Related papers: Quantum Magnets under Pressure: Controlling Elemen…
We consider a $\phi^4$-theory with a position-dependent distance from the critical point. One realization of this model is a classical ferromagnet subject to non-uniform mechanical stress. We find a sharp phase transition where the envelope…
In this paper we review the theory of unconventional quantum critical points that are beyond the Landau's paradigm. Three types of unconventional quantum critical points will be discussed: (1). The transition between topological order and…
A growing body of evidence suggests that the quantum critical behavior at the onset of magnetism in heavy fermion systems can not be understood in terms of a simple quantum spin density wave. This talk will discuss the consequences of this…
A fragile ordered state can be easily tuned by various external parameters. When the ordered state is suppressed to zero temperature, a quantum phase transition occurs, which is often marked by the appearance of unconventional…
In the metallic magnet Nb$_{1-y}$Fe$_{2+y}$, the low temperature threshold of ferromagnetism can be investigated by varying the Fe excess $y$ within a narrow homogeneity range. We use elastic neutron scattering to track the evolution of…
We study the effect of site dilution and quantum fluctuations in an antiferromagnetic spin system on a square lattice within the linear spin-wave approximation. By performing numerical diagonalization in real space and finite-size scaling,…
When a metal undergoes a continuous quantum phase transition, non-Fermi liquid behaviour arises near the critical point. It is standard to assume that all low-energy degrees of freedom induced by quantum criticality are spatially extended,…
Quantum criticality describes the collective fluctuations of matter undergoing a second-order phase transition at zero temperature. Heavy fermion metals have in recent years emerged as prototypical systems to study quantum critical points.…
In certain Mott-insulating dimerized antiferromagnets, triplet excitations of the paramagnetic phase can decay into the two-particle continuum. When such a magnet undergoes a quantum phase transition into a magnetically ordered state, this…
Entanglement underpins quantum information processing and computing, yet its experimental quantification in complex, many-body condensed matter systems remains a considerable challenge. Here, we reveal a highly entangled electronic phase…
We study the fate of spin-1/2 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…
We report on Hall resistivity and electrical resistivity measurements under pressure and magnetic field in UGe_2 with ferromagnetic (FM) tricriticality. The Hall resistivity sensitively detects the first order metamagnetic transition from a…
The heavy-fermion system CeCu_{6-x}Au_x exhibits a quantum critical point at x_c = 0.1 separating nonmagnetic and magnetically ordered ground states. The pronounced non-Fermi-liquid behavior at x_c calls for a search for the relevant…
Recent results on the nature of the quantum critical point between Neel and valence bond solid(VBS) ordered phases of two dimensional quantum magnets are examined by an attack from the VBS side. This approach leads to an appealingly simple…
Quantum-critical behavior of the itinerant electron antiferromagnet (V0.9Ti0.1)2O3 has been studied by single-crystal neutron scattering. By directly observing antiferromagnetic spin fluctuations in the paramagnetic phase, we have shown…
In this chapter we discuss aspects of the quantum critical behavior that occurs at a quantum phase transition separating a topological phase from a conventionally ordered one. We concentrate on a family of quantum lattice models, namely…
The description of quantized collective excitations stands as a landmark in the quantum theory of condensed matter. A prominent example occurs in conventional magnets, which support bosonic magnons - quantized harmonic fluctuations of the…
Collective motions in strongly interacting magnets involve many spins and are often described in terms of integer-spin excitations. However, in certain cases, the collective motion can behave as if these integer excitations break apart into…
Quantum electrodynamics in $2+1$ dimensions (QED$_3$) has been proposed as a critical field theory describing the low-energy effective theory of a putative algebraic Dirac spin liquid or of quantum phase transitions in two-dimensional…
We report a study of the ferromagnetism of ZrZn$_{2}$, the most promising material to exhibit ferromagnetic quantum criticality, at low temperatures $T$ as function of pressure $p$. We find that the ordered ferromagnetic moment disappears…