Quantum magnetism and frustrated magnets
In the following we present selected examples of recent research in the fields of quantum magnetism and frustrated magnets
For a full list of publications of our institute click here
Dispersions of many-body Bethe strings
Complex bound states of magnetic excitations, known as Bethe strings, were predicted almost a century ago to exist in one-dimensional quantum magnets. The dispersions of the string states have so far remained the subject of intensive theoretical studies. By performing neutron scattering experiments on the one-dimensional Heisenberg–Ising antiferromagnet SrCo2V2O8 in high longitudinal magnetic fields, we revealed in detail the dispersion relations of the string states over the full Brillouin zone, as well as their magnetic field dependencies. Furthermore, the characteristic energy, the scattering intensity and linewidth of the observed string states exhibit excellent agreement with our precise Bethe–ansatz calculations. Our results establish the important role of string states in the quantum spin dynamics of one-dimensional systems, and will invoke studies of their dynamical properties in more general many-body systems.
AK Bera, J Wu, W Yang, R Bewley, M Boehm, J Xu, M Bartkowiak, O Prokhnenko, B Klemke, ATMN Islam, JM Law, Z Wang, and B Lake, Nat. Phys. (2020) doi: 10.1038/s41567-020-0835-7
See also the associated highlight report
Order out of a Coulomb Phase and Higgs Transition in Nd2Zr2O7
The pyrochlore material Nd2Zr2O7 with an “all-in-all-out” (AIAO) magnetic order shows novel quantum moment fragmentation with gapped flat dynamical spin ice modes. The parametrized spin Hamiltonian with a dominant frustrated ferromagnetic transverse term reveals a proximity to a U(1) spin liquid. We studied the magnetic excitations of Nd2Zr2O7 above the ordering temperature (TN) using high-energy-resolution inelastic neutron scattering. Our findings relate the magnetic ordering of Nd2Zr2O7 with the Higgs mechanism and provide explanations for several previously reported experimental features.
J Xu, O Benton, ATMN Islam, T Guidi, G Ehlers, and B Lake, Phys. Rev. Lett. 124 (2020) 097203
Field-induced quantum spin-1/2 chains and disorder in Nd2Zr2O7
Neutron scattering study on the quantum spin ice candidate Nd2Zr2O7 in magnetic fields along the (1 1 0) direction give evidence for field-induced one-dimensional correlations and disorder. The pyrochlore lattice is completely separated into orthogonal sets of chains which is in strong contrast to classical spin ice and surprising for an “all-in-all-out” (AIAO) ordered magnet with a non-Ising Hamiltonian. Our mean-field and Monte Carlo simulations reveal that the (1 1 0) field induces a transition from the AIAO order to a “2-in-2-out” disordered state with interactions between the two sets of chains cancelled out, resulting in disorder and quantum spin-1/2 XYZ chains.
Hamiltonian of the S = 1/2 dimerized antiferromagnetic-ferromagnetic quantum spin chain BaCu2V2O8
The novel quantum magnet BaCu2V2O8 was recently discovered to be a rare physical realization of a onedimensional antiferromagnetic-ferromagnetic dimerized chain which displays strongly correlated phenomena at elevated temperatures. Here, static susceptibility and inelastic neutron scattering data are compared to several theoretical models. An analytical relation for the dynamic structure factor of the complex unit cell of BaCu2V2O8 is derived and used to identify the intrachain exchange paths.
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. 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, Open Access Version
See also the associated highlight report
C Balz, B Lake, M Reehuis, ATMN Islam, O Prokhnenko, Y Singh, P Pattison, S Tóth, J. Phys.: Condens. Matter 29 (2017) 225802
J Sonnenschein, C Balz, U Tutsch, M Lang, H Ryll, JA Rodriguez-Rivera, ATMN Islam, B Lake, J Reuther, Physical Review B 100 (2019) 174428
Investigation of the spin-1 honeycomb antiferromagnet BaNi2V2O8 with easy-plane anisotropy
Using dc susceptibility measurements and inelastic neutron scattering techniques, we found that the magnetic excitation spectrum of the 2D, S = 1 honeycomb antiferromagnet BaNi2V2O8 is found to be dispersionless within experimental resolution between the honeycomb layers, while it disperses strongly within the honeycomb plane. The magnetic excitations are compared to linear spin-wave theory allowing the Hamiltonian to be determined. The interplane coupling Jout is four orders of magnitude weaker than the intraplane interactions, confirming the highly two-dimensional magnetic behavior of this compound. The sizes of the energy gaps are used to extract the magnetic anisotropies and reveal substantial easy-plane anisotropy and a very weak in-plane easy-axis anisotropy. Together these results reveal that BaNi2V2O8 is a candidate compound for the investigation of vortex excitations and Berezinsky-Kosterliz-Thouless phenomenon.
ES Klyushina, B Lake, ATMN Islam, JT Park, A Schneidewind, T Guidi, EA Goremychkin, B Klemke, M Månsson Phys. Rev. B 96, 214428 (2017)
Spin dynamics of the ordered dipolar-octupolar pseudospin-1/2 pyrochlore Nd2Zr2O7 probed by muon spin relaxation
A muon spin relaxation study on the Ising pyrochlore Nd2Zr2O7 reveals an “all-in-all-out” magnetic order below 0.4 K. At 20 mK the zero-field muon spin relaxation spectra indicate strong fluctuations of the ordered state. The spectra of the paramagnetic state (below 4.2 K) reveal anomalously slow paramagnetic spin dynamics and show only a small difference with the spectra of the ordered state. We find that the fluctuation rate decreases with decreasing temperature and becomes nearly temperature independent below the transition temperature, indicating persistent slow spin dynamics in the ground state.
J Xu, C Balz, C Baines, H Luetkens, B Lake, Phys. Rev. B 94 (2016) 064425
J Xu, VK Anand, AK Bera, M Frontzek, DL Abernathy, N Casati, K Siemensmeyer, B Lake, Phys. Rev. B 92 (2015) 224430
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.
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)
Field-Induced Magnonic Liquid in the 3D Spin-Dimerized Antiferromagnet Sr3Cr2O8
It is well established [Phys. Rev. Lett. 103, 207203 (2009)] that Sr3Cr2O8 exhibits a magnonic-superfluid phase between 30 and 60 T and below 8 K. By mapping ultrasound and magnetization anomalies as a function of temperature and magnetic field up to 61 T we establish that this superfluid phase is embedded in a domelike phase regime of a high temperature 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
In the pyrohafnate compound Pr2Hf2O7 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. Inelastic neutron scattering data reveal the expected five well-defined magnetic excitations due to crystal-field splitting of the J=4 ground-state multiplet of Pr3+. 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 is replaced by three energy minima at different antiferromagnetic zone centers. Further, the triplet states are split into two branches by single-ion anisotropy.