Evidence of striped electronic phases in a structurally modulated superlattice.

The electronic properties of crystals can be manipulated by superimposing spatially periodic electric, magnetic or structural modulations. Long-wavelength modulations incommensurate with the atomic lattice are particularly interesting1, exemplified by recent advances in two-dimensional (2D) moiré materials2,3. Bulk van der Waals (vdW) superlattices4-8 hosting 2D interfaces between minimally disordered layers represent scalable bulk analogues of artificial vdW heterostructures and present a complementary venue to explore incommensurately modulated 2D states. Here we report the bulk vdW superlattice SrTa2S5 realizing an incommensurate one-dimensional (1D) structural modulation of 2D transition metal dichalcogenide (TMD) H-TaS2 layers. High-quality electronic transport in the H-TaS2 layers, evidenced by quantum oscillations, is made anisotropic by the modulation and exhibits commensurability oscillations paralleling lithographically modulated 2D systems9-11. We also find unconventional, clean-limit superconductivity in SrTa2S5 with a pronounced suppression of interlayer relative to intralayer coherence. The in-plane magnetic field dependence of interlayer critical current, together with electron diffraction from the structural modulation, suggests superconductivity12-14 in SrTa2S5 is spatially modulated and mismatched between adjacent TMD layers. With phenomenology suggestive of pair-density wave superconductivity15-17, SrTa2S5 may present a pathway for microscopic evaluation of this unconventional order18-21. More broadly, SrTa2S5 establishes bulk vdW superlattices as versatile platforms to address long-standing predictions surrounding modulated electronic phases in the form of nanoscale vdW devices12,13 to macroscopic crystals22,23.

Clean 2D superconductivity in a bulk van der Waals superlattice.

Advances in low-dimensional superconductivity are often realized through improvements in material quality. Apart from a small group of organic materials, there is a near absence of clean-limit two-dimensional (2D) superconductors, which presents an impediment to the pursuit of numerous long-standing predictions for exotic superconductivity with fragile pairing symmetries. We developed a bulk superlattice consisting of the transition metal dichalcogenide (TMD) superconductor 2H-niobium disulfide (2H-NbS2) and a commensurate block layer that yields enhanced two-dimensionality, high electronic quality, and clean-limit inorganic 2D superconductivity. The structure of this material may naturally be extended to generate a distinct family of 2D superconductors, topological insulators, and excitonic systems based on TMDs with improved material properties.

New approaches to nanoparticle sample fabrication for atom probe tomography.

Due to their unique properties, nano-sized materials such as nanoparticles and nanowires are receiving considerable attention. However, little data is available about their chemical makeup at the atomic scale, especially in three dimensions (3D). Atom probe tomography is able to answer many important questions about these materials if the challenge of producing a suitable sample can be overcome. In order to achieve this, the nanomaterial needs to be positioned within the end of a tip and fixed there so the sample possesses sufficient structural integrity for analysis. Here we provide a detailed description of various techniques that have been used to position nanoparticles on substrates for atom probe analysis. In some of the approaches, this is combined with deposition techniques to incorporate the particles into a solid matrix, and focused ion beam processing is then used to fabricate atom probe samples from this composite. Using these approaches, data has been achieved from 10-20 nm core-shell nanoparticles that were extracted directly from suspension (i.e. with no chemical modification) with a resolution of better than ± 1 nm.

Precision cutting and patterning of graphene with helium ions.

We report nanoscale patterning of graphene using a helium ion microscope configured for lithography. Helium ion lithography is a direct-write lithography process, comparable to conventional focused ion beam patterning, with no resist or other material contacting the sample surface. In the present application, graphene samples on Si/SiO2 substrates are cut using helium ions, with computer controlled alignment, patterning, and exposure. Once suitable beam doses are determined, sharp edge profiles and clean etching are obtained, with little evident damage or doping to the sample. This technique provides fast lithography compatible with graphene, with approximately 15 nm feature sizes.

Epitaxial Catalyst-Free Growth of InN Nanorods on c-Plane Sapphire.

We report observation of catalyst-free hydride vapor phase epitaxy growth of InN nanorods. Characterization of the nanorods with transmission electron microscopy, and X-ray diffraction show that the nanorods are stoichiometric 2H-InN single crystals growing in the [0001] orientation. The InN rods are uniform, showing very little variation in both diameter and length. Surprisingly, the rods show clear epitaxial relations with the c-plane sapphire substrate, despite about 29% of lattice mismatch. Comparing catalyst-free with Ni-catalyzed growth, the only difference observed is in the density of nucleation sites, suggesting that Ni does not work like the typical vapor-liquid-solid catalyst, but rather functions as a nucleation promoter by catalyzing the decomposition of ammonia. No conclusive photoluminescence was observed from single nanorods, while integrating over a large area showed weak wide emissions centered at 0.78 and at 1.9 eV.

Drug use, travel and HIV risk.

A study was conducted to examine the travel experiences of a community sample of 160 drug users and 44 non-users recruited as part of a study of HIV risk. Of the sample, 47% (96/204) reported intercity travel in the previous ten years. Results showed that men were more likely to travel than women, Anglos more than minorities, and young persons more than old. When travellers testing HIV-seropositive (n = 13) were compared with seronegative travellers, HIV-positive travellers reported more sex while travelling than HIV-negative persons, but virtually all of the difference reported involved sex with condoms. There were no significant differences in sex risk behaviours while travelling between drug users and non-drug users, or in sex risk behaviors between drug injectors and non-injectors. Travellers had fewer injection partners while travelling than they had while at home. There was also a significant difference in number of sex partners with whom a condom was not used, with fewer sex partners while travelling.

Triangulation and adolescent development in the U.S. and Japan.

Using an indirect measure of family structure, relationships between parents and adolescents were studied in 99 U. S. and 60 Japanese families. As two-person relationships tend toward instability under stress, a third person may be drawn in to stabilize the system. Parents, for example, may avoid the tension in the marital relationship by focusing together on an adolescent's problem, or pull the adolescent into a coalition with one parent. Either way the parents are said to have "triangled" the adolescent. In this study, a relationship is found between parents avoiding tension in their own relationship and their tendency to triangle an adolescent. Triangled daughters, in both cultures, had lower scores on ego development, supporting the hypothesis that such patterns can be detrimental to the adolescent's personal development. The discussion includes comments on cross-cultural research.

Relationships and diseases among drug users and nonusers.

Human immunodeficiency virus (HIV) and syphilis are relationship-based diseases that are typically transmitted by the cooperative activities (sex or drug injection) of two persons. A sample of 215 drug users and 52 sociodemographically matched nonusers was collected to examine the behaviors and relationships related to HIV and syphilis transmission. Results showed that, although drug users had more risk opportunities (more sex partners and, of course, more injection partners) than nonusers, actual sex risk behaviors (never using condoms) did not differ appreciably among drug users and nonusers or with opposite-sex partners and same-sex partners. The similarity of sexual risk was supported by the similar levels of syphilis between drug users and nonusers. The unique risk to drug users was drug injection, although drug users were found to engage in fewer risky injection behaviors (sharing of drug injection equipment) than the risky sexual behaviors in which all participants engaged. Although drug users interacted as frequently with partners as nonusers, nonuser relationships were longer lasting and emotionally closer.

Very-short-patch repair in Escherichia coli requires the dam adenine methylase.

Strains of Escherichia coli which lack the dam-encoded adenine methylase are mutators due to a reduction in the efficiency of postreplication mismatch repair. In this study, we show that Dam(-) strains are also defective in very-short-patch repair, the system which corrects T/G mismatches arising from the deamination of 5-methylcytosine. This defect is associated with decreased levels of Vsr, the endonuclease which initiates short-patch repair. We also show that production of the dcm-encoded cytosine methylase is unaffected in Dam(-) strains. Since the dcm and vsr genes are cotranscribed, the regulation of Vsr by Dam is probably posttranscriptional.

Biophysical properties, pharmacology, and modulation of human, neuronal L-type (alpha(1D), Ca(V)1.3) voltage-dependent calcium currents.

Voltage-dependent calcium channels (VDCCs) are multimeric complexes composed of a pore-forming alpha(1) subunit together with several accessory subunits, including alpha(2)delta, beta, and, in some cases, gamma subunits. A family of VDCCs known as the L-type channels are formed specifically from alpha(1S) (skeletal muscle), alpha(1C) (in heart and brain), alpha(1D) (mainly in brain, heart, and endocrine tissue), and alpha(1F) (retina). Neuroendocrine L-type currents have a significant role in the control of neurosecretion and can be inhibited by GTP-binding (G-) proteins. However, the subunit composition of the VDCCs underlying these G-protein-regulated neuroendocrine L-type currents is unknown. To investigate the biophysical and pharmacological properties and role of G-protein modulation of alpha(1D) calcium channels, we have examined calcium channel currents formed by the human neuronal L-type alpha(1D) subunit, co-expressed with alpha(2)delta-1 and beta(3a), stably expressed in a human embryonic kidney (HEK) 293 cell line, using whole cell and perforated patch-clamp techniques. The alpha(1D)-expressing cell line exhibited L-type currents with typical characteristics. The currents were high-voltage activated (peak at +20 mV in 20 mM Ba2+) and showed little inactivation in external Ba2+, while displaying rapid inactivation kinetics in external Ca2+. The L-type currents were inhibited by the 1,4 dihydropyridine (DHP) antagonists nifedipine and nicardipine and were enhanced by the DHP agonist BayK S-(-)8644. However, alpha(1D) L-type currents were not modulated by activation of a number of G-protein pathways. Activation of endogenous somatostatin receptor subtype 2 (sst2) by somatostatin-14 or activation of transiently transfected rat D2 dopamine receptors (rD2(long)) by quinpirole had no effect. Direct activation of G-proteins by the nonhydrolyzable GTP analogue, guanosine 5'-0-(3-thiotriphospate) also had no effect on the alpha(1D) currents. In contrast, in the same system, N-type currents, formed from transiently transfected alpha(1B)/alpha(2)delta-1/beta(3), showed strong G-protein-mediated inhibition. Furthermore, the I-II loop from the alpha(1D) clone, expressed as a glutathione-S-transferase (GST) fusion protein, did not bind Gbetagamma, unlike the alpha(1B) I-II loop fusion protein. These data show that the biophysical and pharmacological properties of recombinant human alpha(1D) L-type currents are similar to alpha(1C) currents, and these currents are also resistant to modulation by G(i/o)-linked G-protein-coupled receptors.

Partner concordance in reports of joint risk behaviors.

OBJECTIVE: To evaluate the accuracy of self reports on sexual and drug use behaviors. METHODS: Data from a network study of HIV transmission among a sample of drug users and nonusers are used to compare reports of sexual and drug use behaviors by partners who engaged in those behaviors. Partner concordance (self-report agreement between two people) was used as an estimate of validity. RESULTS: Results showed that persons are able to recall and report about 85% of their recent partners (15%-20% less for recent drug use partners). For relationships that were reported by both partners, a high degree of concordance existed about recent behaviors (83%-96%) and variable agreement about frequency (0.48

Normative influence on condom use in the personal networks of female cocaine smokers.

Attitudes-norms research (the theories of planned behavior and reasoned action) has been successful in accounting for many types of behavior change. One of the strengths of this approach has been to combine individual beliefs and normative influences in the explanation of behavior change. However, the conceptualization of normative influence in these theories makes very strong assumptions about self-awareness in the selection of normative referents. These assumptions are particularly problematic when applied to female cocaine smokers, who report frequent sex while under duress or while cognitively impaired. In this study the original conceptualization of normative influence and two alternatives (assuming emotion-based and interaction-based selection of normative referents) are operationalized to evaluate stage of change for condom use among women who are heavy crack cocaine users with multiple sex partners. Results show that stage of change for use of condoms with nonmain partners is best accounted for by interaction-based selection of normative referents.

Age and HIV risk in a national sample of injection drug and crack cocaine users.

A national sample of 25,106 out-of-treatment crack cocaine and injection drug users was examined to determine the effect of aging on HIV risk behavior. Injectors and crack cocaine users aged 55 and over were compared to those who were aged 18 to 54 years of age. Results showed that the behavior risk of older persons did not differ substantially from those reported by other age groups, and that injection risk rose steadily along with age for all ages represented in the sample. The authors conclude that older persons who use illegal drugs remain at elevated risk for HIV compared to the general population, and may be in need of more targeted intervention strategies.

Calcium channel beta subunit promotes voltage-dependent modulation of alpha 1 B by G beta gamma.

Voltage-dependent calcium channels (VDCCs) are heteromultimers composed of a pore-forming alpha1 subunit and auxiliary subunits, including the intracellular beta subunit, which has a strong influence on the channel properties. Voltage-dependent inhibitory modulation of neuronal VDCCs occurs primarily by activation of G-proteins and elevation of the free G beta gamma dimer concentration. Here we have examined the interaction between the regulation of N-type (alpha 1 B) channels by their beta subunits and by G beta gamma dimers, heterologously expressed in COS-7 cells. In contrast to previous studies suggesting antagonism of G protein inhibition by the VDCC beta subunit, we found a significantly larger G beta gamma-dependent inhibition of alpha 1 B channel activation when the VDCC alpha 1 B and beta subunits were coexpressed. In the absence of coexpressed VDCC beta subunit, the G beta gamma dimers, either expressed tonically or elevated via receptor activation, did not produce the expected features of voltage-dependent G protein modulation of N-type channels, including slowed activation and prepulse facilitation, while VDCC beta subunit coexpression restored all of the hallmarks of G beta gamma modulation. These results suggest that the VDCC beta subunit must be present for G beta gamma to induce voltage-dependent modulation of N-type calcium channels.

Estimated HIV risk among Hispanics in a national sample of drug users.

OBJECTIVE: To estimate and compare the HIV risks among three Hispanic subpopulations. METHODS: Chronic drug users participating in a nationwide intervention study on drug use were interviewed with regard to drug use and sexual behavior. HIV risk was estimated using information about individuals' HIV-relevant behaviors, the social context (i.e., city) in which such behaviors occur, and published estimates of HIV transmission for various risk behaviors. Multiple linear regression analysis was used to investigate differences in estimated HIV risk between Puerto Rican, Mexican-American, and Mexican drug users, accounting for sociodemographic factors, sexual preference, and geographic region. RESULTS: Puerto Ricans had significantly greater estimated overall HIV risk, estimated injection risk, and in general, significantly greater estimated sexual risk than Mexican Americans and Mexicans. No significant differences were found in any estimated risk between Mexican Americans and Mexicans in this sample. No significant differences were found among any of the subgroups for estimated risk from having anal sex while using a condom, or from having receptive anal sex without using a condom. CONCLUSIONS: These findings suggest that Puerto Ricans who use drugs experience a higher risk of HIV infection than other Hispanic drug users. Research is needed to identify which economic, social, and cultural components account for this increased risk.

Modeling HIV risk.

OBJECTIVE: To provide HIV risk estimates for specific local population subgroups using an HIV risk index combining HIV risk behaviors, prevalence rates, and transmission probabilities. METHODS: A sample of 270 individuals was studied. Respondents described sexual and injection risk behaviors and partners in the previous 30 days. An HIV risk index was computed for each individual, combining reported sexual and injecting risk behaviors, HIV prevalence estimates for partners and HIV transmission probabilities for each of the risk behaviors. Partner HIV prevalence rates were estimated from a national sample, and HIV transmission probabilities were obtained from previously published studies. RESULTS: Projected risk estimates were found to differ a great deal within major demographic categories. Highest 10-year risk was found among African-American male gay injectors (72%) and the lowest among white male heterosexual noninjectors (<.01%). CONCLUSIONS: The method used here for estimating HIV risk can be calculated for specific at-risk population subgroups of any size and composition, including individuals. By understanding which specific subpopulations are at risk, specific interventions and public health campaigns may be better targeted.

The motivation for drug abuse treatment: testing cognitive and 12-step theories.

The purpose of this paper is to evaluate two models of behavior change: cognitive theory and 12-step theory. Research subjects were drawn from three separate, but parallel, samples of adults. The first sample consisted of out-of-treatment chronic drug users, the second consisted of drug users who had applied for treatment at a publicly funded multiple-provider drug treatment facility, and the third consisted of drug users who had applied for treatment at an intensive outpatient program for crack cocaine users. Cognitive theory was supported. Study participants applying for drug abuse treatment reported a higher level of perceived problem severity and a higher level of cognitive functioning than out-of-treatment drug users. Two hypotheses drawn from 12-step theory were not supported. Treatment applicants had more positive emotional functioning than out-of-treatment drug users, and one treatment-seeking sample had higher self-esteem.

The social ecology of drug use: a factor analysis of an urban environment.

Drug use is related to the environment in which drug users live. Using 32 economic, housing, crime, and health variables for 1990 census tracts in Houston, Texas, this study found four identifiable factors: social disorganization, economic success, threat of violence, and chronic disease. Narcotic offenses loaded strongly on one and weakly on two of the factors; this variable did not load on the economic success factor. These results suggest that a textured look at drug use environments finds them to be multidimensional. Urban dwellers must deal with the independent effects of social disorganization, economic conditions, chronic disease, and violence.

Barriers to social services for HIV-infected urban migrators.

Anecdotal accounts suggest that residency requirements often lead to denial of services at a time when HIV positive migrators are most in need of these services. However, this suspicion has never been empirically tested. Using needs assessment data collected for Harris County, Texas, this article examines eligibility and knowledge barriers faced by HIV positive recent migrators into Harris County. Results indicated that migration into the county was a significant predictor of failure to receive government-administered basic services such as food services but was not a significant predictor of failure to receive community based organization (CBO)-administered "specialized" services targeted specifically to HIV positive individuals. Results also indicated that migration was associated with knowledge barriers for all types of services.

Implications of managed care in a publicly funded health care delivery system.

In the light of federal cuts in the Medicaid budget, the Medicaid-funded health delivery system is under severe cost pressures, especially in publicly funded institutions. In the private health insurance industry, managed care innovations have successfully restrained costs. For publicly funded institutions to remain viable, managed care contracts must be implemented for the Medicaid system as well. Managed care holds promise for reducing costs as well as reducing many of the current barriers to quality health care for the indigent.