Stripe Phase Formation Research Articles

Pattern Formation by Electric-Field Quench in a Mott Crystal.

The control of the Mott phase is intertwined with the spatial reorganization of the electronic states. Out-of-equilibrium driving forces typically lead to electronic patterns that are absent at equilibrium, whose nature is however often elusive. Here, we unveil a nanoscale pattern formation in the Ca2RuO4 Mott insulator. We demonstrate how an applied electric field spatially reconstructs the insulating phase that, uniquely after switching off the electric field, exhibits nanoscale stripe domains. The stripe pattern has regions with inequivalent octahedral distortions that we directly observe through high-resolution scanning transmission electron microscopy. The nanotexture depends on the orientation of the electric field; it is nonvolatile and rewritable. We theoretically simulate the charge and orbital reconstruction induced by a quench dynamics of the applied electric field providing clear-cut mechanisms for the stripe phase formation. Our results open the path for the design of nonvolatile electronics based on voltage-controlled nanometric phases.

Ultrafast dynamics of vibrational symmetry breaking in a charge-ordered nickelate.

The ability to probe symmetry-breaking transitions on their natural time scales is one of the key challenges in nonequilibrium physics. Stripe ordering represents an intriguing type of broken symmetry, where complex interactions result in atomic-scale lines of charge and spin density. Although phonon anomalies and periodic distortions attest the importance of electron-phonon coupling in the formation of stripe phases, a direct time-domain view of vibrational symmetry breaking is lacking. We report experiments that track the transient multi-terahertz response of the model stripe compound La1.75Sr0.25NiO4, yielding novel insight into its electronic and structural dynamics following an ultrafast optical quench. We find that although electronic carriers are immediately delocalized, the crystal symmetry remains initially frozen-as witnessed by time-delayed suppression of zone-folded Ni-O bending modes acting as a fingerprint of lattice symmetry. Longitudinal and transverse vibrations react with different speeds, indicating a strong directionality and an important role of polar interactions. The hidden complexity of electronic and structural coupling during stripe melting and formation, captured here within a single terahertz spectrum, opens new paths to understanding symmetry-breaking dynamics in solids.

Magnetic Field Induced Morphological Transitions in Block Copolymer/Superparamagnetic Nanoparticle Composites.

This two-dimensional computational study investigates the effect of external magnetic fields on thin film nanocomposites comprised of superparamagnetic nanoparticles dispersed within block copolymer melts, which display a variety of morphological transitions based on the field orientation, nanoparticle loading, and selectivity of the nanoparticles for the blocks. In-plane magnetic fields lead to chaining of the nanoparticles; when selective for the minority block in a hexagonal block copolymer nanostructure, this chaining results in the formation of stripe phases oriented parallel to the magnetic field. When selective for the majority block of the hexagonal structure, nanoparticle chains of sufficient persistence length drive the orientation of the hexagonal morphology with the ⟨100⟩ direction oriented parallel to the magnetic field. Out-of-plane magnetic fields induce repulsive dipolar interactions between the nanoparticles that annihilate the defects in the hexagonal morphology of the block copolymer when the nanoparticle is selective for the minority block. When the nanoparticles are selective for the majority block and the field is oriented out of plane, repulsive dipolar interactions lead to the formation of honeycomb lattices. In all cases, the nanoparticle size and volume fraction must be chosen to maximize the commensurability with the block copolymer structure to optimize the ordering of the final composite.

Excitations and Stripe Phase Formation in a Two-Dimensional Dipolar Bose Gas with Tilted Polarization

We present calculations of the ground state and excitations of an anisotropic dipolar Bose gas in two dimensions, realized by a nonperpendicular polarization with respect to the system plane. For sufficiently high density, an increase of the polarization angle leads to a density instability of the gas phase in the direction where the anisotropic interaction is strongest. Using a dynamic many-body theory, we calculate the dynamic structure function in the gas phase which shows the anisotropic dispersion of the excitations. We find that the energy of roton excitations in the strongly interacting direction decreases with increasing polarization angle and almost vanishes close to the instability. Exact path integral ground state MonteCarlo simulations show that this instability is indeed a quantum phase transition to a stripe phase, characterized by long-range order in the strongly interacting direction.

The electronic structure change induced by stripe phase formation in cuprates

It has been suggested that the stripe phase shows 1D charge transport in the CuO 2 planes instead of the 2D characteristics of the Zhang–Rice band. X-ray absorption near edge spectroscopy (XANES) has been used to study the electronic structure of the stripe phase compounds La 2− x Sr x CuO 4, and La 2− x− y Nd y Sr x CuO 4 for x around 1/8. The measurements were performed at both 300 and 28 K to examine the possible electronic structure change above and below the temperature of the stripe phase formation.

Experimental studies of the higher Landau levels in the 2D electron system

The high Landau levels have demonstrated numerous interesting phenomena with theoretical models in need of specific experimental support. While the composite fermion picture provides a model for the series of correlated electron liquids states manifesting as the fractional quantum Hall effect in the lowest Landau level, this model finds its limits in the higher Landau levels. Here transport phenomena such as anisotropic conductance at the half-filled levels dominate the magnetoresistance spectra, which is ostensibly due to phase separation into stripes of integral filling. In the B-field range between these anisotropy findings in the high Landau levels and the FQHE of the lowest Landau level, the anomalous state at 5 2 filling factor has presented an enigmatic variation on the standard liquid states normally found exclusively at odd-denominator filling. This unique state has been suggested to result from composite fermion pairing in d or p wave. In this work we show results independently addressing these theoretical models for the striped phase and the 5 2 state. We show magnetoresistance data that specifically indicates striped phase formation in the 2DES, also depicting the direction of the stripes within the sample. At smaller filling factors in surface acoustic wave measurements we show evidence for the composite fermion model of pairing to form the 5 2 state by demonstrating Fermi surface formation at temperatures above those at which the 5 2 is quantized in Hall resistance.

Evidence for a liquid crystal phase transition in two-dimensional electrons in high Landau levels

Transport measurements of high-mohility two-dimensional electron systems at low temperatures have revealed a large resistance anisotropy around half-filling of excited Landau levels. These results have been attributed to electronic stripe-phase formation with spontaneously broken orientational symmetry. Mechanisms which are known to break the orientational symmetry include poorly-understood crystal structure effects and an in-plane magnetic field, $B_{||}$. Here we report that a large $B_{||}$ also causes the transport anisotropy to persist up to much higher temperatures. In this regime, we find that the anisotropic resistance scales sublinearly with $B_{||}/T$. These observations support the proposal that the transition from anisotropic to isotropic transport reflects a liquid crystal phase transition where local stripe order persists even in the isotropic regime.

Effect of spin substitution on stripe order inLa1.875Ba0.125Cu1−yMyO4(M=ZnorNi)

The destruction of stripe order by Zn $(S=0)$ and the formation of a new phase by Ni $(S=1)$ are demonstrated directly from the substitution dependence of the electronic coefficient of specific heat $\ensuremath{\gamma}$ in ${\mathrm{La}}_{1.875}{\mathrm{Ba}}_{0.125}{\mathrm{Cu}}_{1\ensuremath{-}y}{M}_{y}{\mathrm{O}}_{4}$ $(M=\mathrm{Zn}$ or Ni). In the Zn-substitution case, \ensuremath{\gamma}, which is strongly suppressed by stripe phase formation, increases to a value that corresponds to the state where there is no stripe order. In the Ni-substitution case, \ensuremath{\gamma} saturates at nearly half the value of the Zn-substitution case. The difference can be explained by a difference in the spin interactions between the substituent atom and the nearest neighbor Cu atoms, indicating that spin interaction plays an important role in the formation of charge stripes.

Effect of Zn- or Ni-substitution on specific heat in La 1.875Ba 0.125CuO 4: role of spin interaction in stripe phase

We investigated the effect of Zn- or Ni-substitution on specific heat in La 1.875Ba 0.125CuO 4 where the electronic specific heat coefficient, γ, is largely suppressed by the stripe phase formation. The measurements of specific heat were carried out between 1.5 and 7 K for the polycrystalline samples of La 1.875 Ba 0.125 Cu 1−y M y O 4 ( M=Zn or Ni ). From the y dependence of γ in the Zn-doping case the stripe phase is found to be fully destroyed by doping. While the result of the Ni-doping case shows that the stripe phase is not totally broken, but may undergo a new phase. The difference between Zn- and Ni-doping cases indicates that the spin interaction plays an important role for the stripe phase formation.

To what degree is Hg-1201 able to act as an archetype of HTSC cuprate phenomena? From Seebeck results to stripe phase behaviour

Upon closely examining the Seebeck behaviour of Hg-1201 we have discovered that the system, whether stoichiometric or mercury deficient, appears to behave in similar fashion to the LBCO and YBCO-123 systems in that a dip appears in Tc( p) at p~1/8. We have earlier indicated that this concentration of holes marks the point at which stripe phase formation in the organization of charge and spin in the two-subsystem, mixed-valent, HTSC materials passes from one charge wall loading scheme to another. We observe a strong difference in doping characteristics between the mercury-deficient and mercury-stoichiometric systems, and assemble here a plausible scenario of what is involved. While HgBa2CuO4+ indeed does offer very considerable potential advantages as an archetype to the cuprate HTSC phenomenon, excess anion systems like this clearly introduce materials complexities. These will always demand careful attention and perhaps not endear the system to many accustomed to the relative simplicities of a cation-doped system.

Strong enhancement of spin fluctuations in the low-temperature tetragonal phase of antiferromagnetically ordered La2-x-yEuySrxCuO4

Measurements of the static magnetization, susceptibility and electron spin resonance (ESR) of Gd spin probes have been performed to study the properties of antiferromagnetically ordered La2-x-yEuySrxCuO4 (x0.02) with the low-temperature tetragonal structure. According to the static magnetic measurements, the CuO2 planes are magnetically decoupled in this structural phase. The ESR study reveals strong magnetic fluctuations at the ESR frequency which are not present in the orthorhombic phase. It is argued that this drastic enhancement of the spin fluctuations is due to a considerable weakening of the interlayer exchange and a pronounced influence of hole motion on the antiferromagnetic properties of lightly hole-doped La2CuO4. No evidence for the stripe phase formation at small hole doping is obtained in the present study.

ESR studies of doped La{sub 2}CuO{sub 4}

ESR of Gd spin probes in La{sub 2{minus}x{minus}y}Eu{sub y}Sr{sub x}CuO{sub 4} has been employed to study the Cu spin dynamics and its relation to stripe order of spins and charges in the low temperature tetragonal phase of this compound. In the magnetically ordered hole doped samples (0 {lt} x {le} 0.02) a strong enhancement of magnetic fluctuations on the ESR frequency scale is observed in this structural phase. At larger hold dopings (0.08 {le} x {approx{underscore}lt} 0.17) a drastic decrease of the Cu spin fluctuation frequency, competing with bulk superconductivity, is apparent. For the elevated hole concentrations the obtained results are consistent with the stripe model, whereas in the low hole doping regime the data give no evidence for the stripe phase formation.

LEEM Measurements of Step Energies at the (001) Surface of Heavily Boron-Doped Silicon

We have used the low energy electron microscope (LEEM) to investigate step faceting and striped phase formation on heavily boron-doped Si(001) surfaces. Measurements of thermal fluctuations along the steps, equilibrium island shapes, and the angle of triangular facets provide quantitative information on the step formation energies. We find that a simple, systematic temperature dependence of the spatial anisotropy of the free energy of isolated steps is sufficient to account for the observed faceting of the B type steps. These results suggest that the proliferation of straight A step edges at lower temperature is caused by the energy of isolated A steps becoming vanishingly small. We show that rapid cooling of the samples leads to the nucleation and growth of a high density of vacancy islands on the surfaces. Based on these results, we propose a qualitative model for mass transport during stripe formation.

Comparison between competing interaction energies of AF correlation and long range coulomb repulsion in phase separation

We have made calculation to compare the energy changes in the formation of stripe phase due to AF interaction and long range part of Coulomb repulsion. We found the energy loss in AF correlation and energy gain in long range part of Coulomb repulsion are generally of the same scale.