558 research outputs found
String order and adiabatic continuity of Haldane chains and band insulators
The ground state of spin-1 Haldane chains is characterized by the so-called
string order. We show that the same hidden order is also present in ordinary
one-dimensional band insulators. We construct a family of Hamiltonians which
connects adiabatically band insulators to two topologically non-equivalent spin
models, the Haldane chain and the antiferromagnetic spin-1/2 ladder. We observe
that the localized spin-1/2 edge-state characteristic of spin-1 chains is
smoothly connected to a surface-bound state of band insulators and its
emergence is not related to any bulk phase transition. Furthermore, we show
that the string order is absent in any dimensions higher than one.Comment: 6 pages, 7 figures. Appendix about charge string orders added.
Version as publishe
Giant mass and anomalous mobility of particles in fermionic systems
We calculate the mobility of a heavy particle coupled to a Fermi sea within a
non-perturbative approach valid at all temperatures. The interplay of particle
recoil and of strong coupling effects, leading to the orthogonality catastrophe
for an infinitely heavy particle, is carefully taken into account. We find two
novel types of strong coupling effects: a new low energy scale and
a giant mass renormalization in the case of either near-resonant scattering or
a large transport cross section . The mobility is shown to obey two
different power laws below and above . For ,
where is the Fermi wave length, an exponentially large effective
mass suppresses the mobility.Comment: 4 pages, 4 figure
Preprint arXiv: 2208.10487 Submitted on 22 Aug 2022
The physics of long-range interacting quantum systems is currently living a renaissance driven by the fast progress in quantum simulators. In these systems many paradigms of statistical physics do not apply and also the universal long-wavelength physics gets substantially modified by the presence of long-ranged forces. Here we explore the low-energy excitations of several long-range interacting quantum systems, including spin models and interacting Bose gases, in the ordered phase associated with the spontaneous breaking of U(1) and SU(2) symmetries. Instead of the expected Goldstone modes, we find three qualitatively different regimes, depending on the range of the interaction. In one of these regimes the Goldstone modes are gapped, via a generalization of the Higgs mechanism. Moreover, we show how this effect is realized in current experiments with ultracold atomic gases in optical cavities
Fragility of String Orders
One-dimensional gapped systems are often characterized by a 'hidden'
non-local order parameter, the so-called string order. Due to the gap,
thermodynamic properties are robust against a weak higher-dimensional coupling
between such chains or ladders. To the contrary, we find that the string order
is not stable and decays for arbitrary weak inter-chain or inter-ladder
coupling. We investigate the vanishing of the order for three different
systems: spin-one Haldane chains, band insulators, and the transverse-field
Ising model. Using perturbation theory and bosonization, we show that the
fragility of the string order arises from non-local commutation relations
between the non-local order parameter and the perturbation.Comment: 7 pages, 3 figures. Published versio
Thermoelectric Behaviour Near Magnetic Quantum Critical Point
We use the coupled 2d-spin-3d-fermion model proposed by Rosch {\sl et. al.}
(Phys. Rev. Lett. {\bf 79}, 159 (1997)) to study the thermoelectric behaviour
of a heavy fermion compound when it is close to an antiferromagnetic quantum
critical point. When the low energy spin fluctuations are quasi two
dimensional, as has been observed in and , with a typical 2d ordering wavevector and 3d Fermi
surface, the ``hot'' regions on the Fermi surface have a finite area. Due to
enhanced scattering with the nearly critical spin fluctuations, the electrons
in the hot region are strongly renormalized. We argue that there is an
intermediate energy scale where the qualitative aspects of the renormalized hot
electrons are captured by a weak-coupling perturbative calculation. Our
examination of the electron self energy shows that the entropy carried by the
hot electrons is larger than usual. This accounts for the anomalous logarithmic
temperature dependence of specific heat observed in these materials. We show
that the same mechanism produces logarithmic temperature dependence in
thermopower. This has been observed in . We
expect to see the same behaviour from future experiments on .Comment: RevTex, two-column, 7 pages, 2 figure
Low-temperature ordered phases of the spin- XXZ chain system CsCoCl
In this study the magnetic order of the spin-1/2 XXZ chain system
CsCoCl in a temperature range from 50 mK to 0.5 K and in applied
magnetic fields up to 3.5 T is investigated by high-resolution measurements of
the thermal expansion and the specific heat. Applying magnetic fields along a
or c suppresses completely at about 2.1 T. In addition, we find
an adjacent intermediate phase before the magnetization saturates close to 2.5
T. For magnetic fields applied along b, a surprisingly rich phase diagram
arises. Two additional transitions are observed at critical fields T and T, which we propose to
arise from a two-stage spin-flop transition.Comment: 10 pages, 10 figure
Non-Equilibrium Transport through a Kondo-Dot in a Magnetic Field: Perturbation Theory and Poor Man's Scaling
We consider electron transport through a quantum dot described by the Kondo
model in the regime of large transport voltage V in the presence of a magnetic
field B with max(V,B) >> T_K. The electric current I and the local
magnetization M are found to be universal functions of V/T_K and B/T_K, where
T_K is the equilibrium Kondo temperature. We present a generalization of the
perturbative renormalization group to frequency dependent coupling functions,
as necessitated by the structure of bare perturbation theory. We calculate I
and M within a poor man's scaling approach and find excellent agreement with
experiment.Comment: version accepted in PRL, notations changed, parts rewritten, figures
modified, references and some corrections adde
Interplay of disorder and spin fluctuations in the resistivity near a quantum critical point
The resistivity in metals near an antiferromagnetic quantum critical point
(QCP) is strongly affected by small amounts of disorder. In a quasi-classical
treatment, we show that an interplay of strongly anisotropic scattering due to
spin fluctuations and isotropic impurity scattering leads to a large regime
where the resistivity varies as T^alpha, with an anomalous exponent, alpha, 1
<= alpha <= 1.5, depending on the amount of disorder. I argue that this
mechanism explains in some detail the anomalous temperature dependence of the
resistivity observed in CePd_2Si_2, CeNi_2Ge_2 and CeIn_3 near the QCP.Comment: 4 pages, 4 eps figures, published version, only small change
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