A. E. Jacobs

Novel surface state in a class of incommensurate systems

We study the Landau model of the class of incommensurate systems with a scalar order parameter where the modulated phase is driven by a gradient-squared term with negative coefficient. For example, theoretical studies of cholesteric liquid crystals in a field ~electric or magnetic! suggest that such an modulated phase should exist at high chirality. The bulk phase diagram in the presence of a bulk external field which couples linearly to the order parameter exhibits a modulated phase inside a loop in the temperature-field plane, and a homogeneous phase outside. On analyzing the same model for a semi-infinite system, we find a surprising result; the system exhibits surface states in a region where the bulk phase is homogeneous ~but close to the modulated region!. These states are very different from the well-known surface states induced either by a surface field or by enhanced interactions at the surface, for they exist and are energetically favored even when the sole effect of the surface is to terminate the bulk, as expressed by free boundary conditions taken at the surface. Near the surface, the surface-state order parameter is very different from the bulk value ~in fact, it has the opposite sign!. When the temperature or the bulk field are varied to move away from the modulated state, we find a surface phase transition at which the surface states become energetically unfavorable, though they continue to exist as metastable states. We then study how a surface field changes the surface phase diagram.

Quantum fluctuations in the incommensurate phase of CsCuCl 3 in a transverse magnetic field

In zero magnetic field, the stacked, triangular antiferromagnet

Magnetic structures of RbCuCl 3 in a transverse field

{\mathrm{CsCuCl}}_{3}

SINGLE-HOLE PROPERTIES IN THE T-J AND STRONG-COUPLING MODELS

has a helical structure incommensurate (IC) in the chain direction (normal to the planes). A magnetic field applied transverse to the chains distorts the helix, but the IC structure persists up to at least

Hubbard model in the strong-coupling approximation

0.43

Universal incommensurate structures

times the saturation field. The IC wave number

Modulated structures in tilted chiral smectic films

q

Detailed Lanczos study of one- and two-hole band structure and finite-size effects in the t-J model.

(from neutron-diffraction experiments) decreases with increasing field, but then it has an unexpected plateau. Classical theory explains the behavior at small fields, including the temperature dependence, but it fails to explain the plateau, which we ascribe to quantum fluctuations. We find that linear spin-wave (LSW) theory also fails to explain the plateau; in fact, LSW theory fails more severely than classical theory in describing the IC phase. We introduce a phenomenological treatment of quantum fluctuations. After verifying that it describes well some known results, we apply the phenomenological theory to the IC phase of

Modulated structures in Langmuir monolayers and in smectic films.

{\mathrm{CsCuCl}}_{3}

Incommensurate state of CsCuCl3 in a transverse magnetic field.

, finding that it yields a plateau at approximately the observed value of