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arXiv:cond-mat/0601231 · Europhysics Letters (EPL)

Frustrated Heisenberg antiferromagnets: Fluctuation-induced first order vs . deconfined quantum criticality

Frank Krüger, S Scheidl

Published May 4, 2006• 29 citations
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Abstract

Recently it was argued that quantum phase transitions can be radically different from classical phase transitions with, as a highlight, the "deconfined critical points" exhibiting fractionalization of quantum numbers due to Berry phase effects. Such transitions are supposed to occur in frustrated (" J 1 - J 2 ") quantum magnets. We have developed a novel renormalization approach for such systems which is fully respecting the underlying lattice structure. According to our findings, another profound phenomenon is around the corner: a fluctuation-induced (order-out-of-disorder) first-order transition. This has to occur for large spin and we conjecture that it is responsible for the weakly first-order behavior recently observed in numerical simulations for frustrated S = 1/2 systems.

DOI
10.1209/epl/i2006-10039-3

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Recently it was argued that quantum phase transitions can be radically different from classical phase transitions with, as a highlight, the "deconfined critical points" exhibiting fractionalization of quantum numbers due to Berry phase effects. Such transitions are supposed to occur in frustrated (" J 1 - J 2 ") quantum magnets. We have developed a novel renormalization approach for such systems which is fully respecting the underlying lattice structure. According to our findings, another profound phenomenon is around the corner: a fluctuation-induced (order-out-of-disorder) first-order transition. This has to occur for large spin and we conjecture that it is responsible for the weakly first-order behavior recently observed in numerical simulations for frustrated S = 1/2 systems.
Recently, it was argued that quantum phase transitions in frustrated two dimensional antiferromagnets can be radically different from their classical counterparts with as a highlight, the “deconfined critical points” exhibiting fractionalization of quantum numbers due to Berry phase effects. However, field theoretical descriptions rest on a naïve coarse graining of the microscopic lattice model assuming order parameter fluctuations on small scales to be smooth. We have developed a novel renormalization approach for such systems which incorporates fluctuations on small scales in a natural way, and is fully respecting the underlying lattice structure and the frustration mechanism. According to our findings, another profound phenomenon is around the corner: a fluctuation induced first order transition.
Quantum phase transitions in Mott insulators do not fit easily into the Landau-Ginzburg-Wilson paradigm. A recently proposed alternative to it is the so-called deconfined quantum criticality scenario, providing a new paradigm for quantum phase transitions. In this context it has recently been proposed that a second-order phase transition would occur in a two-dimensional spin 1/2 quantum antiferromagnet in the deep easy-plane limit. A check of this conjecture is important for understanding the phase structure of Mott insulators. To this end we have performed large-scale Monte Carlo simulations on an effective gauge theory for this system, including a Berry-phase term that projects out the S=1/2 sector. The result is a first-order phase transition, thus contradicting the conjecture.
Quantum phase transitions in Mott insulators do not fit easily into the Landau-Ginzburg-Wilson paradigm.A recently proposed alternative to it is the so-called deconfined quantum criticality scenario, providing a new paradigm for quantum phase transitions.In this context it has recently been proposed that a second-order phase transition would occur in a two-dimensional spin 1=2 quantum antiferromagnet in the deep easy-plane limit.A check of this conjecture is important for understanding the phase structure of Mott insulators.To this end we have performed large-scale Monte Carlo simulations on an effective gauge theory for this system, including a Berry-phase term that projects out the S 1=2 sector.The result is a first-order phase transition, thus contradicting the conjecture.
We give a short review of our recent works on the first-order quantum phase transitions in frustrated spin chains with orthogonal-dimer structure. When the ratio of the competing antiferromagnetic exchange couplings is varied, a first-order transition occurs between the dimer phase and the plaquette phase, which is accompanied by the discontinuity in the spin excitation gap. We further show that strong frustration triggers the phase transitions in a magnetic field, which exhibit plateaus and jumps in the magnetization curve. The hole-doping effect is also addressed for the orthogonal-dimer chain with linked-tetrahedra structure. It is found that the competing antiferromagnetic interactions result in a firstorder metal-insulator transition upon hole doping. Recently antiferromagnetic quantum spin systems with strong frustration have been studied intensively. A typical example is SrCu2(BO3)2 1) where the spin-1/2 magnetic Cu ions are located on the orthogonal-dimer lattice (the so-called Shastry-Sutherland model). 2) Various interesting phenomena were observed for this compound
We study the fate of spin-1/21/21/2 spiral-ordered two-dimensional quantum antiferromagnets that are disordered by quantum fluctuations. A crucial role is played by the topological point defects of the spiral phase, which are known to have a Z2{Z}_{2}Z2​ character. Previous works established that a nontrivial quantum spin-liquid phase results when the spiral is disordered without proliferating the Z2{Z}_{2}Z2​ vortices. Here, we show that when the spiral is disordered by proliferating and condensing these vortices, valence-bond solid ordering occurs due to quantum Berry phase effects. We develop a general theory for this latter phase transition and apply it to a lattice model. This transition potentially provides a new example of a Landau-forbidden deconfined quantum critical point.
The phase diagrams of highly frustrated quantum spin systems can exhibit first-order quantum phase transitions and thermal critical points even in the absence of any long-ranged magnetic order. However, all unbiased numerical techniques for investigating frustrated quantum magnets face significant challenges, and for generic quantum Monte Carlo methods the challenge is the sign problem. Here we report on a general quantum Monte Carlo approach with a loop-update scheme that operates in any basis, and we show that, with an appropriate choice of basis, it allows us to study a frustrated model of coupled spin-1/2 trimers: simulations of the trilayer Heisenberg antiferromagnet in the spin-trimer basis are sign-problem-free when the intertrimer couplings are fully frustrated. This model features a first-order quantum phase transition, from which a line of first-order transitions emerges at finite temperatures and terminates in a thermal critical point. The trimer unit cell hosts an internal degree of freedom that can be controlled to induce an extensive entropy jump at the quantum transition, which alters the shape of the first-order line. We explore the consequences for the thermal properties in the vicinity of the critical point, which include profound changes in the lines of maxima defined by the specific heat. Our findings reveal trimer quantum magnets as fundamental systems capturing in full the complex thermal physics of the strongly frustrated regime.