Transition from an electron solid to the sequence of fractional quantum Hall states at very low Landau level filling factor

At low Landau level filling of a two-dimensional electron system, typically associated with the formation of an electron crystal, we observe local minima in Rxx at filling factors nu = 2/11, 3/17, 3/19, 2/13, 1/7, 2/15, 2/17, and 1/9. Each of these developing fractional quantum Hall (FQHE) states appears only above a filling-factor-specific temperature. This can be interpreted as the melting of an electron crystal and subsequent FQHE liquid formation. The observed sequence of FQHE states follows the series of composite fermion states emanating from nu = 1/6 and nu = 1/8.

Observation of quantized Hall drag in a strongly correlated bilayer electron system

The frictional drag between parallel two-dimensional electron systems has been measured in a regime of strong interlayer correlations. When the bilayer system enters the excitonic quantized Hall state at total Landau level filling factor nu(T) = 1, the longitudinal component of the drag vanishes but a strong Hall component develops. The Hall drag resistance is observed to be accurately quantized at h/e(2).

Density-induced interchange of anisotropy axes at half-filled high Landau levels

We observe density-induced 90 degrees rotations of the anisotropy axes in transport measurements at half-filled high Landau levels in the two dimensional electron system, where stripe states are proposed ( nu = 9/2, 11/2, etc.). Using a field effect transistor, we find the transition density to be 2.9x10(11) cm(-2) at nu = 9/2. Hysteresis is observed in the vicinity of the transition. We construct a phase boundary in the filling factor magnetic field plane in the regime 4.4<nu<4.6. An in-plane magnetic field applied along either anisotropy axis always stabilizes the low density orientation of the stripes.

Laparoscopic splenectomy has become the gold standard in children

Splenectomy is frequently required in children with various hematologic disorders. The reported advantages of laparoscopic splenectomy (LS) include less pain, shorter hospital stay, and improved cosmesis. This report evaluates the outcome of children undergoing LS at a single children's facility. One hundred twelve children underwent LS by the lateral approach between August 1995 and February 2001. Indications for LS were hereditary spherocytosis in 58, idiopathic thrombocytopenic purpura in 21, sickle cell disease in 19, and other conditions in 14. LS alone was completed in 89 children and LS and cholecystectomy (LSC) in 20. Three required conversion to open splenectomy. Accessory spleens were identified in 19. Complications included ileus (four), acute chest syndrome (four), bleeding (two), pneumonia (one), and diaphragm perforation (one). There was no mortality. An accessory spleen was missed in one child with recurrent anemia. Average operative time for LS was 106 minutes and for LSC 135 minutes. Operative time for LS decreased with experience but the difference was not significant. Average length of stay was 1.51 days (range 1-11) and was longer in sickle cell disease (2.47 days) versus hereditary spherocytosis (1.29 days) and idiopathic thrombocytopenic purpura (1.16 days). We conclude that LS is safe and effective in children with hematologic disorders and is associated with minimal morbidity, zero mortality, and a short length of stay.

Laparoscopic Splenectomy Has Become the Gold Standard in Children

Splenectomy is frequently required in children with various hematologic disorders. The reported advantages of laparoscopic splenectomy (LS) include less pain, shorter hospital stay, and improved cosmesis. This report evaluates the outcome of children undergoing LS at a single children's facility. One hundred twelve children underwent LS by the lateral approach between August 1995 and February 2001. Indications for LS were hereditary spherocytosis in 58, idiopathic thrombocytopenic purpura in 21, sickle cell disease in 19, and other conditions in 14. LS alone was completed in 89 children and LS and cholecystectomy (LSC) in 20. Three required conversion to open splenectomy. Accessory spleens were identified in 19. Complications included ileus (four), acute chest syndrome (four), bleeding (two), pneumonia (one), and diaphragm perforation (one). There was no mortality. An accessory spleen was missed in one child with recurrent anemia. Average operative time for LS was 106 minutes and for LSC 135 minutes. Operative time for LS decreased with experience but the difference was not significant. Average length of stay was 1.51 days (range 1–11) and was longer in sickle cell disease (2.47 days) versus hereditary spherocytosis (1.29 days) and idiopathic thrombocytopenic purpura (1.16 days). We conclude that LS is safe and effective in children with hematologic disorders and is associated with minimal morbidity, zero mortality, and a short length of stay.

Insulating and fractional quantum hall states in the first excited Landau level

The observation of new insulating phases of two-dimensional electrons in the first excited Landau level is reported. These states, which are manifested as reentrant integer quantized Hall effects, exist alongside well-developed even-denominator fractional quantized Hall states at nu = 7/2 and 5/2 and new odd-denominator states at nu = 3+1/5 and 3+4/5.

Experimental demonstration of Fermi surface effects at filling factor 5/2

Using small wavelength surface acoustic waves (SAW) on ultrahigh mobility heterostructures, Fermi surface properties are detected at 5/2 filling factor at temperatures higher than those at which the quantum Hall state forms. An enhanced conductivity is observed at 5/2 by employing sub-0.5-microm SAW, indicating a quasiparticle mean-free path substantially smaller than that in the lowest Landau level. These findings are consistent with the presence of a filled Fermi sea of composite fermions, which may pair at lower temperatures to form the 5/2 ground state.

Tunneling spectroscopy of the elementary excitations in a one-dimensional wire

The collective excitation spectrum of interacting electrons in one dimension has been measured by controlling the energy and momentum of electrons tunneling between two closely spaced, parallel quantum wires in a GaAs/AlGaAs heterostructure while measuring the resulting conductance. The excitation spectrum deviates from the noninteracting spectrum, attesting to the importance of Coulomb interactions. An observed 30% enhancement of the excitation velocity relative to noninteracting electrons with the same density, a parameter determined experimentally, is consistent with theories on interacting electrons in one dimension. In short wires, 6 and 2 micrometers long, finite size effects, resulting from the breaking of translational invariance, are observed.

Current-path properties of the transport anisotropy at filling factor 9/2

To establish the presence and orientation of the proposed striped phase in ultra-high-mobility 2D electron systems at nu = 9/2, current path transport properties are determined by varying the separation and alignment of current and voltage contacts. Contacts aligned orthogonal to the proposed intrinsic striped phase produce voltages consistent with current spreading along the stripes; current driven along the proposed stripe direction results in voltages consistent with channeling along the stripes. Direct comparison is made to current spreading/channeling properties of artificially induced 1D charge modulation systems, which determines the 9/2 stripe direction.

Anisotropic disorder in high-mobility 2D heterostructures and its correlation to electron transport

Surface morphology of high-mobility heterostructures is examined and correlated with dc transport. All samples examined show evidence of lines in the [11-0] direction with roughness ranging from small-amplitude features to severe anisotropic ridges. Transport in these samples is consistent with that in samples having artificially induced 1D charge modulations. The native surface properties reflect a prevalent, anisotropic disorder affecting 2D electron conduction. Importantly, the native lines are orthogonal to the stripes theoretically proposed to explain high Landau level transport anisotropies.

Portal venous air: the poor prognosis persists

BACKGROUND/PURPOSE

The prognostic importance of portal vein air (PVA) in babies with necrotizing enterocolitis (NEC) has been controversial. This study compares the outcome in babies with NEC and PVA treated surgically versus those with medical management.

METHODS

Forty neonates in the neonatal intensive care unit (NICU; 1995 through 1999) had (PVA) during their hospitalization. Babies were analyzed for gestational age (GA), birth weight (BW), and survival after operative versus medical management.

RESULTS

The average GA was 26 weeks, average BW was 1,173 g. Twenty-three patients (57.5%) tolerated full feedings and 8 (20%) partial feedings at diagnosis. All 40 babies required intubation at birth with 23 (57.5%) requiring reintubation with onset of PVA. In all cases, PVA was present within 24 hours of onset of abdominal distension, feeding intolerance, or heme-positive stools. Two cases of PVA "resolved" only to recur later in the patients' courses. Thirty-two patients (80%) manifested pneumatosis intestinalis on abdominal radiographs, and 8 (20%) had perforations. Acidosis was present in 25 (63%) patients, and vasopressor support (dopamine) was required in 15 (38%), with 2 patients requiring support only preoperatively. Initial management consisted of bowel rest, fluid resuscitation, orogastric decompression, and broad-spectrum antibiotics. Operation was performed in 31 (78%). Seventeen underwent resection with ostomy formation with 6 deaths and 11 survivors. Four underwent resection using the clip and drop back method, with one death and 3 requiring an ostomy at second look laparotomy. Ten had NEC totalis and closure of the abdomen only. Overall operative mortality rate was 17 of 31 (54%). Nine seemingly stable patients were treated nonoperatively. Six had progressed disease and died before salvage laparotomy could be performed, whereas 3 (33%) survived without further therapy.

CONCLUSIONS

PVA has been a relative indication for operation. This view has been challenged by the survival of some patients without laparotomy. Although nonoperative therapy seems appealing in hemodynamically stable patients without acidosis, our data confirm the poor prognosis of infants with PVA and NEC.

The utility of lung biopsy in recipients of stem cell transplantation

BACKGROUND/PURPOSE

Pulmonary infiltrates in recipients of stem cell transplantation often present as diagnostic dilemmas. Although lung biopsy may establish the diagnosis of parenchymal disease, it remains unclear whether such a procedure results in a significant change in the patient's treatment and outcome. This study evaluates the efficacy of lung biopsy in recipients of stem cell transplantation.

METHODS

The medical records of 15 stem cell transplant recipients who underwent 18 lung biopsies were reviewed. The indications for stem cell transplantation were leukemia in 10 patients, lymphoma in 2, histiocytosis in 1, neuroblastoma in 1, and Ewing's sarcoma in 1. The results of the lung biopsies were correlated to the clinical management and outcomes.

RESULTS

The overall mortality rate was 67% (10 patients). Eight of the 9 patients who required mechanical ventilatory support at the time of lung biopsy died. The pathologic diagnoses were pneumonitis in 6 biopsies, fibrosis in 6, brochiolitis obliterans organizing pneumonia in 3, hemorrhage in 2, and infarction in 1. Therapy was changed in 1 patient who improved after a course of steroids for bronchiolitis obliterans organizing pneumonia. Lung biopsy cultures were positive in 6 patients but rarely resulted in changes in antibiotic therapy.

CONCLUSIONS

Results of very few lung biopsies performed in stem cell transplant recipients redirected therapy. Furthermore, the ultimate outcome of these patients were not improved by the results of lung biopsies.

Observation of a linearly dispersing collective mode in a quantum hall ferromagnet

Double-layer two-dimensional electron systems can exhibit a fascinating collective phase believed to display both quantum ferromagnetism and excitonic superfluidity. This unusual phase has recently been found to exhibit tunneling phenomena reminiscent of the Josephson effect. A key element of the theoretical understanding of this bizarre quantum fluid is the existence of linearly dispersing Goldstone collective modes. Using the method of tunneling spectroscopy, we have demonstrated the existence of these modes. We find the measured velocity to be in reasonable agreement with theoretical estimates.

Spectroscopic evidence for the localization of Skyrmions near nu = 1 as T --> 0

Optically pumped nuclear magnetic resonance measurements of 71Ga spectra were carried out in an n-doped GaAs/Al(0.1)Ga0.9As multiple quantum well sample near the integer quantum Hall ground state nu = 1. As the temperature is lowered (down to T approximately 0.3 K), a "tilted plateau" emerges in the Knight shift data, which is a novel experimental signature of quasiparticle localization. The dependence of the spectra on both T and nu suggests that the localization is a collective process. The frozen limit spectra appear to rule out a 2D lattice of conventional Skyrmions.

Four-terminal resistance of a ballistic quantum wire

The electrical resistance of a conductor is intimately related to the relaxation of the momentum of charge carriers. In a simple model, the accelerating force exerted on electrons by an applied electric field is balanced by a frictional force arising from their frequent collisions with obstacles such as impurities, grain boundaries or other deviations from a perfect crystalline order. Thus, in the absence of any scattering, the electrical resistance should vanish altogether. Here, we observe such vanishing four-terminal resistance in a single-mode ballistic quantum wire. This result contrasts the value of the standard two-probe resistance measurements of h/2e2 approximately 13 kOmega. The measurements are conducted in the highly controlled geometry afforded by epitaxial growth onto the cleaved edge of a high-quality GaAs/AlGaAs heterostructure. Two weakly invasive voltage probes are attached to the central section of a ballistic quantum wire to measure the inherent resistance of this clean one-dimensional conductor.

Observation of multiple magnetorotons in the fractional quantum Hall effect

Magnetorotons in the dispersions of collective gap excitation modes of fractional quantum Hall liquids are measured in resonant inelastic light scattering experiments. Two deep magnetoroton minima are observed at nu = 2/5, while a single deep minimum is resolved at nu = 1/3. The observations are the first evidence of multiple roton minima in gap excitations of the quantum liquids. The results support Chern-Simons and composite fermion calculations that predict multiple roton minima for states with nu>1/3.

Resonant tunneling into a biased fractional quantum Hall edge

We observe resonant tunneling into a voltage biased fractional quantum Hall effect (FQHE) edge, using atomically sharp tunnel barriers unique to cleaved-edge overgrown devices. The resonances demonstrate different tunnel couplings to the metallic lead and the FQHE edge. Weak coupling to the FQHE edge produces clear non-Fermi liquid behavior with a sixfold increase in resonance area under bias arising from the power law density of states at the FQHE edge. A simple device model uses the resonant tunneling formalism for chiral Luttinger liquids to successfully describe the data.

Geometry-dependent dephasing in small metallic wires

Temperature dependent weak localization is measured in metallic nanowires in a previously unexplored size regime down to width w = 5 nm. The dephasing time, tau(phi), shows a low temperature T dependence close to quasi-1D theoretical expectations (tau(phi) approximately T(-2/3)) in the narrowest wires, but exhibits a relative saturation as T-->0 for wide samples of the same material, as observed previously. As only sample geometry is varied to exhibit both suppression and divergence of tau(phi), this finding provides a new constraint on models of dephasing phenomena.

Pectus excavatum repair: experience with standard and minimal invasive techniques

BACKGROUND/PURPOSE

The Nuss procedure is a minimally invasive pectus repair that helps avoid cartilage resection and osteotomy. This report compares outcomes in patients undergoing a standard pectus repair to patients with the Nuss procedure.

METHODS

One hundred three children (ages 5 to 20 years) with severe pectus excavatum underwent repair. Patients were evaluated for type of repair performed, associated anomalies, cardiopulmonary function, operating time, analgesia requirements, complications, length of hospital stay, hospital and operative charges, and cosmetic result. Statistical analysis was performed using the Mann-Whitney rank sum test.

RESULTS

There were 68 patients (average age, 12.6 years) in the standard group and 35, (average age, 9.5 years) in the Nuss group. Associated anomalies were found in 6 standard group and 2 Nuss group patients. Average operating time in Nuss was 3.3 hours and in open procedures, 4.7 hours. Postoperative complications occurred in 13 (20%) standard repair patients and 15 (43%) after the Nuss. In the standard group, 14 patients received intrathecal and 3 received epidural analgesia, while 35 (52%) required an intravenous patient-controlled anesthetic device (PCA; average, 1.8 days). In the Nuss group, 25 patients (71%) received epidural anesthesia (average, 3 days), and 31 (89%) utilized PCA (average 3.8 days). Four (6%) standard patients and 8 Nuss patients (29%) required reoperation. Length of stay averaged 4.0 days (range 2 to 30) in the standard group and 4.8 days (range, 2 to 11) in the Nuss group. Average operating room charge was $8,325 in the standard group and $9,480 in the Nuss group. Average hospital charge was $4,137 for the standard patient and $4,044 for the Nuss group. Analgesic requirements and length of hospital stay were increased (P <.05). The complication rate and operative and hospital charges were similar between groups.

CONCLUSIONS

Although the Nuss repair is associated with shorter operating time, smaller incisions, and less dissection, early results indicate few other advantages. Drawbacks of the Nuss procedure include high complication and reoperation rates and lack of efficacy in older teenagers and those with connective tissue disorders. Long-term follow-up will be necessary to determine final cosmetic and functional outcomes and define the overall risks and benefits of this procedure as compared with the standard technique.

Plateau Behavior in the Chiral Luttinger Liquid Exponent

The current-voltage power law exponent, alpha, for electron tunneling into chiral Luttinger liquids at the fractional quantum Hall edge is found to exhibit a plateaulike structure at alpha close to 3 as the filling factor, nu, is varied. The presence of a plateau near alpha = 3 strongly suggests a fundamental connection between alpha and the structure of the underlying quantum ground states associated with the robust incompressible nu = 1/3 Hall fluid. However, the position in the inverse filling factor where the plateau occurs can vary between samples and appears shifted to values higher than expected from theory.

Inflammatory pseudotumor of the alimentary tract: clinical and surgical experience

BACKGROUND/PURPOSE

Initially described in 1937, inflammatory pseudotumor (IPT) inflammatory myofibroblastic tumor (IMT) or plasma cell granulomas are synonymous for an inflammatory solid tumor that contains spindle cells, myofibroblasts, plasma cells, and histocytes. Common sites of presentation include lung, mesentary, liver, and spleen; intestinal presentations are rare, and the etiology remains obscure. This report details the clinical and surgical experiences in 4 children with alimentary tract IPT at a single institution.

METHODS

A retrospective chart review was conducted of pediatric patients with the pathologic diagnosis of IPT.

RESULTS

Between 1990 and 1999, 4 patients (4 girls, ages 5 to 15 years) were identified with gastrointestinal tract origins of IPT. Symptoms at presentation included anemia (n = 4), intermittent abdominal pain (n = 3), fever (n = 3), weight loss (n = 2), diarrhea (n = 2), dysphagia (n = 1). Two patients had comorbid conditions of juvenile rheumatoid arthritis and mature B cell lymphoma. Three of 4 patients had elevated sedimentation rates. The sites of origin were the gastroesophageal junction, the colon, the rectum, and the appendix, with the referral diagnosis achalasia, perforated appendix, inflammatory bowel disease, and recurrent lymphoma, respectively. All were treated with aggressive surgical resection, and 3 girls have had no recurrences since the initial surgery. One patient had 3 recurrences within 8 months of presentation; she remains disease free 8 years later.

CONCLUSIONS

IPT, although rare in the gastrointestinal tract, mimics more common problems. Successful surgical management is possible even in cases of multiple recurrences.

Gastroschisis: a plea for risk categorization

BACKGROUND

The incidence of gastroschisis has increased in the past decade. A differing clinical course between "complex" (those with atresias, perforation, or stenosis) and "simple" cases has prompted a review of risk assessment factors.

METHODS

A retrospective chart review was conducted of 103 infants with gastroschisis over 5 years (1992 to 1997).

RESULTS

Of 103 infants, 52 were girls and 51 were boys. Seventy-one infants (69%) had a simple defect, and 32 (31%) were complex. The simple group had an average estimated gestational age of 37.5 weeks (range, 26 to 40), and a birth weight of 3.0 kg (range, 1.7 to 3.8). A total of 71% underwent primary repair, whereas 29% required a silo. Mechanical ventilation averaged 6.8 days (range, 1 to 19). Enteral feedings were initiated at 15 days (range, 3 to 27) with full enteral intake achieved by 22.4 days (range, 5 to 40). Three infants required home parenteral nutrition. The average length of stay (LOS) was 26.4 days (range, 10 to 57). Complications occurred in 26 infants (36%), including intravenous catheter sepsis (n = 15), pneumatosis (n = 2), pneumonia (n = 1), bowel obstruction (n = 7), wound infection (n = 5), and SVC thrombosis (n = 1). Survival rate was 100%. Thirty-two infants had complex defects; 27 patients had atresias, stenosis, or perforations; and 3 had volvulus. The average estimated gestational age was 34 weeks (range, 26 to 38), and birth weight was 2.0 kg (range, 0.9 to 4.0). Primary repair was performed in 65% and silo placement in 35%. Mechanical ventilation was required for 22.3 days (range, 2 to 14). Enteral feedings were initiated at 22.5 days (range, 6 to 56) with full feedings achieved at 50 days (range, 21 to 113). Fourteen infants required home total parenteral nutrition (TPN). The LOS was 85.4 days (range, 24 to 270). A total of 47 complications occurred in the complex group including catheter sepsis (n = 15), short bowel syndrome (n = 7), pneumatosis (n = 3), bowel obstruction (n = 4), pneumonia (n = 2), superior vena cava thrombosis (n = 1), enterocutaneous fistula (n = 1), and 9 deaths (28% mortality rate).

CONCLUSIONS

These data indicate gastroschisis can be divided into low-risk (simple) and high-risk (complex) categories. These 2 groups have significant differences in clinical behavior, postsurgical complications, LOS, and mortality rate (0 v 28%). Although the overall survival rate was 91% (94 of 103), parents, referring physicians, and insurers must be made aware of the impact of risk categorization on the estimated cost, LOS, and outcomes.

Reorientation of anisotropy in a square well quantum hall sample

We have measured magnetotransport at half-filled high Landau levels in a quantum well with two occupied electric subbands. We find resistivities that are isotropic in perpendicular magnetic field but become strongly anisotropic at nu = 9/2 and 11/2 on tilting the field. The anisotropy appears at an in-plane field, B(ip) approximately 2.5 T, with the easy-current direction parallel to B(ip) but rotates by 90 degrees at B(ip) approximately 10 T and points now in the same direction as in single-subband samples. This complex behavior is in quantitative agreement with theoretical calculations based on a unidirectional charge density wave state model.

Localization in artificial disorder: two coupled quantum dots

Using single electron capacitance spectroscopy, we study electron additions in quantum dots containing two potential minima separated by a shallow barrier. Analysis of the addition spectra in the magnetic field allows us to distinguish between electrons delocalized over the entire dot and those localized in either of the potential minima. We demonstrate that a high magnetic field abruptly splits up a low-density droplet into two smaller fragments, each residing in a potential minimum. An unexplained cancellation of electron repulsion between electrons in these fragments gives rise to paired electron additions.