Quantum magnetism and frustrated magnets
In the following we present selected examples of recent research in the fields of quantum magnetism and frustrated magnets
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Physical realization of a quantum spin liquid based on a complex frustration mechanism
Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. In the case of magnets with isotropic interactions, spin-liquid behaviour is sought in simple lattices with antiferromagnetic interactions that favour antiparallel alignments of the magnetic moments and are incompatible with the lattice geometries. Despite an extensive search, experimental realizations remain very few. Here we investigate the novel, unexplored magnet Ca10Cr7O28, which has a complex Hamiltonian consisting of several different isotropic interactions and where the ferromagnetic couplings are stronger than the antiferromagnetic ones. We show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid. Thus spin-liquid behaviour in isotropic magnets is not restricted to the simple idealized models currently investigated, but can be compatible with complex structures and ferromagnetic interactions.
C Balz, B Lake, J Reuther, H Luetkens, R Schönemann, T Herrmannsdörfer, Y Singh, ATMN Islam, EM Wheeler, JA Rodriguez-Rivera, T Guidi, GG Simeoni, C Baines, H Ryll, Nature Physics 12 (2016) 942
See also the associated highlight report
C Balz, B Lake, ATMN Islam, Y Singh, JA Rodriguez-Rivera, T. Guidi, EM Wheeler, GG Simeoni, H Ryll, Phys. Rev. B 95 (2017) 174414
C Balz, B Lake, M Reehuis, ATMN Islam, O Prokhnenko, Y Singh, P Pattison, S Tóth, J. Phys.: Condens. Matter 29 (2017)
Spinon confinement in the one-dimensional Ising-like antiferromagnet SrCo2V2O8
For quasi-one-dimensional quantum spin systems theory predicts the occurrence of a confinement of spinon excitation due to interchain couplings. Here we investigate the system SrCo2V2O8, a realization of the weakly coupled Ising-like XXZ antiferromagnetic chains, by terahertz spectroscopy with and without applied magnetic field. At low temperatures a series of excitations is observed, which split in a Zeeman-like fashion in an applied magnetic field. These magnetic excitations are identified as the theoretically predicted spinon-pair excitations. Using a one-dimensional Schrödinger equation with a linear confinement potential imposed by weak interchain couplings, the hierarchy of the confined spinons can be fully described.
A.K. Bera, B. Lake, F.H.L. Essler, L. Vanderstraeten, C. Hubig, U. Schollwöck, A.T.M.N. Islam, A. Schneidewind, and D.L. Quintero-Castro, Phys. Rev. B 96, (2017) 054423, Editor’s choice
Z Wang, M Schmidt, AK Bera, ATMN Islam, B Lake, A Loidl, J Deisenhofer, Phys. Rev. B 91 (2015) 140404(R)
Magnetic excitations in the S=1/2 antiferromagnetic-ferromagnetic chain compound BaCu2V2O8 at zero and finite temperature
Unlike most quantum systems which rapidly become incoherent as temperature is raised, strong correlations persist at elevated temperatures in S=1/2 dimer magnets, as revealed by the unusual asymmetric line shape of their excitations at finite temperatures. Here, we quantitatively explore and parametrize the strongly correlated magnetic excitations at finite temperatures using high-resolution inelastic neutron scattering of the model compound BaCu2V2O8 which we show to be an alternating antiferromagnetic-ferromagnetic spin−1/2 chain. Comparison to state of the art computational techniques shows excellent agreement over a wide temperature range. Our findings hence demonstrate the possibility to quantitatively predict coherent behavior at elevated temperatures in quantum magnets.
ES Klyushina, AC Tiegel, B Fauseweh, ATMN Islam, JT Park, B Klemke, A Honecker, GS Uhrig, SR Manmana, B Lake, Phys. Rev. B 93 (2016) 241109(R)
Field-Induced Magnonic Liquid in the 3D Spin-Dimerized Antiferromagnet Sr3Cr2O8
We have performed ultrasound and magnetization studies in three-dimensional, spin-dimerized Sr3Cr2O8 as a function of temperature and external magnetic field up to 61 T. It is well established [Phys. Rev. Lett. 103, 207203 (2009)] that this system exhibits a magnonic-superfluid phase between 30 and 60 T and below 8 K. By mapping ultrasound and magnetization anomalies as a function of magnetic field and temperature we establish that this superfluid phase is embedded in a domelike phase regime of a hightemperature magnonic liquid extending up to 18 K. Compared to thermodynamic results, our study indicates that the magnonic liquid could be characterized by an Ising-like order but has lost the coherence of the transverse components.
Physical properties of the candidate quantum spin-ice system Pr2Hf2O7
Physical properties of a pyrohafnate compound Pr2Hf2O7 have been investigated by various techniques on polycrystalline as well as single-crystal samples. Inelastic neutron scattering (INS) was used to determine the crystal-field energy-level scheme and wave functions. Synchrotron XRD data confirm the ordered cubic pyrochlore structure without any noticeable site mixing or oxygen deficiency. No clear evidence of long-range magnetic ordering is observed down to 90 mK, however the ac susceptibility evidences slow spin dynamics revealed by a frequency dependent broad peak associated with spin freezing. The INS data reveal the expected five well-defined magnetic excitations due to crystal-field splitting of the J=4 ground-state multiplet of the Pr3+. The crystal-field parameters and ground-state wave function of Pr3+ have been determined. The Ising anisotropic nature of the magnetic ground state is inferred from the INS as well as χ(T) and M(H) data. Together these properties make Pr2Hf2O7 a candidate compound for quantum spin-ice behavior.
VK Anand, L Opherden, J Xu, DT Adroja, ATMN Islam, T Herrmannsdörfer, J Hornung, R Schönemann, M Uhlarz, HC Walker, N Casati, B Lake, Phys. Rev B 94 (2016) 144415
Consequences of critical interchain couplings and anisotropy on a Haldane chain
Effects of interchain couplings and anisotropy on a Haldane chain have been investigated by single-crystal inelastic neutron scattering and density functional theory (DFT) calculations on the model compound SrNi2V2O8. Significant effects on low-energy excitation spectra are found where the Haldane gap (0 ≈ 0.41J, where J is the intrachain exchange interaction) is replaced by three energy minima at different antiferromagnetic zone centers due to the complex interchain couplings. Further, the triplet states are split into two branches by single-ion anisotropy. Quantitative information on the intrachain and interchain interactions as well as on the single-ion anisotropy is obtained from the analyses of the neutron scattering spectra by the random-phase approximation method. The presence of multiple competing interchain interactions is found from the analysis of the experimental spectra and is also confirmed by the DFT calculations. The interchain interactions are two orders of magnitude weaker than the nearest-neighbor intrachain interaction J = 8.7 meV.
AK Bera, B Lake, ATMN Islam, O Janson, H Rosner, A Schneidewind, JT Park, E Wheeler, S Zander, Phys. Rev. B 91, 144414 (2015)
Quantum spin chain as a potential realization of the Nersesyan-Tsvelik model
It is well established that long-range magnetic order is suppressed in magnetic systems whose interactions are low-dimensional. The prototypical example is the S-1/2 Heisenberg antiferromagnetic chain (S-1/2 HAFC) whose ground state is quantum critical. In real S-1/2 HAFC compounds interchain coupling induces long-range magnetic order although with a suppressed ordered moment and reduced Neel temperature compared to the Curie-Weiss temperature. Recently, it was suggested that order can also be suppressed if the interchain interactions are frustrated, as for the Nersesyan-Tsvelik model. We have studied the new S-1/2 HAFC, (NO)[Cu(NO3)3]. This material shows extreme suppression of order which furthermore is incommensurate revealing the presence of frustration consistent with the Nersesyan-Tsvelik model
C Balz, B Lake, H Luetkens, C Baines, T Guidi, M Abdel-Hafiez, AUB Wolter, B Büchner, IV Morozov, EB Deeva, OS Volkova, AN Vasiliev, Phys. Rev. 90, 060409(R) (2014)
Magnetic correlations of the quasi-one-dimensional half-integer spin-chain antiferromagnets SrM2V2O8 (M =Co, Mn)
Magnetic correlations of two isostructural quasi-one-dimensional (1D) antiferromagnetic spin-chain compounds SrM2V2O8 (M = Co, Mn) have been investigated by magnetization and powder neutron diffraction. Two different collinear antiferromagnetic (AFM) structures, characterized by the propagation vectors, k = (0 0 1) and k = (0 0 0), have been found below ∼5.2 and ∼42.2 K for the Co and Mn compounds, respectively. For the Mn compound, AFM chains (along the c axis) order ferromagnetically within the ab plane, whereas, for the Co compound, AFM chains order ferromagnetically/antiferromagnetically along the a/b direction. The critical exponent study confirms that the Co andMn compounds belong to the Ising and Heisenberg universality classes, respectively. For both compounds, short-range spin-spin correlations are present over a wide temperature range above TN. The reduced ordered moments at base temperature (1.5 K) indicate the presence of quantum fluctuations in both compounds due to the quasi-1D magnetic interactions.
AK Bera, B Lake, WD Stein, S Zander, Phys. Rev. B 89, 094402 (2014)
Multispinon Continua at Zero and Finite Temperature in a Near-Ideal Heisenberg Chain
The space-and time-dependent response of many-body quantum systems is the most informative aspect of their emergent behavior. The dynamical structure factor, experimentally measurable using neutron scattering, can map this response in wave vector and energy with great detail, allowing theories to be quantitatively tested to high accuracy. Here, we present a comparison between neutron scattering measurements on the one-dimensional spin-1/2 Heisenberg antiferromagnet KCuF3, and recent state-of-the-art theoretical methods based on integrability and density matrix renormalization group simulations. The unprecedented quantitative agreement shows that precise descriptions of strongly correlated states at all distance, time, and temperature scales are now possible, and highlights the need to apply these novel techniques to other problems in low-dimensional magnetism.
B Lake, DA Tennant, J-S Caux, T Barthel, U Schollwöck, SE Nagler, CD Frost, Phys. Rev. Lett. 111, 137205 (2013)
Asymmetric thermal line shape broadening in a gapped 3D antiferromagnet
It is widely believed that magnetic excitations become increasingly incoherent as the temperature is raised due to random collisions which limit their lifetime. This picture is based on spin-wave calculations for gapless magnets in 2 and 3 dimensions and is observed experimentally as a symmetric Lorentzian broadening in energy. We have investigated a three-dimensional dimer antiferromagnet and have found unexpectedly that the broadening is asymmetric - indicating that far from thermal decoherence, the excitations behave collectively like a strongly correlated gas. This result suggests that a temperature activated coherent state of quasiparticles is not confined to special cases like the highly dimerized spin-1/2 chain but is found generally in dimerized antiferromagnets of all dimensionalities and perhaps gapped magnets in general.
DL Quintero-Castro, B Lake, ATMN Islam, EM Wheeler, C Balz, M Månsson, KC Rule, S Gvasaliya, and A Zheludev, Phys. Rev. Lett. 109, 127206 (2012)