Magnetic excitations in theS=12antiferromagnetic-ferromagnetic chain compoundBaCu2V2O

Abstract

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 lineshape of their excitations at finite temperatures. Here we quantitatively explore and parameterize the strongly correlated magnetic excitations at finite temperatures using the high resolution inelastic neutron scattering on 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.In the study of unconventional states of matter, quantum magnetic materials with their strong correlations play a crucial role [1][2][3][4][5]. Quantum mechanical coherence and entanglement are intrinsic to these systems, both being relevant for potential applications in quantum devices [6,7]. However, the question arises for their persistence when increasing temperature. Intuitively, one expects temperature to suppress quantum behavior, as typically encountered in the study of quantum criticality [8]. Interestingly, this is not always the case, and in certain systems, e.g. in the presence of disorder, coherent behavior is not simply suppressed by temperature, but rather an interesting interplay develops [9,10], which can lead to counterintuitive behavior such as the increase of conductance through molecules with temperature [11].Another example is the extraordinary coherence of the magnetic excitations at elevated temperatures. This was theoretically predicted for 1-dimensional (1D) gapped quantum dimer antiferromagnets (AFM) by using integrable quantum field theory [12] and was experimentally confirmed on the strongly dimerized spin−1/2 AFM alternating chain compound copper nitrate, which has a spin-singlet ground state and gapped triplet excitations (henceforth referred to as triplons [13]) confined within a narrow band [14]. Here, the triplons interact strongly via the AFM interdimer coupling and also via an effective repulsive interaction due to the hard-core constraint. The resulting strong correlations lead to the experimentally observed asymmetric broadening of the lineshape with temperature [14,15]. So far, such experimental data was compared to exact diagonalization data from small systems and to results from low-temperature expansion around the strongly dimerized limit of Heisenberg spin-1/2 chains [16,17]. Further experimental studies revealed that the strongly correlated behavior at elevated temperatures is not restricted to 1D systems. It was recently observed that the lineshape in the 3-dimensional (3D) coupled-dimer antiferromagnet Sr 3 Cr 2 O 8 also becomes asymmetric and increasingly weighted towards the center of the band as temperature increases [18,19]. So far, no reliable theoretical approaches on the microscopic ...

Effects of interactions on dynamic correlations of hard-core bosons at finite temperatures

“…Interestingly, the shifts stemming from the additional interactions will improve the agreement between the diagrammatic approach and the peak positions computed numerically by density-matrix renormalization, see Supplemental Material of Ref. 19. We note that the band narrowing [5-9, 18, 49-51] is reduced at high temperatures and even inverted at lower temperatures.…”

“…In order to be able to apply bosonic perturbation theory this hard-core constraint is incorporated as on-site infinite repulsion U → ∞. The Brückner approach was applied to quantitatively explain the experimental data for two one-dimensional (1D) materials [19,20]. It was also applied to predict such data in a two-dimensional (2D) material [21].…”

Time-dependent correlations in quantum magnets at finite temperature

“…However, recent investigations of one-dimensional [9,10] and three-dimensional [11] materials of coupled spin dimers * benedikt.fauseweh@tu-dortmund.de † felix.groitl@psi.ch ‡ goetz.uhrig@tu-dortmund.de using inelastic neutron scattering (INS) show that the lineshape in the frequency domain develops an asymmetric tail. It has been argued that such tails may be found in a broad range of quantum systems.…”

“…In this Rapid Communication we use the diagrammatic Brückner approach [10,22,28] to capture thermal fluctuations in the system. This approach has been explicitly developed to calculate finite temperature effects on the single particle spectrum in gapped systems.…”

“…The concomitant asymmetry cannot be described by a pure Lorentzian at finite temperatures. Hence, the details of the scattering processes have a considerable impact on the correlations, and more sophisticated theories have been employed to explain this phenomenon [10,[12][13][14][15][16][17][18][19][20][21][22].…”