arXiv subjects

Jan Zaanen

Publications and source records attributed to Jan Zaanen.

86 records · Page 5Linked to original sources

arXiv2007

Towards an Experimental Test of Gravity-induced Quantum State Reduction

According to the hypothesis of Penrose and Diosi, quantum state reduction is a manifestation of the incompatibilty of general relativity and the unitary time evolution of quantum physics. Dimensional analysis suggests that Schrodinger cat type states should collapse on measurable time scales when masses and lengths of the order of bacterial scales are involved. We analyze this hypothesis in the context of modern developments in condensed matter and cold atoms physics, aimed at realizing macroscopic quantum states. We first consider 'micromechanical' quantum states, analyzing the capacity of an atomic force microscopy based single spin detector to measure the gravitational state reduction, but we conclude that it seems impossible to suppress environmental decoherence to the required degree. We subsequently discuss 'split' cold atom condensates to find out that these are at present lacking the required mass scale by many orders of magnitude. We then extent Penrose's analysis to superpositions of mass current carrying states, and we apply this to the flux quantum bits realized in superconducting circuits. We find that the flux qubits approach the scale where gravitational state reduction should become measurable, but bridging the few remaining orders of magnitude appears to be very difficult with present day technology.

cond-mat.str-el↗

arXiv2007

Metallic stripes and the universality of the anomalous half-breathing phonon in high-Tc cuprates

We demonstrate that the strong anomalies in the high frequency LO-phonon spectrum in cuprate superconductors can in principle be explained by the enhanced electronic polarizability associated with the self-organized one dimensionality of metallic stripes. Contrary to the current interpretation in terms of transversal stripe fluctuations, the anomaly should occur at momenta parallel to the stripes. The doping dependence of the anomaly is naturally explained, and we predict that the phonon line-width and the spread of the anomaly in the transverse momentum decrease with increasing temperature while high resolution measurements should reveal a characteristic substructure to the anomaly.

cond-mat.str-el↗

arXiv2006

On the relation between decoherence and spontaneous symmetry breaking

We have recently shown that there is a limit to quantum coherence in many-particle spin qubits due to spontaneous symmetry breaking. These results were derived for the Lieb-Mattis spin model. Here we will show that the underlying mechanism of decoherence in systems with spontaneous symmetry breaking is in fact more general. We present here a generic route to finding the decoherence time associated with spontaneous symmetry breaking in many particle qubits, and subsequently we apply this approach to two model systems, indicating how the continuous symmetries in these models are spontaneously broken and discussing the relation of this symmetry breaking to the thin spectrum. We then present in detail the calculations that lead to the limit to quantum coherence, which is due to energy shifts in the thin spectrum.

cond-mat.str-el↗

arXiv2005

Spin-orbit coupling induced magnetism in the $d$-density wave phase of La$_{2-x}$Ba$_x$CuO$_4$ superconductors

We study the effects of spin-orbit coupling in the $d$-density wave (DDW) phase. In the low-temperature orthorhombic phase of La$_{2-x}$Ba$_x$CuO$_4$, we find that spin-orbit coupling induces a ferromagnetic moment in the DDW phase, which is polarized along the [110] direction with a considerable magnitude. This effect does not exist in the superconducting phase. On the other hand, if the $d$-density wave order does not exist at zero field, a magnetic field along the [110] direction always induces such a staggered orbital current. We discuss experimental constraints on the DDW states in light of our theoretical predictions.

cond-mat.supr-con↗

arXiv2005

Trapping of Quantized Electrical Charge in Superfluid 3He-B via the Electrodynamics of Spin-orbit Coupled Systems

Exploiting analogies between spin-orbit coupled spin superfluids and non-Abelian Yang-Mills theory, we argue that machines can be built capable of trapping a quantized amount of electric linear charge density, while the line charge quantum itself is surprisingly large. The required conditions might well hold in superfluid 3He-B, for which we propose an experimental realization of this phenomenon.

cond-mat.soft↗

arXiv2005

An intrinsic limit to quantum coherence due to spontaneous symmetry breaking

We investigate the influence of spontaneous symmetry breaking on the decoherence of a many-particle quantum system. This decoherence process is analyzed in an exactly solvable model system that is known to be representative of symmetry broken macroscopic systems in equilibrium. It is shown that spontaneous symmetry breaking imposes a fundamental limit to the time that a system can stay quantum coherent. This universal timescale is $t_{spon} \simeq 2πN \hbar / (k_B T)$, given in terms of the number of microscopic degrees of freedom $N$, temperature $T$, and the constants of Planck ($\hbar$) and Boltzmann ($k_B$).

cond-mat.str-el↗

arXiv2000

Stripes And Nodal Fermions As Two Sides Of The Same Coin

One of the central problems in High T_{c} superconductivity is to reconcile the abundant evidence for stripe-like physics at `short' distances with the equally convincing evidence for BCS like physics at large distance scales (the `nodal fermions'). Our central hypothesis is that the duality notion applies: the superconductor should be viewed as a condensate of topological excitations associated with the fully ordered stripe phase. As we will argue, the latter are not only a form of `straighforward' spin and charge order but also involve a form of `hidden' or `topological' long range order which is also responsible for the phenomenon of spin-charge separation in 1+1D. The topological excitation associated with the destruction of this hidden order is of the most unusual kind. We suggest that the associated disorder field theory has a geometrical, gravity like structure concurrent with topological phases with no precedent elsewhere.

cond-mat.str-el↗

arXiv2000

Do shifting Bragg peaks of cuprate stripes reveal fractionally charged kinks?

The stripe phases found in correlated oxides can be viewed as an ordering of the solitons associated with doping the Mott-insulating state. Inspired by the recent observation that the stripes tilt away from the main axis of the crystal lattice in the regime x = 1/8, we propose that a new type of stripe phase is realized in the large doping regime. This new phase should be viewed as a doped version of the microscopically insulating x = 1/8 stripes. The topological excitations associated with the extra doping are fractionally charged kinks along the stripes whose motions make the stripe fluctuate. We argue that the directional degree of freedom of the kinks might order, causing the stripe phase to tilt. Quantitative predictions follow for the doping dependence of the tilt angle, which in turn can be used to determine the fundamental charge quantum associated with the stripe phase.

cond-mat.supr-con↗

arXiv1999

Quantum melting of magnetic long-range order near orbital degeneracy. Classical phases and Gaussian fluctuations

We study the effective spin-orbital model derived for the d9 ions in a three-dimensional perovskite lattice, as in KCuF_3, where at each site the doubly degenerate eg orbitals contain a single hole. The model describes the superexchange interactions that depend on the pattern of orbitals occupied. We present the ground state properties of this model, depending on the splitting between the eg orbitals E_z, and the Hund's rule coupling in the excited d8 states, J_H. The classical phase diagram consists of six magnetic phases which all have different orbital ordering: two antiferromagnetic (AF) phases with G-AF order and either x2-y2 or 3z2-r2 orbitals occupied, two phases with mixed orbital (MO) patterns and A-AF order, and two other MO phases with either C-AF or G-AF order. All of them become degenerate at the multicritical point M=(E_z,J_H)=(0,0). Using a generalization of linear spin-wave theory we study both the transverse excitations which are spin-waves and spin-and-orbital-waves, as well as the longitudinal (orbital) excitations. The transverse modes couple to each other, and the spin-and-orbital-wave turns into a soft mode near the M point. Therefore, quantum corrections to the long-range-order parameter are drastically increased near the orbital degeneracy, and classical order is suppressed in a crossover regime between the G-AF and A-AF phases in the (E_z,J_H) plane. This behavior is reminiscent of that found in frustrated spin models, and we conclude that orbital degeneracy provides a new and physically realizable mechanism which stabilizes a spin liquid ground state due to inherent frustration of magnetic interactions. We also point out that such a disordered magnetic phase is likely to be realized in LiNiO_2.

cond-mat.str-el↗

arXiv1998

Spontaneous orientation of a quantum lattice string

Using exact diagonalization and quantum Monte-Carlo techniques we study a quantum lattice string model introduced as a model for a single cuprate struipe. We focus on the ground state properties of the string. Our results shows that, in the physically relevant region of the parameter space, a zero temperature spontaneous symmetry breaking occures. The string spontaneously orients itself along one direction in space and become directed. We introduce an order parameter for the directedness and show that at zero temperature this order parameter reaches its saturation value.

cond-mat.str-el↗

arXiv1998

Quantum disorder versus order-out-of-disorder in the Kugel-Khomskii model

The Kugel-Khomskii model, the simplest model for orbital degenerate magnetic insulators, exhibits a zero temperature degeneracy in the classical limit which could cause genuine quantum disorder. Khaliullin and Oudovenko [Phys. Rev. B 56, R14 243 (1997)] suggested recently that instead a particular classical state could be stabilized by quantum fluctuations. Here we compare their approach with standard random phase approximation and show that it strongly underestimates the strength of the quantum fluctuations, shedding doubts on the survival of any classical state.

cond-mat.str-el↗

arXiv1998

Quantum magnetism in the stripe phase: bond- versus site order

It is argued that the spin dynamics in the charge-ordered stripe phase might be revealing with regards to the nature of the anomalous spin dynamics in cuprate superconductors. Specifically, if the stripes are bond ordered much of the spin fluctuation will originate in the spin sector itself, while site ordered stripes require the charge sector as the driving force for the strong quantum spin fluctuations.

cond-mat.str-el↗

arXiv1997

Charged domain walls as quantum strings living on a lattice

A generic lattice cut-off model is introduced describing the quantum meandering of a single cuprate stripe. The fixed point dynamics is derived, showing besides free string behavior a variety of partially quantum disordered phases, bearing relationships both with quantum spin-chains and surface statistical physics.

cond-mat.str-el↗

arXiv1997

Quantum Melting of Magnetic Order due to Orbital Fluctuations

We have studied the phase diagram and excitations of the spin-orbital model derived for a three dimensional perovskite lattice, as in KCuF_3. The results demonstrate that the orbital degeneracy drastically increases quantum fluctuations and suppresses the classical long-range order near the multicritical point in the mean-field phase diagram. This generates a qualitatively new spin liquid, providing the first example of a valence bond ground state in three dimensions.

cond-mat.str-el↗