depending on the oxygen content will enable the study of the spin dynamics of field-induced order states in this model magnetic compound of high current interest with only one type of dimer layers, which shows the same distance between the Cu atoms, in the structure.

cond-mat.mtrl-sci↗

arXiv2017

Impurities in weakly coupled quantum spin chains Sr$_{2}$CuO$_{3}$ and SrCuO$_{2}$

We study the effect of non-magnetic Zn$^{2+}$ (spin-$0$) and magnetic Ni$^{2+}$ (spin-$1$) impurities on the ground state and low-lying excitations of the quasi-one-dimensional spin-$1/2$ Heisenberg antiferromagnet Sr$_{2}$CuO$_{3}$ using inelastic neutron scattering, specific heat and bulk magnetization measurements. We show that 1 \% Ni$^{2+}$ doping in Sr$_2$CuO$_3$ results in a sizable spin gap in the spinon excitations, analogous to the case of Ni-doped SrCuO$_2$ previously reported [ref. 1]. However, a similar level of Zn$^{2+}$ doping in SrCuO$_2$, investigated here for comparison, did not reveal any signs of a spin gap. Magnetic ordering temperature was found to be suppressed in the presence of both Zn$^{2+}$ and Ni$^{2+}$ impurities, however, the rate of suppression due to Ni$^{2+}$ was found to be much more pronounced than for Zn$^{2+}$. Effect of magnetic field on the ordering temperature is investigated. We found that with increasing magnetic field, not only the magnetic ordering temperature gradually increases but the size of specific heat anomaly associated with the magnetic ordering also progressively enhances, which can be qualitatively understood as due to the field induced suppression of quantum fluctuations.

cond-mat.str-el↗

arXiv2017

Strong exchange anisotropy in YbMgGaO$_4$ from polarized neutron diffraction

We measured the magnetic correlations in the triangular lattice spin-liquid candidate material YbMgGaO$_4$ via polarized neutron diffraction. The extracted in-plane and out-of-plane components of the magnetic structure factor show clear anisotropy. We found that short-range correlations persist at the lowest measured temperature of 52 mK and neutron scattering intensity is centered at the $M$ middle-point of the hexagonal Brillouin-zone edge. Moreover, we found pronounced spin anisotropy, with different correlation lengths for the in-plane and out-of-plane spin components. When comparing to a self-consistent Gaussian appoximation, our data clearly support a model with only first-neighbor coupling and strongly anisotropic exchanges.

cond-mat.str-el↗

arXiv2016

Electromagnon dispersion probed by inelastic x-ray scattering

Inelastic x-ray scattering with meV energy resolution (IXS) is an ideal tool to measure collective excitations in solids and liquids. In non-resonant scattering condition, the cross section is strongly dominated by lattice vibrations (phonons). However, it is possible to probe additional degrees of freedom such as magnetic fluctuations that are strongly coupled to the phonons. The IXS spectrum of the coupled system will contain not only the phonon dispersion (majority component) but also the so far undetected magnetic correlation function (minority component). Here we report the discovery of strong magnon-phonon coupling in LiCrO$_2$ that enables the measurement of magnetic correlations throughout the Brillouin-zone via IXS. We found electromagnon excitations and electric dipole active two-magnon excitations in the magnetically ordered phase and paraelectromagnons in the paramagnetic phase of LiCrO$_2$. We predict that the numerous group of (frustrated) magnets with dominant direct exchange and non-collinear magnetism shows similarly strong coupling and surprisingly large and measurable IXS cross section for magnons and multi-magnon processes.

cond-mat.str-el↗

arXiv2016

Spiral spin-liquid and the emergence of a vortex-like state in MnSc$_2$S$_4$

Spirals and helices are common motifs of long-range order in magnetic solids, and they may also be organized into more complex emergent structures such as magnetic skyrmions and vortices. A new type of spiral state, the spiral spin-liquid, in which spins fluctuate collectively as spirals, has recently been predicted to exist. Here, using neutron scattering techniques, we experimentally prove the existence of a spiral spin-liquid in MnSc$_2$S$_4$ by directly observing the 'spiral surface' - a continuous surface of spiral propagation vectors in reciprocal space. We elucidate the multi-step ordering behavior of the spiral spin-liquid, and discover a vortex-like triple-q phase on application of a magnetic field. Our results prove the effectiveness of the $J_1$-$J_2$ Hamiltonian on the diamond lattice as a model for the spiral spin-liquid state in MnSc$_2$S$_4$, and also demonstrate a new way to realize a magnetic vortex lattice.

cond-mat.str-el↗

arXiv2016

Observation of a 4-spin Plaquette Singlet State in the Shastry-Sutherland compound SrCu2(BO3)2

The study of interacting spin systems is of fundamental importance for modern condensed matter physics. On frustrated lattices, magnetic exchange interactions cannot be simultaneously satisfied, and often give rise to competing exotic ground states. The frustrated 2D Shastry-Sutherland lattice realized by SrCu2(BO3)2 is an important test to our understanding of quantum magnetism. It was constructed to have an exactly solvable 2-spin dimer singlet ground state within a certain range of exchange parameters and frustration. While the exact dimer state and the antiferromagnetic order at both ends of the phase diagram are well known, the ground state and spin correlations in the intermediate frustration range have been widely debated. We report here the first experimental identification of the conjectured plaquette singlet intermediate phase in SrCu2(BO3)2. It is observed by inelastic neutron scattering after pressure tuning at 21.5 kbar. This gapped plaquette singlet state with strong 4-spin correlations leads to a transition to an ordered Neel state above 40 kbar, which can realize a deconfined quantum critical point.

cond-mat.str-el↗