22 research outputs found
Spin-strain coupling in NiCl2-4SC(NH2)2
We report results of ultrasonic investigations of the quantum S = 1 spin-chain magnet NiCl2-4SC(NH2)2, also known as DTN, in magnetic fields up to 18 T and temperatures down to 0.3 K. A field H along the [001] direction induces a transition into an antiferromagnetic phase with T(N)max â 1.2 K. Accordingly, at T = 0 there are two quantum critical points at ~2.1 T and at ~12.6 T. The acoustic c33 mode, propagating along the spin chains, shows a pronounced softening close to the phase transition, accompanied by energy dissipation of the sound wave. The H-T phase diagram obtained from our measurements is compared with results from other experimental investigations and the low-temperature acoustic anomalies are traced up to T > T(N). We also report frequency-dependent effects, which open the possibility to investigate the spin fluctuations in the critical regions. Our observations show an important role of the spin-phonon coupling in DTN
Interplay of quantum and classical fluctuations near quantum critical points
For a system near a quantum critical point (QCP), above its lower critical
dimension , there is in general a critical line of second order phase
transitions that separates the broken symmetry phase at finite temperatures
from the disordered phase. The phase transitions along this line are governed
by thermal critical exponents that are different from those associated with the
quantum critical point. We point out that, if the effective dimension of the
QCP, ( is the Euclidean dimension of the system and the
dynamic quantum critical exponent) is above its upper critical dimension ,
there is an intermingle of classical (thermal) and quantum critical
fluctuations near the QCP. This is due to the breakdown of the generalized
scaling relation between the shift exponent of the critical
line and the crossover exponent , for by a \textit{dangerous
irrelevant interaction}. This phenomenon has clear experimental consequences,
like the suppression of the amplitude of classical critical fluctuations near
the line of finite temperature phase transitions as the critical temperature is
reduced approaching the QCP.Comment: 10 pages, 6 figures, to be published in Brazilian Journal of Physic
Bose-Einstein Condensation in Magnetic Insulators
The elementary excitations in antiferromagnets are magnons, quasiparticles
with integer spin and Bose statistics. In an experiment their density is
controlled efficiently by an applied magnetic field and can be made finite to
cause the formation of a Bose-Einstein condensate (BEC). Studies of magnon
condensation in a growing number of magnetic materials provide a unique window
into an exciting world of quantum phase transitions (QPT) and exotic quantum
states.Comment: 17 pages, 3 figure
Ag(nic)2 (nic = nicotinate): a spin-canted quasi-2D antiferromagnet composed of square-planar S = 1/2 Ag(II) ions.
Square-planar S = 1/2 Ag(II) ions in polymeric Ag(nic)(2) are linked by bridging nic monoanions to yield 2D corrugated sheets. Long-range magnetic order occurs below T(N) = 11.8(2) K due to interlayer couplings that are estimated to be about 30 times weaker than the intralayer exchange interaction
Neutron study of the magnetism in NiCl2âą4SC(NH2)2
We study the strongly anisotropic quasi-one-dimensional S = 1 quantum magnet NiCl2 center dot 4SC(NH2)(2) using elastic and inelastic neutron scattering. We demonstrate that a magnetic field splits the excited doublet state and drives the lower doublet state to zero energy at a critical field H-c1. For H-c1 H-c2, and we demonstrate the presence of an AF interaction between Ni2+ on the two interpenetrating sublattices. In the antiferromagnetically ordered phase, the spin-waves that develop from the lower-energy doublet are split into two modes. This is most likely the result of the presence of the AF interaction between the interpenetrating lattices.© 2013 IOP Publishing LTD
Stress-Induced Domain Wall Motion in a Ferroelastic Mn<sup>3+</sup> Spin Crossover Complex
Domain wall motion is detected for the first time during the transition to a ferroelastic and spinâstate ordered phase of a spin crossover complex. Single crystal Xâray diffraction and resonant ultrasonic spectroscopy (RUS) revealed two distinct symmetryâbreaking phase transitions in the mononuclear Mn 3+ compound [Mn(3,5âdiBrâsal 2 (323))]BPh 4 , 1. The first at 250 K, involves the space group change Cc â Pc and is thermodynamically continuous, while the second, Pc â P1 at 85 K, is discontinuous and related to spin crossover and spinâstate ordering. Stressâinduced domain wall mobility was detected as softening of the phonon modes at the Pc â P1 transition