Field-induced Quantum Phase Transition Research Articles

Field-dependent spin and heat conductivities of dimerized spin-12chains

We study the spin and heat conductivity of dimerized spin-1/2 chains in homogeneous magnetic fields at finite temperatures. At zero temperature, the model undergoes two field-induced quantum phase transitions from a dimerized, into a Luttinger, and finally into a fully polarized phase. We search for signatures of these transitions in the spin and heat conductivities. Using exact diagonalization, we calculate the Drude weights, the frequency dependence of the conductivities, and the corresponding integrated spectral weights. As a main result, we demonstrate that both the spin and heat conductivity are enhanced in the gapless phase and most notably at low frequencies. In the case of the thermal conductivity, however, the field-induced increase seen in the bare transport coefficients is suppressed by magnetothermal effects, caused by the coupling of the heat and spin current in finite magnetic fields. Our results complement recent magnetic transport experiments on spin ladder materials with sufficiently small exchange couplings allowing access to the field-induced transitions.

Exact calculation of the magnetocaloric effect in the spin-12XXZchain

We calculate the entropy and cooling rate of the antiferromagnetic spin-1/2 XXZ chain under an adiabatic demagnetization process using the quantum transfer-matrix technique and non-linear integral equations. The limiting case of the Ising chain (corresponding to infinitely large anisotropy) is presented for comparison. Our exact results for the Heisenberg chain are used as a crosscheck for the numerical exact diagonalization as well as Quantum Monte Carlo simulations and allow us to benchmark the numerical methods. Close to field-induced quantum phase transitions we observe a large magnetocaloric effect. Furthermore, we verify universal low-temperature power laws in the cooling rate and entropy, in particular linear scaling of entropy with temperature T in the gapless Luttinger-liquid state and scaling as \sqrt{T} at field-induced transitions to gapped phases.

Field-induced quantum phase transitions in the spin-1/2 triangular-lattice antiferromagnet Cs2CuBr4

In classical magnetic spin systems, geometric frustration leads to a large number of states of identical energy. We report here evidence from magnetocaloric and related measurements that in Cs2CuBr4 — a geometrically frustrated Heisenberg S = 1/2 triangular antiferromagnet — quantum fluctuations stabilize a series of gapped collinear spin states bounded by first-order transitions at simple increasing fractions of the saturation magnetization for fields directed along the c axis. Only the first of these quantum phase transitions has been theoretically predicted, suggesting that quantum effects continue to dominate at fields much higher than previously considered.

Quantum phase transitions in a two-dimensional quantumXYXmodel: Ground-state fidelity and entanglement

A systematic analysis is performed for quantum phase transitions in a two-dimensional anisotropic spin-1/2 antiferromagnetic XYX model in an external magnetic field. With the help of an innovative tensor network algorithm, we compute the fidelity per lattice site to demonstrate that the field-induced quantum phase transition is unambiguously characterized by a pinch point on the fidelity surface, marking a continuous phase transition. We also compute an entanglement estimator, defined as a ratio between the one-tangle and the sum of squared concurrences, to identify both the factorizing field and the critical point, resulting in a quantitative agreement with quantum Monte Carlo simulation. In addition, the local order parameter is "derived" from the tensor network representation of the system's ground-state wave functions.

Apparent Violation of the Wiedemann-Franz Law near a Magnetic Field Tuned Metal-Antiferromagnetic Quantum Critical Point

The temperature dependences of the interlayer electrical and thermal resistivity in a layered metal are calculated for Fermi liquid quasiparticles which are scattered inelastically by two-dimensional antiferromagnetic spin fluctuations. Both resistivities have a linear temperature dependence over a broad temperature range. Extrapolations to zero temperature made from this linear-T range give values that appear to violate the Wiedemann-Franz law. However, below a low-temperature scale, which becomes small close to the critical point, a recovery of this law occurs. Our results describe recent measurements on CeCoIn5 near a magnetic field-induced quantum phase transition. Hence, the experiments do not necessarily imply a non-Fermi liquid ground state.

Effects of the staggered Dzyaloshinskii-Moriya interaction on entanglement in antiferromagnetic Heisenberg chain

By using the density-matrix renormalization group method, the spin entanglement in the antiferromagnetic Heisenberg chain with the staggered Dzyaloshinskii-Moriya (DM) interactions is investigated. It is found that the staggered DM interactions remove the field-induced second order quantum phase transition of the system at the magnitude of magnetic field H＝2, and lead to the vanishing of the anomalous behavior of quantum entanglement. Also, the staggered DM interactions eliminate the divergence of entanglement range. It means the disappearance of the factorizing point in the model studied. Due to the presence of the staggered DM interactions, the system will never come to the state of saturated ferromagnetism under finite magnetic fields, thus maintaining the spin entanglement. The staggered DM interactions favor the tuning of the strength and range of spin entanglement.

Quantum magnetization plateau and sign change of the magnetocaloric effect in a ferrimagnetic spin chain.

The magnetocaloric effect in the $(5∕2,1)$ ferrimagnetic chain is investiagted using a finite-cluster solver with central spin selection to reduce finite-size effects. We found that an enhanced magnetocaloric effect occurs at the boundary of the ground-state magnetization plateau. The calculated magnetic Gr\uneisen parameter ${\ensuremath{\Gamma}}_{H}$ exhibits strong signatures of a field-induced quantum phase transition at the critical magnetic field ${H}_{c}$ destroying the plateau. We obtain the characteristic increase ${\ensuremath{\Gamma}}_{H}\ensuremath{\sim}{T}^{\ensuremath{-}1∕\ensuremath{\nu}z}$ near ${H}_{c}$ and the sign change ${\ensuremath{\Gamma}}_{H}\ensuremath{\approx}\ensuremath{-}{G}_{r}∕(H\ensuremath{-}{H}_{c})$, in accordance with scaling theory predictions.

Magnetic Fluctuations at a Field-Induced Quantum Phase Transition

We report an inelastic neutron-scattering study at the field-induced magnetic quantum phase transition of CeCu5.8Au0.2. The data can be described better by the spin-density-wave scenario than by a local quantum critical point, while the latter scenario was shown to be applicable to the zero-field concentration-tuned quantum phase transition in CeCu6-xAux for x=0.1. This constitutes direct microscopic evidence for a difference in the quantum fluctuation spectra at a magnetic quantum critical point driven by different tuning parameters.

Spin-glass state of individual magnetic vortices inYBa2Cu3OyandLa2−xSrxCuO4below the metal-to-insulator crossover

Highly disordered magnetism confined to individual weakly interacting vortices is detected by muon spin rotation in two different families of high-transition-temperature superconductors, but only in samples on the low-doping side of the low-temperature normal state metal-to-insulator crossover (MIC). The results support an extended quantum phase transition (QPT) theory of competing magnetic and superconducting orders that incorporates the coupling between CuO2 planes. Contrary to what has been inferred from previous experiments, the static magnetism that coexists with superconductivity near the field-induced QPT is not ordered. Our findings unravel the mystery of the MIC and establish that the normal state of high-temperature superconductors is ubiquitously governed by a magnetic quantum critical point in the superconducting phase.

Quantum spin flop transition in nanowire ferrimagnets

Recently a ferrimagnetic quantum spin chain consisting of Ni and Mn ions has been synthesized to create a new type of magnet with a long life time, based on the Glauber's slow dynamics. Its magnetic properties are well described by the ( s , S ) = ( 1 , 2 ) spin alternating chain model, with single-ion anisotropies D 1 and D 2 at Ni and Mn sites, respectively. Thus we investigate the magnetization process of this model, using the numerical exact diagonalization and finite-size scaling technique. It predicts a new field-induced quantum phase transition between two different Tomonaga–Luttinger liquid phases; TLL1 and TLL2 for some negative values of D 1 and D 2 . This transition corresponds to a quantum version of the spin flop. Some typical phase diagrams are presented.

Field-induced quantum phase transition in the anisotropic Kondo necklace model

The anisotropic Kondo necklace model in 2D and 3D is treated as a genuine model for magnetic to Kondo singlet quantum phase transitions in the heavy fermion (HF) compounds. The variation of the quantum critical point (QCP) with anisotropy parameters has been investigated previously in the zero field case [1]. Here we extend the treatment to finite fields using a generalised bond operator representation including all triplet states. The variation of critical tc with external field H and the associated phase diagram is derived. The influence of anisotropies and the different g-factors for localised and itinerant spins on tc(H) is also investigated. It is found that three different types of behaviour may appear: (i) Destruction of antiferromangetism and appearance of a singlet state above a critical field. (ii) The inverese behaviour, namely field induced antiferromagnetism out of the Kondo singlet phase. (iii) Reentrance behaviour of the Kondo singlet phase as function of field strength.

Magnetostriction in the Bose-Einstein condensate quantum magnet NiCl2–4SC(NH2)2 (Invited)

The quantum magnet NiCl2–4SC(NH2)2 is a candidate for observing Bose-Einstein condensation of spin degrees of freedom in applied magnetic fields. An XY antiferromagnetic ordered state occurs in a dome-shaped region of the temperature-field phase diagram between Hc1=2.1T and Hc2=12.6T and below 1.2K. Bose-Einstein condensation corresponds to the field-induced quantum phase transition into the ordered state. We investigate magnetostriction in single crystals of this compound at dilution refrigerator temperatures in magnetic fields up to 18T, and as a function of magnetic field angle. We show that significant changes in the lattice parameters are induced by magnetic fields, and argue that these result from antiferromagnetic couplings between the Ni spins along the tetragonal c axis. The magnetic phase diagram as a function of temperature, field, and field angle can be extracted from these data. We discuss the implications of these results to Bose-Einstein condensation in this system.

Quantum criticality of a d-dimensional spin-S XXZ ferromagnetic model with longitudinal infinite-range interactions

A d-dimensional spin-S XXZ ferromagnetic model with uniform long-range interactions in the z-direction is mapped into an effective XY model in a transverse field. Then, using the two-time Green function equation of motion method, the zero-temperature properties for a periodic cubic lattice are exactly obtained close to the quantum critical point signaling the field-induced quantum phase transition from the fully polarized state into a phase with easy-plane spin ordering. We determine the relevant critical exponents which have a mean-field character for any d and S.

Magnetotransport across the field-induced quantum phase transition in [formula omitted

We report on the Hall resistivity ρ xy ( T , B ) and magnetoresistivity ρ xx ( T , B ) for CeCu 5.8 Au 0.2 with magnetic field B applied along the b -axis (hard direction) and current I along the a -axis. CeCu 5.8 Au 0.2 orders antiferromagnetically below T N = 0.25 K . Using magnetic field as a control parameter, CeCu 5.8 Au 0.2 may continuously be driven through a quantum critical point at a critical magnetic field value B c ≈ 3.6 T . While no change in ρ xy ( T , B = 0 ) is seen across T N , a gradual decrease of ρ xy ( T = const ., B ) is observed around B c .

On Bose–Einstein condensation in magnetic systems

We study magnetic field induced quantum phase transitions in insulating systems. We use a generalized scaling theory to obtain the temperature dependence of several physical quantities along the quantum critical trajectory ( H = H C , T → 0 ) where H C is the critical magnetic field. We consider transitions from a spin liquid into an antiferromagnetic phase at a critical field H C 1 and from a fully polarized paramagnet into a phase with transverse components of magnetization at H C 2 . The transitions at H C 1 and H C 2 can be viewed as Bose–Einstein condensations of magnons, however, they belong to different universality classes since they are associated with different values of the dynamic critical exponent z.

Quantum phase transition in field-induced ordering phases of anisotropic Haldane systems

Being motivated by the novel phase transition found in the Haldane compound, Ni ( C 5 H 14 N 2 ) 2 N 3 ( PF 6 ) , we have investigated the field-induced quantum phase transitions in the anisotropic S = 1 Haldane system by means of the density matrix renormalization group method. With increasing magnetic fields, in addition to the Haldane to ordered phase transition, the spin-reorientation transition between the ordered phases is predicted to occur in the case where the magnetic field is inclined from the principal axes of the anisotropy. Physical consequences of this transition are discussed in connection with the experimental result.

Suppression of magnetic order in [formula omitted] by magnetic fields

The heavy-fermion compound YbNiSi 3 orders antiferromagnetically below the Néel temperature T N = 5.1 K . The magnetic order can be suppressed either by applying a magnetic field of B c = 8.3 T (for B ∥ b ) or by replacing Si with Ge. The latter is equivalent to a negative lattice pressure. In search of a field-induced quantum phase transition, we studied the disappearance of magnetic order in YbNiSi 3 at B c , using specific-heat and resistivity measurements. Apart from a slightly enhanced specific heat and resistivity at B c , no signs for quantum criticality could be identified down to 0.35 K. Our data indicate that YbNiSi 3 belongs to the heavy-fermion compounds in which the magnetic field gradually reduces the amount of antiferromagnetic order until the magnetic entropy vanishes continuously at B c and the system changes smoothly to a field-polarized paramagnet.

Bose–Einstein condensation and entanglement in magnetic systems

We present a study of magnetic field induced quantum phase transitions ininsulating systems. A generalized scaling theory is used to obtain the temperaturedependence of several physical quantities along the quantum critical trajectory(H = HC,) where H is a longitudinal external magnetic field andHC thecritical value at which the transition occurs. We consider transitions from a spin liquid at a critical fieldHC1 and from a fullypolarized paramagnet, at HC2, into phases with long range order in the transverse components. The transitions atHC1 and HC2 can be viewed as Bose–Einstein condensations of magnons which howeverbelong to different universality classes since they have different values of thedynamic critical exponent. Finally, we use that the magnetic susceptibility isan entanglement witness to discuss how this type of correlation sets in as thesystem approaches the quantum critical point along the critical trajectory,H = HC2, .

Anisotropic Hall effect in single crystal heavy fermion YbAgGe

Temperature- and field-dependent Hall effect measurements (down to T = 50 mK and up to H = 18 T ) are reported for YbAgGe, a heavy fermion compound exhibiting a field-induced quantum phase transition. Whereas the temperature-dependent Hall coefficient of YbAgGe shows behavior similar to what has been observed in a number of heavy fermion compounds, the low-temperature, field-dependent measurements reveal well defined, sudden changes with applied field. Distinct features in the anisotropic field-dependent Hall resistivity sharpen on cooling down and at the base temperature are close to the respective critical fields for the field-induced quantum critical point. At low-temperatures, the functional form of the field-dependent Hall coefficient is field-direction-dependent and complex beyond simple existing models, thus reflecting the multi-component Fermi surface of the material and its non-trivial modification at the quantum critical point.

Quantum Monte Carlo Study of Entanglement in Quantum Spin Systems

We perform an extensive quantum Monte Carlo investigation of entanglement properties in quantum spin systems close to or at a quantum critical point. Making use of the Stochastic Series Expansion method, we can systematically estimate the bipartite entanglement of the ground-state wavefunction in a large class of anisotropic spin models on unfrustrated lattices and in a uniform magnetic field. The behavior of the entanglement estimators as a function of the field shows remarkable universal features independent of the lattice dimensionality, marking both the occurrence of a field-induced quantum phase transition and of an exactly factorized state.