Magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs2CuBr4

We report on high-field electron spin resonance (ESR) studies of magnetic excitations in the spin-1/2 triangular-lattice antiferromagnet Cs2CuBr4. Frequency-field diagrams of ESR excitations are measured for different orientations of magnetic fields up to 25 T. We show that the substantial zero-field energy gap, Δ ≈ 9.5 ?> K, observed in the low-temperature excitation spectrum of Cs2CuBr4, (Zvyagin et al 2014 Phys. Rev. Lett.112 077206) is present well above TN. Noticeably, the transition into the long-range magnetically ordered phase does not significantly affect the size of the gap, suggesting that even below TN the high-energy spin dynamics in Cs2CuBr4 is determined by short-range-order spin correlations. The experimental data are compared with results of model spin-wave-theory calculations for spin-1/2 triangular-lattice antiferromagnet.

Spin-1/2 Heisenberg antiferromagnets (AFs) on triangular lattices form an important class of low-dimensional (low-D) spin systems to probe effects of quantum fluctuations, magnetic order, and frustrations.In 1973, developing the idea of the "resonating valence bond" ground state, P. W. Anderson proposed that quantum fluctuations can be sufficiently strong to destroy the classical 120 • order in such a system [1].As consequence, a two-dimensional spin-liquid phase may be realized.This phase can be regarded as a 2D fluid of resonating spinsinglet pairs, with the excitation spectrum formed by fractionalized mobile quasiparticles.Subsequent numerical studies have, however, confirmed the presence of the semiclassical 120 • magnetic ordering albeit with strongly reduced ordered moments [2][3][4][5].In recent years, the topic of spin-1/2 Heisenberg AFs on a triangular lattice has received particular attention due to the rich phase diagram, whose details are still actively debated [6][7][8][9][10][11].
Cs 2 CuCl 4 and Cs 2 CuBr 4 are two prominent members of this family of frustrated spin systems.Inelastic neutron-scattering experiments in Cs 2 CuCl 4 revealed the presence of a highly dispersive continuum of excited states [12].These excitations were initially identified as spinons in the 2D frustrated spin liquid.Later on, the data have been re-interpreted in the frame of the 1D spinliquid scenario [13,14] with interchain bound spinon excitations (triplons) as a signature of the quasi-1D nature of magnetic correlations in this material [15,16].Electron spin resonance (ESR) studies provided additional support for the proposed quasi-1D Heisenberg AF chain model with the uniform Dzyaloshinskii-Moriya (DM) interaction, opening an energy gap, ∆ = 14 GHz, at the Γ point [17].
Cs 2 CuBr 4 realizes a distorted triangular lattice with orthorhombic crystal structure, space group P nma, and the room-temperature lattice parameters a = 10.195Å, b = 7.965 Å, and c = 12.936 Å (Z = 4) [18].Compared to J ′ /J ≃ 0.3 [19,20] for Cs 2 CuCl 4 , the J ′ /J ratio for Cs 2 CuBr 4 is somewhat larger, J ′ /J ≃ 0.41 [19], that places this compound further away from the decoupled AF chain limit and makes it more frustrated.This difference is thought to be related to the 1/3 magnetization plateau and the cascade of field-induced phase transitions, observed in Cs 2 CuBr 4 [21,22].In spite of intensive theoretical and experimental efforts, very little is known about the spin dynamics in Cs 2 CuBr 4 .Inelastic neutron-scattering studies have been reported in Ref. [23]; unfortunately, due to the limited spectral resolution many important details of the magnetic excitation spectrum appear missing.Nevertheless, ESR experiments on Cs 2 CuBr 4 in the fully spin-polarized phase above H sat ∼ 30 T [19] detected the exchange mode, allowing to estimate parameters of the effective spin Hamiltonian, J/k B = 14.9 K, J ′ /k B = 6.1 K (where J and J ′ are the exchange coupling parameters along the horizontal and zigzag bonds, respectively, see Fig. 1, inset).Besides, the ESR experiments revealed a substantial zero-field gap, ∆ ≈ 200 GHz, whose nature has so far remained unclear.In this work, we continue high-field ESR studies of Cs 2 CuBr 4 , evidenced by its remarkable spin dynamics.
Cs 2 CuBr 4 single crystals were synthesized by slow evaporation of aqueous solutions of CsBr and CuBr 2 similar to the procedure described in Ref. [18].Experiments were done using high-field ESR installations at the Dresden High Magnetic Field Laboratory (HLD, Germany), the Center for Advanced High Magnetic Field Science (Osaka University, Japan), and the National High Magnetic Field Laboratory (Florida State University, Tallahassee, USA) [24].The experiments were done in the frequency range of 100 -800 GHz, using tunable sources of millimeter-wave radiation sources backward wave oscillators (product of ISTOK, Russia), VDI transmit systems (product of Virginia Diodes Inc., USA), and MVNA vector network analyzer (product of AB Millimetre, France).
A single-line ESR absorption was detected in the paramagnetic phase (T Pronounced evolution of the ESR spectrum was observed upon cooling. The frequency-field diagram of magnetic excitations in Cs 2 CuBr 4 measured between 0.4 and 4 K for magnetic fields, H, applied along the b axis is shown in Fig. 1.Some examples of ESR spectra of magnetic excitations at different frequencies are presented in Fig. 2. For this orientation, a single gapped mode (labeled B) was observed, whose frequency-field dependence can be described using the equation: where ν is the resonance frequency, h is the Planck constant, µ B is the Bohr magneton, g * is the paramagnetic g factor (the g factor measured at T = 77 K, g * = 2.11, was used for the fit and calculations), and ∆ is the zero-field excitation gap.The best fit was obtained using ∆ = 199(4) GHz (which corresponds to 9.5 K) for T = 1.5 K.
The temperature dependence of the ESR field position at a frequency of 295.2 GHz with magnetic field applied along the b axis is shown in Fig. 3 by squares.Using these data and Eq. ( 1), the gap size ∆ can be calculated for different temperatures (circles in Fig. 3).We found that the gap in Cs 2 CuBr 4 persists up to relatively high temperatures (T ∼ J/k B and above), suggesting the presence of short-range-order spin correlations, responsible for the gap opening, well above T N .
At T N = 1.4 K Cs 2 CuBr 4 undergoes a transition into a 3D long-range ordered phase.Noticeably, the transition leaves the gap size almost unchanged (Fig. 3).Similar effects were observed in a number of quantum AFs [25][26][27][28], whose ground states below T N are 3D magnetically ordered and the low-energy excitation spectra are determined by 3D long-range-order correlations, while the high-energy (∼ J) spin dynamics is still determined by low-D effects.Remarkably, no ESR line-width anomaly was observed in Cs 2 CuBr 4 at the transition into the 3D magnetically ordered state (Fig. 4).The temperature evolution of ESR spectra taken at 295.2 GHz with magnetic field applied along the b axis is shown in Fig. 5.
Overall, the ESR properties of Cs 2 CuBr 4 in many respects appear very similar to those obtained for Cs 2 CuCl 4 [17].On the other hand, contrary to Cs 2 CuCl 4 with the zero-field energy gap ∆ ∼ D ≈ 0.15J, the size of the gap in Cs 2 CuBr 4 (∆ ≈ 0.6J) is too large to be explained in terms of the quasi-1D Heisenberg AF chain model with the uniform DM interaction D. This strongly suggests that, compared to Cs 2 CuCl 4 , the effect of frustrated interactions on the spin dynamics (and more generally, on the magnetic properties) in Cs 2 CuBr 4 can be different.
To get an insight into the spin dynamics of Cs 2 CuBr 4 ,  we did simple spin-wave calculations for the ordered state of an orthorhombically-distorted triangular-lattice antiferromagnet.The minimal spin Hamiltonian includes only exchange interactions where the nearest-neighbor exchange constant is J ij = J or J ′ for horizontal or zigzag bonds, respectively (see the inset in Fig. 1) [29,30].The classical ground state of the spin model ( 2) is a planar spiral magnetic structure with the ordering wave-vector Q = (0, Q, 0) and cos(Qb/2) = −J ′ /2J.At zero temperature, the energy of magnetic excitations in a spiral AF is given by the standard expression [31,32]: where J k = j J ij e ik(ri−rj ) is the Fourier transform of the exchange interactions.
In the framework of the formulated toy model, the origin of the gap may be understood if real geometry of DM interactions in Cs 2 CuBr 4 is taken into account [6,19].
The staggered component of the DM interaction reduces the translational symmetry of the spin Hamiltonian (2) producing a doubling of the unit cell in the bc plane and, consequently, folding of the Brillouin zone.As a consequence, an excitation with a momentum k mixes with another magnon at k + q * c , where q c * = (0, 0, 2π/c) is a new reciprocal lattice vector.The actual shift of the magnon energy (3) produced by the DM interactions may be quite small because of smalness of D/J.However, the presence of a nonzero mixing matrix element leads to a finite ESR response for the high-energy magnon at the M point (q c * ) in the unfolded Brillouin zone, see Fig. 6.It is worthwhile to mention that ESR transitions at the Brillouin zone boundary were observed in a number of low-D quantum magnets (see, e.g., Ref. [33][34][35][36]), while the corresponding selection rules were theoretically studied in Ref. [37].Remarkably, as revealed experimentally [35,36], such nominally forbidden ESR transitions (which become allowed in the presence of the DM interaction) can be very intensive, exhibiting, on the other hand, a very pronounced polarization dependence.
The corresponding ESR gap is calculated using Eq. ( 3) as With J/k B = 14.9K and J ′ /k B = 6.1 K determined in the high-field ESR experiments [19], the gap for Cs 2 CuBr 4 is estimated to be ∆ = 0.84 meV, which corresponds to 9.7 K or 203 GHz.This value perfectly agrees with our experimental observations.This agreement between the harmonic spin-wave theory for the exchange model and the experimental results might be somewhat fortuitous, as we completely neglected quantum fluctuations and the DM interactions.The observed agreement may indicate that these additional effects partially compensate each other [38].The above consideration also predicts one more ESR mode arising from excitation of magnons with momenta k = ±Q + q * b = (0, 2π/b ± Q, 0) (M ′ point in Fig. 6).The corresponding energy is Using the above J and J ′ [19] we find ∆ ′ ≈ 1 meV.As further calculations show, these excitations appear well inside the two-magnon continuum (Fig. 6), thus having a finite lifetime even at zero temperature.Therefore, they cannot be observed in ESR experiments.This conclusion is in agreement with the existing theoretical results for the damping of high-energy magnons in the spin-1/2 Heisenberg AF on a perfect triangular lattice [39][40][41].
For H a and H c, two pairs of ESR modes were observed (A, A ′ , and C, C ′ , respectively); the corresponding frequency-field ESR diagrams and ESR spectra are proaching the frequency 200 GHz, modes A ′ and C became broader and weaker, so that no ESR was observed below 200 GHz.
It is important to mention, that due to the development of long-range 3D correlations below T N one would expect the emergence of lower-frequency antiferromagnetic resonance (AFMR) magnon modes.Theory predicts the presence of at most three relativistic Goldstone AFMR modes as a consequence of the complete breaking of the rotational SO(3) symmetry in anisotropic spin systems with a noncollinear ground state [42].Such AFMR modes have been recently observed in the isostructural compound Cs 2 CuCl 4 below T N = 0.62 K [43].
In conclusion, our work provides new insights into the unconventional spin dynamics in the spin-1/2 triangularlattice antiferromagnet Cs 2 CuBr 4 , studied by means of high-field ESR spectroscopy.In particular, the temperature behavior of the observed zero-field energy gap is investigated in a broad temperature range, indicating that the gap is a characteristics of short-range-order spin correlations, which appear to persist in Cs 2 CuBr 4 down to well below T N .Based on the proposed spin-1/2 distorted triangle-lattice antiferromagnet model, our findings might suggest, that Cs 2 CuBr 4 exhibits a peculiar combination of nearly classical spin dynamics and static magnetic ordering [38] with pronounced quantum effects as revealed, e.g., by the 1/3 magnetization plateau.This calls for further experimental quantification of the role of quantum fluctuations in Cs 2 CuBr 4 , in particular, for neutron measurements of the ordered magnetic moments and the magnon modes in zero magnetic field.

FIG. 1 :FIG. 2 :
FIG. 1: (color online) Frequency-field diagram of ESR excitations obtained for Cs2CuBr4 with magnetic field applied along the b axis at different temperatures (mode B).Fit results for 1.5 K using Eq.(1) are shown by the solid line.The frequency-field dependence of the paramagnetic resonance with g b = 2.11 is shown by the dashed line.The inset shows a schematic picture of exchange paths in the bc plane of Cs2CuBr4.

FIG. 3 :
FIG.3:(color online) Temperature dependence of the resonance field (squares; the data are taken at 295.2 GHz with magnetic field applied along the b axis) and the zero-field gap size (circles) calculated using Eq.(1) (see the text for details).Lines are guides for the eye.

FIG. 4 :
FIG. 4: (color online) Temperature dependence of the ESR linewidth measured at a frequency of 295.2 GHz with magnetic field applied along the b axis.Line is guide for the eye.

FIG. 5 :
FIG. 5: (color online) Temperature evolution of ESR spectra taken at 295.2 GHz with magnetic field applied along the b axis (mode B).

FIG. 7 :FIG. 8 :FIG. 9 :
FIG. 7: (color online) Frequency-field diagrams of ESR excitations with magnetic field applied along a (a) and c (b) axes.The dashed lines correspond to paramagnetic resonances with ga = 2.15 (H a, T = 77 K) and gc = 2.26 (H c, T = 77 K).Solid lines are guides for the eye.Insets show examples of the corresponding ESR spectra.The data are obtained at 1.5 K.