Disorder-Induced Revival of the Bose-Einstein Condensation inNi(Cl1−xBrx

Abstract

Building on recent NMR experiments [A. Orlova et al., Phys. Rev. Lett. 118, 067203 (2017)], we theoretically investigate the high magnetic field regime of the disordered quasi-one-dimensional S = 1 antiferromagnetic material Ni(Cl1−xBrx)2-4SC(NH2)2. The interplay between disorder, chemically controlled by Br-doping, interactions, and the external magnetic field, leads to a very rich phase diagram. Beyond the well-known antiferromagnetically ordered regime, analog of a Bose condensate of magnons, which disappears when H ≥ 12.3 T, we unveil a resurgence of phase coherence at higher field H ∼ 13.6 T, induced by the doping. Interchain couplings stabilize finite temperature long-range order whose extension in the field -temperature space is governed by the concentration of impurities x. Such a "mini-condensation" contrasts with previously reported Bose-glass physics in the same regime, and should be accessible to experiments.Introduction.-Interacting quantum systems in the presence of disorder have been intensively studied for several decades, leading to fascinating physics, e.g. the Kondo effect [1], the many-body localization transition [2], or the superfluid to Bose-glass (BG) [3,4] transition at finite disorder for lattice bosons [5][6][7]. While counterintuitive, in some situations disorder may enhance long-range order, as discussed for inhomogeneous superconductors [8][9][10]. Perhaps even more surprisingly, doping gapped antiferromagnets with a finite concentration of magnetic or non-magnetic impurities can fill up the bare spin gap with localized levels [11-13] which may eventually order, in the strict sense of macroscopic long-range order (LRO) at low temperature. Such an impurity-induced ordering mechanism of the type "order from disorder" [14,15] [21]. Nevertheless, only a few studies have focused on the effect of an external field [22][23][24][25][26][27].In this Letter, building on recent nuclear magnetic resonance (NMR) experiments [28], we achieve a realistic theoretical study of the high magnetic field regime of Ni(Cl 1−x Br x ) 2 -4SC(NH 2 ) 2 (DTNX): a threedimensional antiferromagnetic (AF) system made of weakly coupled chains of S = 1 spins subject to singleion anisotropy [panels (b-c) of Fig. 1]. Note that the S = 1 chains are not of Haldane type, due to the large anisotropy D [29]. In the absence of chemical disorder (x = 0), NiCl 2 -4SC(NH 2 ) 2 (DTN) provides a very good realization of magnetic field-induced Bose-Einstein condensation (BEC) in a quantum spin system [30][31][32][33] between two critical field H clean

Nuclear Magnetic Resonance Reveals Disordered Level-Crossing Physics in the Bose-Glass Regime of the Br-Doped Ni(Cl1−xBrxOrlova1, Blinder2, Kermarrec3 et al. 2017Phys. Rev. Lett.Self Cite152390View full textAdd to dashboardCite

“…Above H c2 , many-body physics is thus controlled by the effective pairwise interaction between such localized impurity states. This sheds a new light on the BG physics expected in this system above H c2 [13]; while a BG regime implies the absence of any long-range order down to T = 0, in the related theoretical work [23] it is shown that around H * the impurity states order at low T , thereby profoundly changing the phase diagram.The present investigation of DTNX is based on previous NMR work on DTN samples [24,25], enabling prearXiv:1607.02360v3 [cond-mat.str-el] …”

“…Including quantum dynamics provides crucial information on the (de)localization of the spin around the impurities [23]. For a single impurity, this can be studied both analytically or numerically by exact diagonalization (ED).…”

“…We have thus built a basis to theoretically access the many-body correlations between impurities. This enabled a critical reexamination of the phase diagram of DTNX above H c2 : the related theoretical work [23] determines the (doping-dependent) extension of the field range where an inhomogeneous, ordered, BEC-type phase, related to the level crossing, is replacing a true Bose-glass regime. Our work thus sets a microscopic basis and proper delimitations of the impurityinduced BG regime for all further studies.…”

Spin waves near the edge of halogen substitution induced magnetic order in Ni(Cl1−xBrx)2·4Mannig1, Povarov2, Ollivier3 et al. 2018Phys. Rev. B103191View full textAdd to dashboardCite

“…In turn, interactions are strongly modified, especially J c for which the twohalide bond is the primary superexchange mediator [17]. Recent NMR [25] and theoretical [26] studies suggest that these modification of the Hamiltonian (1) parameters are very local.…”

Quantum criticality in a three-dimensional spin system at zero field and pressure

“…The false color plot shows scattering intensity versus energy transfer ω and momentum transfer along Q = (0.5, 0.5, l) reciprocal space rod, the antiferromagnetic zone center (0.5, 0.5, 0.5) being the global dispersion minimum where the gap is observed directly. The Br-substituted material Ni(Cl 1−x Br x ) 2 ·4SC(NH 2 ) 2 has attracted a great deal of interest in the context of effects of disorder on the field-induced quantum phase transitions [12,15,16] and magnetic excitations [11,17]. However, as was clearly shown by neutron spectroscopy studies, the most obvious effect of Br substitution can be understood simply in terms of its influence on average exchange and anisotropy constants [11].…”