Field-induced Quantum Phase Transition Research Articles

Field-induced magnetic instability and quantum criticality in the antiferromagnet CeCu2Ge2.

The magnetic quantum criticality in strongly correlated electron systems has been considered to be closely related with the occurrence of unconventional superconductivity. Control parameters such as magnetic field, pressure or chemical doping are frequently used to externally tune the quantum phase transition for a deeper understanding. Here we report the research of a field-induced quantum phase transition using conventional bulk physical property measurements in the archetypal antiferromagnet CeCu2Ge2, which becomes superconductive under a pressure of about 10 GPa with Tc ~ 0.64 K. We offer strong evidence that short-range dynamic correlations start appearing above a magnetic field of about 5 T. Our demonstrations of the magnetic instability and the field-induced quantum phase transition are crucial for the quantum criticality, which may open a new route in experimental investigations of the quantum phase transition in heavy-fermion systems.

Critical properties of coupled anisotropic Haldane spin chains in a magnetic field

We use inelastic neutron scattering to measure magnetic fluctuations as a function of transverse magnetic field and temperature in a single crystal of the coupled Haldane chain compound $\mathrm{SrNi}{}_{2}{\text{V}}_{2}{\text{O}}_{8}$ with uniaxial anisotropy. At the base temperature (2 K), spin excitations confirm a field-induced quantum phase transition from the disordered Haldane (gapped singlet) state to a gapped long-range ordered state through a quantum critical point (QCP) (${\ensuremath{\mu}}_{0}{H}_{c}\ensuremath{\approx}11.5$ T). At elevated temperatures in the vicinity of the QCP, both the energy and linewidth of the lowest triplet mode scale linearly with temperature, in accordance with quantum criticality. We also demonstrate that the experimental field dependence of the triplet excitation modes at the base temperature is in agreement with the bosonic and macroscopic models, but is in contrast to the fermionic and perturbative models.

Exotic Field Induced Quantum Phase Transition of the Kagome Lattice Antiferromagnet

The magnetization process of the S=1/2 kagome-lattice antiferromagnet is investigated using the numerical diagonalization up to 42-spin clusters. The critical exponent analysis confirms the unconventional magnetization behavior around the 1/3 magnetization plateau-like anomaly; the field derivative is infinite at the low-field side, while it is zero at the high-field side. We also confirm that a magnetization jump appears at 1/3 of the saturation magnetization in distorted kagome-lattice antiferromagnets.

Novel Field Induced Quantum Phase Transition of the Kagome Lattice Antiferromagnet

The S = 1/2 kagomelattice antiferromagnet is investigated by the numerical diagonalization. We successfully obtained the ground-state full magnetization curve for the 42-spin cluster. Based on the finite-size scaling alnalysis including this new result, we consider the quatnum critical magnetization behavior around the 1/3 of the saturation magnetization. As a result, some unconventional features of the 1/3 magnetization ramp, which was proposed by our previous work, are clarified.

EffectiveS=12description of theS=1chain with strong easy-plane anisotropy

We present a study of the one-dimensional S=1 antiferromagnetic spin chain with large easy plane anisotropy, with special emphasis on field-induced quantum phase transitions. Temperature and magnetic field dependence of magnetization, specific heat, and thermal conductivity is presented using a combination of numerical methods. In addition, the original S=1 model is mapped into the low-energy effective S=1/2 XXZ Heisenberg chain, a model which is exactly solvable using the Bethe ansatz technique. The effectiveness of the mapping is explored, and we show that all considered quantities are in qualitative, and in some cases quantitative, agreement. The thermal conductivity of the considered S=1 model is found to be strongly influenced by the underlying effective description. Furthermore, we elucidate the low-lying electron spin resonance spectrum, based on a semi--analytical Bethe ansatz calculation of the effective S=1/2 model.

Susceptibility, high-field magnetization and ESR studies in a spin-5/2 triangular-lattice antiferromagnet Ba3MnSb2O9

The polycrystalline Ba3MnSb2O9 system as a spin-5/2 quasi-two dimensional triangular-lattice antiferromagnet (TLAF), is investigated by means of magnetic susceptibility, specific heat, high field magnetization and electron spin resonance (ESR) measurements. The ac susceptibility and specific heat studies reveal that it undergoes an AFM transition at TN=10.2K rather than the spin-glass one. High-field magnetization at 4.2K exhibits a weak 1/3 magnetization step near ~16.5T indicative of the field-induced quantum phase transition behavior. The analysis of temperature dependence of ESR spectra suggests that the short-range antiferromagnetic (AFM) correlation appear at temperatures well above TN. Further high-field ESR measurement up to 25T at 4.2K shows a broad resonance line, and the obtained g value is 2.094±0.012. Combined with the magnetic susceptibility and specific heat, the magnetic state below TN is explained by an AFM long rang order or a well developed AFM short range order with small anisotropy.

Theory of field-induced quantum phase transition in spin dimer system Ba3Cr2O8

Motivated by recent experiments on Ba3Cr2O8, we propose a theory describing low-temperature properties in magnetic field h of dimer spin-1/2 systems on a stacked triangular lattice with spatially anisotropic exchange interactions. Considering the interdimer interaction as a perturbation we derive in the second order the elementary excitations (triplon) spectrum and the effective interaction between triplons at the quantum critical point h=hc separating the paramagnetic phase (h<hc) and a magnetically ordered one (hc<h<hs, where hs is the saturation field). Expressions are derived for hc(T) and the staggered magnetization M⊥(h) at h≳hc. We apply the theory to Ba3Cr2O8 and determine exchange constants of the model by fitting the triplon spectrum obtained experimentally. It is demonstrated that in accordance with experimental data the system follows the 3D BEC scenario at T<1K only due to a pronounced anisotropy of the spectrum near its minimum. Our expressions for hs, hc(T) and M⊥(h) fit well with available experimental data.

Magnetic Field Induced Transition in Vanadium Spinels

We study vanadium spinels AV2O4 (A = Cd,Mg) in pulsed magnetic fields up to 65 T. A jump in magnetization at μ0H≈40 T is observed in the single-crystal MgV2O4, indicating a field induced quantum phase transition between two distinct magnetic orders. In the multiferroic CdV2O4, the field induced transition is accompanied by a suppression of the electric polarization. By modeling the magnetic properties in the presence of strong spin-orbit coupling characteristic of vanadium spinels, we show that both features of the field induced transition can be successfully explained by including the effects of the local trigonal crystal field.

Novel field-induced quantum phase transition of the kagome-lattice antiferromagnet

The magnetization process of the S = 1/2 kagome-lattice quantum antiferromagnet is investigated using the numerical exact diagonalization up to 36-site clusters. Our previous finite-size scaling analysis with rhombic clusters indicated the “magnetization ramp” as a novel field-induced quantum phase transition at 1/3 the saturation magnetization. As another possible exotic behavior, we focus on the feature of the magnetization curve at 2/3 the saturation. The critical exponent analysis indicates that a different singular behavior occurs at the 2/3 magnetization.

Novel Field-Induced Quantum Phase Transition of the Kagome-Lattice Antiferromagnet

The magnetization process of the S = 1/2 kagome-lattice quantum antiferromagnet is investigated using the numerical exact diagonalization up to 39-site clusters. The finite-size scaling analysis with the rohmbic clusters indicated the “magnetization ramp” as a novel field-induced quantum phase transition at 1/3 of the saturation magnetization. The magnetization ramp is characterized by the following properties; (i) the field derivative dm/dH is divergent at the lower-field side, (ii) dm/dH is vanishing at the higher-field side, and (iii) there is a single critical field Hc (no plateau). Applying the critical exponent analysis for the non-rohmbic clusters, as well as the rohmbic ones, we confirmed the above properties of the magnetization at 1/3 of the saturation for the kagome lattice antiferromagnet.

Field induced quantum phase transition in the anisotropic spin ladder

We have studied the quantum phase transition between antiferromagnetic and spin liquid phases for field induced quasi-one dimensional spin ladder model Hamiltonian. Using bond operator formalism, the original spin Hamiltonian is mapped to bosonic one. Green's function approach has been implemented to get the low energy spectrum and the corresponding structure factor. The critical field (Bc) for a fixed coupling exchange between two rungs is found based on the Bose–Einstein condensation of quasi-particles (triplons) which takes place when the spin excitation spectrum vanishes at the antiferromagnetic wave vector. We have investigated the effect of both intersite (δ) and local (Δ) anisotropy on the critical field, critical coupling exchange and transverse static structure factor of the field induced quasi-one dimensional antiferromagnetic anisotropic spin ladder model at zero temperature. We have also studied the divergent behavior of static spin structure factor when magnetic field approaches the critical point.

Quantum Critical Magnetization Behaviors of the Kagome- and Triangular-Lattice Antiferromagnets

The magnetization process of the S=1/2 Kagome-lattice quantum antiferromagnet is investigated using the numerical exact diagonalization up to 39-site clusters, comparing with the triangular-lattice antiferromagnet. Our previous finite-size scaling analysis with the rhombic clusters indicated the “magnetization ramp” as a novel field-induced quantum phase transition at 1/3 of the saturation magnetization. As another possible exotic behavior, we focus on the feature of the magnetization curve at the 2/3 of the saturation. The same critical exponent analysis indicates that a different singular behavior occurs at the 2/3 magnetization of the Kagome-lattice antiferromagnet, while no anomaly in the triangular-lattice one.

Thermodynamic properties of the one-dimensional extended quantum compass model in the presence of a transverse field

The presence of a quantum critical point can significantly affect the thermodynamic properties of a material at finite temperatures. This is reflected, e.g., in the entropy landscape S(T; c) in the vicinity of a quantum critical point, yielding particularly strong variations for varying the tuning parameter c such as magnetic field. In this work we have studied the thermodynamic properties of the quantum compass model in the presence of a transverse field. The specific heat, entropy and cooling rate under an adiabatic demagnetization process have been calculated. During an adiabatic (de)magnetization process temperature drops in the vicinity of a field-induced zero-temperature quantum phase transitions. However close to field-induced quantum phase transitions we observe a large magnetocaloric effect.

Field-induced criticality in a gapped quantum magnet with bond disorder

Neutron diffraction and calorimetric measurements are used to study the field-induced quantum phase transition in piperazinium-Cu$_2$(Cl$_{1-x}$Br$_x$)$_6$ ($x=0$, x=3.5% and x=7.5%), a prototypical quantum antiferromagnet with random bonds. The critical indexes $\phi$ and $\beta$ are determined. The findings contradict some original predictions for Bose Glass based on the assumption $z=d$, but are consistent with recent theoretical results implying $z<d$. Inelastic neutron experiments reveal that disorder has a profound effect on the lowest-energy magnetic gap excitation in the system.

Magnetic Excitation and Electric Polarization in Strongly Coupled Spin Monomer and Dimer System Cu3Mo2O9

Magnetic excitation in strongly coupled spin monomer and dimer system is studied theoretically focusing on Cu 3 Mo 2 O 9 as an typical example, where Cu 3 Mo 2 O 9 shows an antiferromagnetic long-range order at zero field. Introducing formfactors of dimers, we present the formulation for magnetic excitations. Under an external magnetic field, an additional field-induced quantum phase transition takes place due to the stabilization of a triplet state of the dimer. The recent experimental study revealed that Cu 3 Mo 2 O 9 shows an multiferroic behavior with an electric polarization in the magnetically ordered state. According to the field-induced transition, we find that the electric polarization should change drastically.

Magnetic excitations in the spin-1 anisotropic antiferromagnet NiCl2-4SC(NH2)2

The spin-1 anisotropic antiferromagnet NiCl_2-4SC(NH2)_2 exhibits a field-induced quantum phase transition that is formally analogous to Bose-Einstein condensation. Here we present results of systematic high-field electron spin resonance (ESR) experimental and theoretical studies of this compound with a special emphasis on single-ion two-magnon bound states. In order to clarify some remaining discrepancies between theory and experiment, the frequency-field dependence of magnetic excitations in this material is reanalyzed. In particular, a more comprehensive interpretation of the experimental signature of single-ion two-magnon bound states is shown to be fully consistent with theoretical results. We also clarify the structure of the ESR spectrum in the so-called intermediate phase.

Destruction of the Kondo effect in the cubic heavy-fermion compound Ce3Pd20Si6

How ground states of quantum matter transform between one another reveals deep insights into the mechanisms stabilizing them. Correspondingly, quantum phase transitions are explored in numerous materials classes, with heavy-fermion compounds being among the most prominent ones. Recent studies in an anisotropic heavy-fermion compound have shown that different types of transitions are induced by variations of chemical or external pressure, raising the question of the extent to which heavy-fermion quantum criticality is universal. To make progress, it is essential to broaden both the materials basis and the microscopic parameter variety. Here, we identify a cubic heavy-fermion material as exhibiting a field-induced quantum phase transition, and show how the material can be used to explore one extreme of the dimensionality axis. The transition between two different ordered phases is accompanied by an abrupt change of Fermi surface, reminiscent of what happens across the field-induced antiferromagnetic to paramagnetic transition in the anisotropic YbRh2Si2. This finding leads to a materials-based global phase diagram--a precondition for a unified theoretical description.

Disorder instability of the magnon condensate in a frustrated spin ladder

The effect of disorder is studied on the field-induced quantum phase transition in the frustrated spin-ladder compound H${}_{8}$C${}_{4}$SO${}_{2}\ifmmode\cdot\else\textperiodcentered\fi{}$Cu${}_{2}$(Cl${}_{1\ensuremath{-}x}$Br${}_{x}$)${}_{4}$ using bulk magnetic and thermodynamic measurements. The parent material ($x=0$) is a quantum spin liquid, which in applied fields is known to form a magnon condensate with long-range helimagnetic order. We show that bond randomness introduced by a chemical substitution on the nonmagnetic halogen site destroys this phase transition at very low concentrations, already for $x=0.01$. The extreme fragility of the magnon condensate is attributed to random frustration in the incommensurate state.

Magnetic-field-induced quantum phase transitions in the two-impurity Anderson model

In the two-impurity Anderson model, the inter-impurity spin exchange interaction favors a spin singlet state between two impurities leading to the breakdown of the Kondo effect. We show that a local uniform magnetic field can delocalize the quasiparticles to restore the Kondo resonance. This transition is found to be continuous, accompanied by not only the divergence of the staggered (antiferromagnetic) susceptibility, but also the divergence of the uniform spin susceptibility. This may imply that the magnetic field induced quantum phase transitions in Kondo systems are in favor of the local critical type.

Field-Induced Phase Transitions of Repulsive Spin-1 Bosons in Optical Lattices

We study the phase diagram of repulsively interacting spin-1 bosons in optical lattices at unit filling, showing that an externally induced quadratic Zeeman effect may lead to a rich physics characterized by various phases and phase transitions. We find that the main properties of the system may be described by an effective field model, which provides the precise location of the phase boundaries for any dimension, in excellent agreement with our numerical calculations for one-dimensional (1D) systems. Thus, our work provides a quantitative guide for the experimental analysis of various types of field-induced quantum phase transitions in spin-1 lattice bosons. These transitions, which are precluded in spin-1/2 systems, may be realized by using an externally modified quadratic Zeeman coupling, similar to recent experiments with spinor condensates in the continuum.