Field-Induced Magnon Decay, Magnon Shadows, and Roton-like Excitations in the Honeycomb Antiferromagnet YbBr$_3$

Abstract

Although the search for quantum many-body phenomena in magnetic materials has a strong focus on highly frustrated systems, even unfrustrated quantum magnets show a multitude of unconventional phenomena in their spin excitation spectra. YbBr$_3$ is an excellent realization of the $S = 1/2$ antiferromagnetic Heisenberg model on the honeycomb lattice, and we have performed detailed spectroscopic experiments with both unpolarized and polarized neutrons at all applied magnetic fields up to saturation. We observe extensive excitation continua, which cause strong renormalization and the decay of single magnons at higher fields, while coherent features include field-induced ``shadows'' of the single magnons and the spectacular emergence of a roton-like excitation. To guide and interpret our experiments, we performed systematic calculations by the method of cylinder matrix-product states that provide quantitative agreement with the neutron scattering data and a qualitative benchmark for the spectral signatures of strong quantum fluctuations even in the absence of magnetic frustration.

Figures (12)

• Figure 1: Crystal structure and interaction parameters. (a) Crystal structure of a single YbBr$_3$ layer. Edge-sharing Br$^{-}$ octahedra surround the magnetic Yb$^{3+}$ ions. (b) Reciprocal space for the honeycomb lattice, showing the Brillouin zone (BZ, black hexagon) and irreducible BZ (orange hexagon). (c) Complete magnetization response deduced from neutron diffraction measurements of the ordered moment at $T = 55$ mK. The orange line is the result calculated by MPS. (d) Measurements at the $\Gamma$ point showing the Larmor-mode frequency, $\hbar \omega_{\rm L} = g_{zz} \mu_{\rm{B}} B_z$. The red line is the result calculated by linear spin-wave theory (LSWT). The vertical dashed line marks the deduced saturation field, $B_{\rm sat} = 8.78(2)$ T. (e-f) INS spectra measured in the fully polarized phase, at $B_z = 9.73$ T, along the two high-symmetry directions in the BZ. Red lines indicate the optimal fit by LSWT used to extract the interaction parameters. The widths of the LSWT lines are proportional to the calculated intensities.
• Figure 2: Field-dependence of magnetic excitations. (a) INS measurements and (b) MPS calculations of the field-induced spectral function at the M$^*$ point. (c) Quantitative comparison of INS and MPS at M$^*$ for all fields applied in experiment. (d) INS measurements and (e) MPS calculations at the K point. (f) Comparison of INS and MPS at K. Red lines in panels (b) and (e) show the magnon branches of LSWT. The labels (i), (ii), (iii), and (iv) in panels (b) and (e) refer to the specific excitation features discussed in the text.
• Figure 3: Zero-field spectrum. (a) INS intensity measured along the paths $\Gamma^*$-M and K-M$^*$-$\Gamma^*$ by INS. (b-e) Quantitative comparison of INS (symbols) and MPS results (beige shading) for constant-${\bf Q}$ spectra measured and computed at M (b), M$^*$ (c), Z$^*$ (d), and K (e).
• Figure 4: Roton-like excitation. (a-c) MPS calculations of the longitudinal (non-spin-flip) spectral response, $I^{\rm NSF} (\mathbf{Q},E)$, along the $\Gamma^*$-M direction for fields of 3, 4, and 5 T, showing the field-induced evolution of a roton-like feature. Red lines show the magnon branches within LSWT. (d) Constant-$|{\bf Q}|$ cuts through the INS and MPS spectral functions at 4 T for eight wave vectors. Error bars on the INS data are smaller than the symbol sizes. (e-f) Intensity in the NSF channel measured by polarized INS at $T = 66$ mK, showing the roton-like mode at 4 and 5 T.
• Figure S1: Nuclear scattering and stacking faults in YbBr3. (a) Unpolarized neutron diffraction data measured in the paramagnetic phase and shown for the $(h0l)$ scattering plane. The modulated rods of nuclear intensity reveal the lack of structural correlations along the stacking direction. (b) Representation of the faulted stacking sequence AaB-Ca$^\prime$A. (c) Projection of the two layers around this fault on the $(\boldsymbol{ab})$ plane. The Yb$^{3+}$ ions in the honeycomb a (a$^\prime$) layer are shown in orange (red) and the A(B)-layer Br$^-$ ions in dark (light) blue. The stacking-fault translation vector taking AaB to Ca$^\prime$A is $\tfrac{2}{3}\boldsymbol{a}+\tfrac{2}{3}\boldsymbol{b}+\tfrac{1}{3}\boldsymbol{c}$, which places all the Yb$^{3+}$ ions in layer a$^\prime$ equidistant from two nearest-neighbor Yb$^{3+}$ ions in layer a, as shown in the projection on the plane perpendicular to $(\boldsymbol{ac})$ in panel (d). The Yb-Yb distances for the $J_1$ and $J^\prime$ bonds in panel (d) are $r(J_1) = 4.028$ Å and $r(J^\prime) = 6.783$ Å. In panel (b) we show the Yb$^{3+}$ and Br$^-$ ions with the correct ratio of their ionic radii, whereas in panels (c) and (d) we have reduced the Br$^-$ radii for a clearer visualization of the honeycomb planes and interlayer geometry.