We report a novel crossover behavior in the long-range-ordered phase of a prototypical spin-$1/2$ Heisenberg antiferromagnetic ladder compound $mathrm{(C_7H_{10}N)_2CuBr_4}$. The staggered order was previously evidenced from a continuous and symmetri
c splitting of $^{14}$N NMR spectral lines on lowering temperature below $T_csimeq 330$ mK, with a saturation towards $simeq 150$ mK. Unexpectedly, the split lines begin to further separate away below $T^*sim 100$ mK while the line width and shape remain completely invariable. This crossover behavior is further corroborated by the NMR relaxation rate $T_1^{-1}$ measurements. A very strong suppression reflecting the ordering, $T_1^{-1}sim T^{5.5}$, observed above $T^*$, is replaced by $T_1^{-1}sim T$ below $T^*$. These original NMR features are indicative of unconventional nature of the crossover, which may arise from a unique arrangement of the ladders into a spatially anisotropic and frustrated coupling network.
We present NMR measurements of a strong-leg spin-1/2 Heisenberg antiferromagnetic ladder compound (C7H10N)2CuBr4 under magnetic fields up to 15 T in the temperature range from 1.2 K down to 50 mK. From the splitting of NMR lines we determine the phas
e boundary and the order parameter of the low-temperature (3-dimensional) long-range-ordered phase. In the Tomonaga-Luttinger regime above the ordered phase, NMR relaxation reflects characteristic power-law decay of spin correlation functions as 1/T1 T^(1/2K-1), which allows us to determine the interaction parameter K as a function of field. We find that field-dependent K varies within the 1<K<2 range which signifies attractive interaction between the spinless fermions in the Tomonaga-Luttinger liquid.
Using $^{63,65}$Cu nuclear magnetic resonance (NMR) in magnetic fields up to 30 T we study the microscopic properties of the 12-site valence-bond-solid ground state in the pinwheel kagome compound Rb$_2$Cu$_3$SnF$_{12}$. We find that the ground state
is characterized by a strong transverse staggered spin polarization whose temperature and field dependence points to a mixing of the singlet and triplet states. This is further corroborated by the field dependence of the gap $Delta (H)$, which has a level anticrossing with a large minimum gap value of $approx Delta (0)/2$, with no evidence of a phase transition down to 1.5,K. By the exact diagonalization of small clusters, we show that the observed anticrossing is mainly due to staggered tilts of the $g$-tensors defined by the crystal structure, and reveal symmetry properties of the low-energy excitation spectrum compatible with the absence of level crossing.
At low temperatures, weakly coupled spin chains develop a magnetic order that reflects the character of gapless spin fluctuations along the chains. Using nuclear magnetic resonance, we identify and characterize two ordered states in the gapless regio
n of the antiferromagnetic, Ising-like spin-chain system BaCo2V2O8, both arising from the incommensurate fluctuations along the chains. They correspond to the columnar and ferromagnetic ordered states of the frustrated J1-J2 spin model on a square lattice, where the spins are encoded in original spin chains. As a result of field-dependent incommensurate fluctuations and frustrated interchain interaction, J1 can be tuned continuously with the magnetic field, and its value with respect to a fixed J2 selects the ordered state. Spin-chain systems can thus be used as tunable simulators of frustrated planar magnetism.
We present a 14N nuclear magnetic resonance study of a single crystal of CuBr4(C5H12N)2 (BPCB) consisting of weakly coupled spin-1/2 Heisenberg antiferromagnetic ladders. Treating ladders in the gapless phase as Luttinger liquids, we are able to full
y account for (i) the magnetic field dependence of the nuclear spin-lattice relaxation rate 1/T_1 at 250 mK and for (ii) the phase transition to a 3D ordered phase occuring below 110 mK due to weak interladder exchange coupling. BPCB is thus an excellent model system where the possibility to control Luttinger liquid parameters in a continuous manner is demonstrated and Luttinger liquid model tested in detail over the whole fermion band.
We present magnetic torque measurements on the Shastry-Sutherland quantum spin system SrCu$_2$(BO$_3$)$_2$ in fields up to 31 T and temperatures down to 50 mK. A new quantum phase is observed in a 1 T field range above the 1/8 plateau, in agreement w
ith recent NMR results. Since the presence of the DM coupling precludes the existence of a true Bose-Einstein condensation and the formation of a supersolid phase in SrCu$_2$(BO$_3$)$_2$, the exact nature of the new phase in the vicinity of the plateau remains to be explained. Comparison between magnetization and torque data reveals a huge contribution of the Dzyaloshinskii-Moriya interaction to the torque response. Finally, our measurements demonstrate the existence of a supercooling due to adiabatic magnetocaloric effects in pulsed field experiments.
A nuclear magnetic resonance (NMR) study is reported of multiple (30) Al$_{0.13}$Ga$_{0.87}$As quantum well (QW) sample near the Landau level filling factor $ u =1$. In these Al$_{0.13}$Ga$_{0.87}$As QWs the effective $g$ factor is nearly zero. This
can lead to two effects: vanishing electronic polarization $(P)$ and skyrmionic excitations composed of a huge number of spins. As small $P$ values cause an overlap of the NMR signals from the QW and barriers, a special technique was employed to allow these two signals to be distinguished. The QW signal corresponds to a small, negative, and very broad distribution of spin polarization that exhibits thermally induced depolarization. Such a distribution can be attributed to sample inhomogeneities and/or to large skyrmions, the latter possibility being favored by observation of a very fast $T_{2}^{-1}$ rate.
A 59Co NMR study of CoO2, the x=0 end member of AxCoO2 (A = Na, Li...) cobaltates, reveals a metallic ground state, though with clear signs of strong electron correlations: low-energy spin fluctuations develop at wave vectors q different from 0 and a
crossover to a Fermi-liquid regime occurs below a characteristic temperature T*~7 K. Despite some uncertainty over the exact cobalt oxidation state n this material, the results show that electronic correlations are revealed as x is reduced below 0.3. The data are consistent with NaxCoO2 being close to the Mott transition in the x -> 0 limit.
Crystallographic, magnetic and NMR properties of a NaxCoO2 single crystal with x~1 are presented. We identify the stoichiometric Na1CoO2 phase, which is shown to be a non-magnetic insulator, as expected for homogeneous planes of Co3+ ions with S=0. I
n addition, we present evidence that, because of slight average Na deficiency, chemical and electronic phase separation leads to a segregation of Na vacancies into the well-defined, magnetic, Na0.8CoO2 phase. The importance of phase separation is discussed in the context of magnetic order for x~0.8 and the occurrence of a metal-insulator transition for x->1.
