Quantum Phase Transition of a Magnet in a Spin Bath

Rønnow, Henrik M.; Parthasarathy, Raghuveer; Jensen, J.; Aeppli, G.; Rosenbaum, T. F.; McMorrow, D. F.

doi:10.1126/science.1108317

Cited by 133 publications

(182 citation statements)

References 26 publications

(30 reference statements)

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“…As with the pump-probe measurements shown in Fig. 1, the Fano line shape in the off-diagonal response suggests that the underlying physical mechanism again consists of a resonance due to a set of tightly bound clusters, and coupling between the resonance and the continuum of excitations in the spin bath the physical origin of which is most likely the nuclear spin degrees of freedom [11,12]. The inherent linewidth of the resonance, ÿ r , was found to be approximately 0.25 K, with no significant variation due to T. By comparison, q, the coupling between the bath and the resonance is 1.3 at T 0:15 K and T 0:11 K, falling to 0.8 for T 0:07 K. The susceptibility from rotations in the transverse plane represents approximately 1% of the total spectral intensity.…”

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confidence: 58%

Quantum Projection in an Ising Spin Liquid

et al. 2007

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A transverse magnetic field is used to scan the diagonal and off-diagonal susceptibility of the uniaxial quantum magnet, LiHo 0:045 Y 0:955 F 4 . Clusters of strongly coupled spins act as the primary source for the response functions, which result from a field-induced quantum projection of the system into a classically forbidden (meaning non-Ising) regime. Calculations based on spin pairs reproduce only some features of the data and fail to predict the measured off-diagonal response, providing evidence of a multispin collective state. DOI: 10.1103/PhysRevLett.99.057203 PACS numbers: 75.45.+j, 75.50.Dd, 75.50.Lk, 76.30.Kg A basic feature of quantum mechanics is the superposition of states and the projection of these superpositions into observable quantities. From the Stern-Gerlach experiment of 1922, where quantum projection was first demonstrated with angular momentum states in silver atoms [1], to spin echoes in nuclear magnetic resonance [2], where a series of radio frequency pulses induce the precession of nuclear spins projected into classically inaccessible states, strict experimental protocols have been derived for quantum manipulation at the atomic level. For quantum information purposes, it would be advantageous to extend these techniques to coherent clusters of atoms and to explore such effects in solids. In this Letter, we pioneer measurements of the off-diagonal susceptibility to observe quantum mixing effects in a uniaxial magnetic salt, where we are able to rotate clusters of spins into a classically forbidden (meaning non-Ising) state, transverse to the spin axis. The spins can be manipulated by a combination of ac and dc magnetic fields, detected by a multiaxis susceptometer, and parsed for their quantum-mechanical (xy) or classical (polarized along z, the Ising axis) nature by sweeping transverse magnetic field or temperature, respectively. Our technique takes full advantage of the tensor nature of the magnetic susceptibility, revealing the fundamental role played by the off-diagonal terms [3] in the Hamiltonian.Comparison of the results with calculations including only single ions and pairs of ions shows that the off-diagonal response is not only large, but that it also displays the characteristics of a quantum spin liquid [3].The results of the technique depend crucially on two properties of our system: (i) the ability to introduce quantum mechanics via a controllable, external tuning parameter, and (ii) the predilection of disorder to form spin clusters that may occur rarely but can dominate the physics. In particular, the random distribution of magnetic Ho ions in the diluted, spin-1=2 Ising system LiHo 0:045 Y 0:955 F 4 gives rise to regions where the local density of Ho is comparable to that of the pure compound, and hence are locally ferromagnetic or antiferromagnetic depending on the displacement between nearest neighbors. The Ising spins are constrained to point up or down along the crystallographic c axis, and can be controllably mixed with the application of a magnetic field H t tr...

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confidence: 58%

Quantum Projection in an Ising Spin Liquid

et al. 2007

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“…from recent experiments, we now use a Monte Carlo method based on Glauber dynamics to calculate C(t) from equation (1). First, we demonstrate that the slow long-time, low-temperature decay we have found in the experimental data stems from rare-ordered clusters with very slow dynamics.…”

Section: Griffiths Phase Having Established the Functional Form Of C(t)mentioning

confidence: 73%

Unreachable glass transition in dilute dipolar magnet

Biltmo

Henelius

2012

Nat Commun

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In magnetic systems the combined effects of disorder and frustration may cause the moments to freeze into a disordered state at a spin-glass transition. Recent experiments have shown that the rare earth compound LiHo 0.045 Y 0.955 F 4 freezes, but that the transition is unreachable because of dynamics that are 10 7 times slower than in ordinary spin-glass materials. This conclusion refutes earlier investigations reporting a speed-up of the dynamics into an exotic anti-glass phase caused by entanglement of quantum dipoles. Here we present a theory, backed by numerical simulations, which describes the material in terms of classical dipoles governed by Glauber dynamics. The dipoles freeze and we find that the ultra-slow dynamics are caused by rare, strongly ordered clusters, which give rise to a previously predicted, but hitherto unobserved, Griffths phase between the paramagnetic and spin-glass phases. In addition, the hyperfine interaction creates a high energy barrier to flipping the electronic spin, resulting in a clear signature in the dynamic correlation function.

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“…In some cases unexpected subtleties can arise at very low temperatures, as in a recent experiment involving LiHoF 4 [24]. In this case, the QCP dictated by the electronic spins is forestalled by the hyperfine coupling of electronic and nuclear spins.…”

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confidence: 96%

Criticality in correlated quantum matter

2005

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At quantum critical points (QCP) [1,2,3,4,5,6,7] there are quantum fluctuations on all length scales, from microscopic to macroscopic lengths, which, remarkably, can be observed at finite temperatures, the regime to which all experiments are necessarily confined. A fundamental question is how high in temperature can the effects of quantum criticality persist? That is, can physical observables be described in terms of universal scaling functions originating from the QCPs? Here we answer these questions by examining exact solutions of models of correlated systems and find that the temperature can be surprisingly high. As a powerful illustration of quantum criticality, we predict that the zero temperature superfluid density, ρs(0), and the transition temperature, Tc, of the cuprates are related by Tc ∝ ρs(0) y , where the exponent y is different at the two edges of the superconducting dome, signifying the respective QCPs. This relationship can be tested in high quality crystals.Do quantum critical points (QCPs) provide a powerful framework for understanding complex correlated many body problems? Do they shed new light on quantum mechanics of macroscopic systems, providing, for example, a deeper understanding of entanglement? Answers to many such questions require a precise recognition of the experimentally observable regime of quantum criticality. An important class of QCPs are analogs of classical critical points, but in a dimension higher than the actual spatial dimension of the system; that is, a quantum critical point in d spatial dimensions is equivalent to a classical critical point in (d + z), dimensions, where z is the dynamic critical exponent. We shall be primarily concerned with z = 1, although an example involving z = 1 is given below. This extra dimension (imaginary time) is the sine qua non of the effect of quantum mechanics.It seems paradoxical that often the region of classical critical fluctuations is small and tends to zero as the temperature, T , tends to zero, while the region of quantum critical fluctuations fans out as T increases. The reason is that the quantum critical region is determined by the dominant microscopic energy scale in the Hamiltonian, whereas the classical critical region is determined by the ratio of the transition temperature, T c , to the same scale raised to a positive power (the dimensionality). Since this ratio is usually small compared to unity (it is 10 −5 in a conventional superonductor), the classical critical region is small as well. As a classical critical point is driven to zero by tuning a parameter to reach the QCP, T c itself vanishes as ξ −z , provided the T = 0 spatial correlation length, ξ, is large compared to the lattice spacing, further amplifying the contrast between classical and quantum criticalities.How high in temperature does the effect of a QCP persist? Consider perhaps the simplest possible example of a quantum phase transition described by the Hamiltonian of an Ising model in a transverse field [8] in one dimension,where the usual Pauli matrices,...

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Quantum Phase Transition of a Magnet in a Spin Bath

Cited by 133 publications

References 26 publications

Quantum Projection in an Ising Spin Liquid

Quantum Projection in an Ising Spin Liquid

Unreachable glass transition in dilute dipolar magnet

Criticality in correlated quantum matter

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