The tunable quantum spin ladder Cu(Qnx)(Cl(1−x)Brx)2

Abstract

We report magnetic, specific heat and ESR measurements on a series of S = 1/2 spin ladder compounds Cu(Qnx)(Cl (1−x) Brx)2. Down to T = 2 K all the observables can be described by the spin ladder model with about 1% of S = 1/2 impurities in the background, which are present even in a nominally pure Cu(Qnx)Cl2 and Cu(Qnx)Br2, for the whole range of 0 ≤ x ≤ 1. We extract ladder exchange constants J l (x) and Jr(x) as well as the gap value ∆(x) by comparing the experimental data to quantum Monte-Carlo simulations. As the ESR measurements show the g-factor of impurities to be precisely equal to g-factor of the ladder spins, we suppose the impurities to be the consequence of structural defects resulting in a broken ladder ends.

Hybrid Fibrillar Xerogels with Unusual Magnetic Properties

“…It is worth stressing that the material considered here is a randomly dispersed array of fibrils, which is at variance with the usual magnetic spin gap or gapless compounds that possess highly organized structures (crystals). Provided that the formation thermodynamics is controlled, we believe that our elaboration approach via physical processes (gelation, nucleation, and so forth) could be a very promising alternative to tune the spin gap or other physical properties of supramolecular assemblies instead of the approaches using chemical engineering − or external stimuli. ,, Finally, this hybrid material is very easy to prepare from its basic constituents and can be further processed in many ways. For instance, a highly porous material can be obtained once the solvent has been properly extracted, , which opens up new possible paths for applications in material sciences.…”

Raman study of spin excitations in the tunable quantum spin ladder Cu(Qnx)(Cl$_{1-x}$Br$_x$)$_2$

“…In Fig. 7b, 2∆ estimated from bulk magnetic measurements 17 and neutron scattering experiments 15 is plotted in circles and open triangles respectively for a direct comparison with our result. Through the entire concentration range, the Raman data reproduces the trends observed by other techniques rather well.…”

“…We then used a linear interpolation of E + /J || vs. J ⊥ /J || to obtain E + for the actual exchange constants in Cu(Qnx)(Cl 1−x Br x ) 2 , as estimated by bulk magnetometry. 17 The resulting estimate for E + is plotted in semi-filled circles in Fig. 7a.…”

“…As Cl is gradually replaced by Br, the most significant crystalographic change is the increase of the lattice constant c, which affects the Cu-Cl/Br-Cu bond angle, and thereby results in a change of the rung exchange constant. 17 Interestingly, the magnetic properties are modified more strongly along the leg. This is because of a complex orbital overlap through Qnx molecules.…”

“…and BPCB -(C 5 H 12 N) 2 CuBr 4 [7][8][9][10][11] for the strong-leg and strong-rung case, respectively. The recently characterized family of materials Cu(Qnx)(Cl 1−x Br x ) 2 , [12][13][14][15][16][17] where Qnx stands for quinoxaline (C 8 H 6 N 2 ), opened a new opportunity of continuously varying the leg to rung exchange ratio, albeit in a rather narrow range. Most previous studies of these materials concentrated on the bulk measurements and provided rough estimates of these parameters: J || = 1.61 meV, J ⊥ = 2.95 meV (J ⊥ /J || = 1.83) and J || = 1.99 meV, J ⊥ = 3.26 meV (J ⊥ /J || = 1.64) for x = 0 and x = 1, respectively.…”