Quantum criticality among entangled spin chains

Abstract

An important challenge in magnetism is the unambiguous identification of a quantum spin liquid 1,2 , of potential importance for quantum computing. In such a material, the magnetic spins should be fluctuating in the quantum regime, instead of frozen in a classical long-range-ordered state. While this requirement dictates systems 3,4 wherein classical order is suppressed by a frustrating lattice 5 , an ideal system would allow tuning of quantum fluctuations by an external parameter. Conventional three-dimensional antiferromagnets can be tuned through a quantum critical point-a region of highly fluctuating spins-by an applied magnetic field. Such systems suffer from a weak specific-heat peak at the quantum critical point, with little entropy available for quantum fluctuations 6 . Here we study a different type of antiferromagnet, comprised of weakly coupled antiferromagnetic spin-1/2 chains as realized in the molecular salt K 2 PbCu(NO 2 ) 6 . Across the temperature-magnetic field boundary between three-dimensional order and the paramagnetic phase, the specific heat exhibits a large peak whose magnitude approaches a value suggestive of the spinon Sommerfeld coefficient of isolated quantum spin chains. These results demonstrate an alternative approach for producing quantum matter via a magnetic-field-induced shift of entropy from one-dimensional short-range order to a three-dimensional quantum critical point.Previous work on field-tuning of quantum fluctuations has focused on the transverse-field Ising model where the magnetic field (H) couples to a sector of the Hamiltonian not directly modifying the order parameter. While this paradigm has been explored in the three-dimensional (3D) dipolar ferromagnet LiHoF 4 (refs 7,8 ), and the 1D system CoNb 2 O 6 (ref. ), the need for a unique Ising-axis normal restricts the number of potential quantum-spin-liquid host materials. For the much broader class of 3D antiferromagnets (AFs), H can indeed tune the Néel temperature (T N ) into the quantum regime. Within the ordered state, however, on increasing H from zero one first encounters a spin-flop transition (for finite spin anisotropy), followed by a gradual reorientation of those spins (Fig. 1a). Hence, in destroying Néel order, H decreases the transverse components of the staggered moment to a value that is vanishingly small near the transition to the field-polarized state, leaving little entropy to be lost near T = 0, and thus low spectral weight available for quantum entanglement 6 . Here we demonstrate a different approach to tuning through a quantum critical point (QCP). The quasi-1D spin-1/2 AF K 2 PbCu(NO 2 ) 6 orders classically at 0.68 K (ref. 10; Fig. 1b). At lower temperatures within the Néel state, applied H values less than the intra-chain mean field cause little change in the specific heat C(T, H). At the phase boundary, however, a large amount of entropy is released, leading to a peak in C/T (Fig. 1c), the value of which (~ 2 J mol -1 K -2 at the lowest temperatures studied) is suggestive of the spinon ...

Tomonaga–Luttinger liquid behavior and spinon confinement in YbAlO3

“…This is consistent with the expectation of the 1D TLL behavior as discussed in ref. 34 . Defining this peak position as the crossover temperature T * into the quantum critical regime, the linear dependence with νz = 1 for T ≥ 0.3 K again agrees with the free fermion fix point (Fig.…”

Magnetic Field Effects in Low-Dimensional Quantum Magnets

“…There are also many materials that do not display net magnetization, but are well described by localized magnetic moments with short-range (usually antiferromagnetic) interactions. For example, I am currently engaged in a collaboration with the group of Arthur Ramirez at University of California, Santa Cruz studying materials which behave as three-dimensional arrays of coupled spin chains (with antiferromagnetic Heisenberg-like interactions that are strong within the chains and weak between them) in the presence of an external field [54] where I am using my simulations to directly compare to their experiments.…”

Persistence of Ising-like easy-axis spin correlations in the paramagnetic state of the spin-1 chain compound NiTe2O5

“…A S = 1 2 Heisenberg antiferromagnetic (AFM) chain with a nearest-neighbor (NN) intrachain interaction is well established to harbor a gapless disordered state [1] with fractional spinon excitations [4][5][6]. When there exist perturbations beyond an nn interaction, exotic ground states such as frustrated spin liquids [7,8] and gapped spin-singlet states [9][10][11][12] emerge, often displaying quantum critical behaviors [13][14][15] generated by strong 1D quantum fluctuations. For S = 1 Heisenberg chains, on the other hand, spin correlations and spin excitations are quite distinct from those for S = 1 2 ones in nature.…”