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arXiv:1705.07166 · Physical review. B./Physical review. B

Competing Bose-glass physics with disorder-induced Bose-Einstein condensation in the doped S=1 antiferromagnet Ni(Cl1−xBrx)2−4SC(NH2)2 at high magnetic fields

Maxime Dupont, Sylvain Capponi, M. Horvatić, Nicolas Laflorencie

Published Jul 26, 2017• 10 citations
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Abstract

We study the interplay between disorder and interactions for emergent bosonic degrees of freedom induced by an external magnetic field in the Br-doped spin-gapped antiferromagnetic material $\mathrm{Ni}{({\mathrm{Cl}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{2}\ensuremath{-}4\mathrm{SC}{({\mathrm{NH}}_{2})}_{2}$ ($\mathrm{DTN}X$). Building on nuclear magnetic resonance experiments at high magnetic field [A. Orlova et al., Phys. Rev. Lett. 118, 067203 (2017)], we describe the localization of isolated impurity states, providing a realistic theoretical modeling for $\mathrm{DTN}X$. Going beyond single impurity localization we use quantum Monte Carlo simulations to explore many-body effects from which pairwise effective interactions lead to a (impurity-induced) Bose-Einstein condensation (BEC) revival [M. Dupont, S. Capponi, and N. Laflorencie, Phys. Rev. Lett. 118, 067204 (2017)]. We further address the question of the existence of a many-body localized Bose-glass (BG) phase in $\mathrm{DTN}X$, which is found to compete with a series of a new kind of BEC regimes made out of the multi-impurity states. The global magnetic field--temperature phase diagram of $\mathrm{DTN}X$ reveals a very rich structure for low impurity concentration, with consecutive disorder-induced BEC minidomes separated by intervening many-body localized BG regimes. Upon increasing the impurity level, multiple mini-BEC phases start to overlap, while intermediate BG regions vanish.

DOI
10.1103/physrevb.96.024442

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•Jul 5, 2018•1 citation
We study the interplay between disorder and interactions for emergent bosonic degrees of freedom induced by an external magnetic field in the Br-doped spin-gapped antiferromagnetic material Ni(Cl1\ensuremath−xBrx)2\ensuremath−4SC(NH2)2\mathrm{Ni}{({\mathrm{Cl}}_{1\ensuremath{-}x}{\mathrm{Br}}_{x})}_{2}\ensuremath{-}4\mathrm{SC}{({\mathrm{NH}}_{2})}_{2}Ni(Cl1\ensuremath−x​Brx​)2​\ensuremath−4SC(NH2​)2​ (DTNX\mathrm{DTN}XDTNX). Building on nuclear magnetic resonance experiments at high magnetic field [A. Orlova et al., Phys. Rev. Lett. 118, 067203 (2017)], we describe the localization of isolated impurity states, providing a realistic theoretical modeling for DTNX\mathrm{DTN}XDTNX. Going beyond single impurity localization we use quantum Monte Carlo simulations to explore many-body effects from which pairwise effective interactions lead to a (impurity-induced) Bose-Einstein condensation (BEC) revival [M. Dupont, S. Capponi, and N. Laflorencie, Phys. Rev. Lett. 118, 067204 (2017)]. We further address the question of the existence of a many-body localized Bose-glass (BG) phase in DTNX\mathrm{DTN}XDTNX, which is found to compete with a series of a new kind of BEC regimes made out of the multi-impurity states. The global magnetic field--temperature phase diagram of DTNX\mathrm{DTN}XDTNX reveals a very rich structure for low impurity concentration, with consecutive disorder-induced BEC minidomes separated by intervening many-body localized BG regimes. Upon increasing the impurity level, multiple mini-BEC phases start to overlap, while intermediate BG regions vanish.
The coupled spin-1 chains material NiCl_{2}-4SC(NH_{2})_{2} (DTN) doped with Br impurities is expected to be a perfect candidate for observing many-body localization at high magnetic field: the so-called "Bose glass," a zero-temperature bosonic fluid, compressible, gapless, incoherent, and short-range correlated. Using nuclear magnetic resonance, we critically address the stability of the Bose glass in doped DTN, and find that it hosts a novel disorder-induced ordered state of matter, where many-body physics leads to an unexpected resurgence of quantum coherence emerging from localized impurity states. An experimental phase diagram of this new "order-from-disorder" phase, established from nuclear magnetic resonance T_{1}^{-1} relaxation rate data in the 13±1% Br-doped DTN, is found to be in excellent agreement with the theoretical prediction from large-scale quantum Monte Carlo simulations.
We numerically investigate the phase diagram of two-dimensional site-diluted coupled dimer systems in an external magnetic field. We show that this phase diagram is characterized by the presence of an extended Bose glass, not accessible to mean-field approximation, and stemming from the localization of two distinct species of bosonic quasiparticles appearing in the ground state. On the one hand, nonmagnetic impurities doped into the dimer-singlet phase of a weakly coupled dimer system are known to free up local magnetic moments. The deviations of these local moments from full polarization along the field can be mapped onto a gas of bosonic quasiparticles, which undergo condensation in zero and very weak magnetic fields, corresponding to transverse long-range antiferromagnetic order. An increasing magnetic field lowers the density of such quasiparticles to a critical value at which a quantum phase transition occurs, corresponding to the quasiparticle localization on clusters of local magnets (dimers, trimers, etc.) and to the onset of a Bose glass. Strong finite-size quantum fluctuations hinder further depletion of quasiparticles from such clusters, and thus lead to the appearance of pseudoplateaus in the magnetization curve of the system. On the other hand, site dilution hinders the field-induced Bose-Einstein condensation of triplet quasiparticles on the intact dimers, and it introduces instead a Bose glass of triplets. A thorough numerical investigation of the phase diagram for a planar system of coupled dimers shows that the two above-mentioned Bose glass phases are continuously connected, and they overlap in a finite region of parameter space, thus featuring a two-species Bose glass. The quantum phase transition from Bose glass to magnetically ordered phases in two dimensions is marked by novel universal exponents ($z\ensuremath{\approx}2$, $\ensuremath{\beta}\ensuremath{\approx}0.9$, and $\ensuremath{\nu}\ensuremath{\approx}1$). Hence we conclude that doped quantum antiferromagnets in a field represent an ideal setting for the study of fundamental dirty-boson physics.
We study theoretically the effects of disorder on Bose–Einstein condensates (BEC) of bosonic triplon quasiparticles in doped dimerized quantum magnets. The condensation occurs in a magnetic field, where the concentration of bosons in the random potential is sufficient to form the condensate. The effect of doping is partly modeled by a δ -correlated distribution of impurities, which (i) leads to a uniform renormalization of the system parameters and (ii) produces disorder in the system with renormalized parameters. This approach can explain qualitatively the available magnetization data on the Tl 1− x K x CuCl 3 compound taken as an example. In addition to the magnetization, we found that the speed of the Bogoliubov mode has a maximum as a function of x . No evidence of the pure Bose glass phase has been found in the BEC regime.
We study theoretically the effects of disorder on Bose–Einstein condensates (BEC) of bosonic triplon quasiparticles in doped dimerized quantum magnets. The condensation occurs in a magnetic field, where the concentration of bosons in the random potential is sufficient to form the condensate. The effect of doping is partly modeled by a δ-correlated distribution of impurities, which (i) leads to a uniform renormalization of the system parameters and (ii) produces disorder in the system with renormalized parameters. This approach can explain qualitatively the available magnetization data on the Tl1−xKxCuCl3 compound taken as an example. In addition to the magnetization, we found that the speed of the Bogoliubov mode has a maximum as a function of x. No evidence of the pure Bose glass phase has been found in the BEC regime.
Condensed matter physics, and especially strongly correlated systems provide some of the most challenging problems of modern physics. In these systems, the many-body interactions and correlations between quantum particles cannot be neglected; otherwise, the models would simply fail to capture the relevant physics at play and phenomena ensuing. In particular, the work presented in this manuscript deals with quantum magnetism and addresses several distinct questions through computational approaches and state-of-the-art numerical methods. The interplay between disorder (i.e. impurities) and interactions is studied regarding a specific magnetic compound, where instead of the expected many-body localized phase at high magnetic fields, a novel disorder-induced ordered state of matter is found, with a resurgence of quantum coherence. Furthermore, the dynamical response of quantum magnets to an external perturbation, such as it is accessed and measured in nuclear magnetic resonance and inelastic neutron scattering experiments is investigated.