bluuebunny/crossref_metadata_2025 huggingface dataset:
Evolution of Arctic sea ice concentration trends and the role of atmospheric circulation forcing, 1979–2007
Authors: Deser Clara , Teng Haiyan
Published: January 2008

The retreat of Arctic sea ice in recent decades is a pre‐eminent signal of climate change. What role has the atmospheric circulation played in driving the sea ice decline? To address this question, we document the evolution of Arctic sea ice concentration trends during the period January 1979–April 2007 in light of changing atmospheric circulation conditions, in particular an upward trend in the wintertime Northern Annular Mode during the first half of the record and a downward trend during the second half. The results indicate that concurrent atmospheric circulation trends contribute to forcing winter and summer sea ice concentration trends in many parts of the marginal ice zone during both periods. However, there is also an emerging signal of overall Arctic sea ice decline since 1979 in both winter and summer that is not directly attributable to a trend in the overlying atmospheric circulation.
Multiple thermal fronts near the Patagonian shelf break
Authors: Franco Bárbara C. , Piola Alberto R. , Rivas Andrés L. , Baldoni Ana , Pisoni Juan P.
Published: January 2008

Eighteen year (1985–2002) sea surface temperature (SST) data are used to study the intraseasonal variability of the Patagonian shelf break front (SBF) in the SW South Atlantic Ocean between 39° and 44°S. The cross‐shelf break SST gradients reveal distinct, previously undocumented thermal fronts located both, offshore and inshore of the SBF. Throughout the year the main SBF, identified as a band of negative SST gradient maxima (relatively strong offshore temperature decrease), forms a persistent feature located closed to the 200 m isobath, while two distinct negative gradient maxima are located inshore and offshore of this location. Daily SST images reveal the presence of three branches of cold waters whose edges delineate the above mentioned fronts. The two offshore branches closely follow lines of constant potential vorticity (f/h) and appear to be associated with the Malvinas Current, while a third branch, located further onshore, is not steered by the bottom topography. South of 40°S the onshore branch forms a quasi permanent front parallel to the SBF.
Magnetism of intermediate hydromaghemite in the transformation of 2‐line ferrihydrite into hematite and its paleoenvironmental implications
Authors: Liu Qingsong , Barrón Vidal , Torrent José , Eeckhout Sigrid G. , Deng Chenglong
Published: January 2008

A maghemite‐like phase referred to here as “hydromaghemite” was obtained as an intermediate product in the hydrothermal transformation of phosphated 2‐line ferrihydrite into hematite. In this study, we used magnetic and non‐magnetic (e.g., X‐ray Absorption Near Edge Structure, XANES) techniques in combination to characterize a series of the intermediate products obtained in the aging of phosphated ferrihydrite (P/Fe atomic ratio = 0.03) at 150°C for 120 days. Particle size calculated from both the specific surface area and average unblocking temperature increased with time. XANES spectra revealed the presence of some tetrahedrally coordinated iron, which is consistent with the formation of hydromaghemite (the dominant magnetic phase in the intermediate products). Thus grain size in newly formed hydromaghemite particles increased with time from the initial values in the superparamagnetic region to others in the single‐domain region. Further transformation of hydromaghemite into hematite, which was complete by day 120, was probably due to hydromaghemite becoming unstable relative to hematite when the surface to volume ratio fell below a given threshold. The relationships between pedogenically produced maghemite and hematite contents in various soils and paleosols suggest that the ferrihydrite → hydromaghemite → hematite transformation may constitute a major pathway accounting for the magnetic enhancement in many soils.
What causes the excessive response of clear‐sky greenhouse effect to El Niño warming in Community Atmosphere Models?
Authors: Zhang Tao , Sun De‐Zheng
Published: January 2008

To diagnose the causes of an excessive response of the clear‐sky greenhouse effect to El Niño warming in the Community Atmosphere Models (CAMs), the response of both water vapor and temperature to El Niño warming in the models is examined as a function of height. The percentage response of water vapor to El Niño warming in the models is considerably stronger than the response in the NCEP reanalysis in the middle and upper troposphere (700–300 mbar). The maximum discrepancy with NCEP data at 500 mbar reaches 18%/K in CAM3. The discrepancy in the temperature response between the models and NCEP data at all tropospheric levels is within 0.3 K/K, with the maximum discrepancy occurring in the immediate neighborhood of 600 mbar. The comparison between the models and ERA‐40 reanalysis leads to the similar results. Employing a radiative model, we have calculated the contributions of the excessive water vapor response in the middle and upper troposphere and the contributions from the differences in the lapse rate response to the discrepancies seen in the clear‐sky greenhouse effect. The results confirm that the main cause of the excessive response of the clear‐sky greenhouse effect is an excessive response of water vapor in the middle and upper troposphere. The excessive response of upper tropospheric water vapor is found to be accompanied with an excessive response in the upper cloud cover and vertical motion. Biases in both phases of ENSO contribute to these excessive responses to ENSO.
Concepts for the development of a quantitative theory of clonal selection and class regulation using lessons from the original
Authors: Hodgkin Philip D
Published: January 2008

When Burnet wrote his modification of Jerne's theory he simply wrote—‘It is postulated that when antigen‐(receptor) contact takes place on the surface of a lymphocyte the cell is activated to settle in an appropriate tissue… and there undergo proliferation to produce a variety of descendants’.1 But studies of ‘activation’ and ‘proliferation’ following receptor contact have turned out to be surprisingly complex—much more complex than the theory apparently requires. It is timely, on this 50th anniversary, to take a closer look at the complexity and highlight some lesser‐known general features of the clonal selection theory and of B cell activation that can be useful in formulating a quantitative version of this theory that includes class regulation and cellular heterogeneity.
Antihydrogen Makes a Fleeting Debut
Authors: Levi Barbara Goss
Published: January 2008

Like a high-profile wedding of movie stars, the recent mating of positrons with antiprotons at CERN commanded worldwide attention. But even by Hollywood standards the marriage was short-lived: The antihydrogen lasted a mere 37 nanoseconds before the positrons were annihilated by collisions with electrons in the detector. That time was too short and the number of antiatoms formed was too few for researchers to conduct high-precision comparisons between hydrogen and its antimatter counterpart. Such studies would tell us whether nature obeys CPTinvariance, that is, whether phenomena remain invariant under the simultaneous operations of charge conjugation, parity inversion and time reversal, and whether matter and antimatter feel the same pull of gravity. The recent experiment at CERN fired imaginations by demonstrating the reality of antihydrogen, but antiatoms must be produced nearly at rest, rather than in flight as in the CERN experiment, before they can be studied in detail.
Researchers Vie to Achieve a Quantum-Dot Laser
Authors: Levi Barbara Goss
Published: January 2008

If semiconductor quantum-well lasers are so successful, shouldn't quantum-wire or quantum-dot lasers be even better? That's a question that many researchers have set out to answer. Quantum wells confine the charge carriers in one dimension; wires confine them in two; and dots, in three. (Quantum dots are clusters of semiconductor material with a few thousand atoms.) As a result of their confinement, the charge carriers occupy discrete energy levels similar to those of electrons in an atom. Energy pumped into the system raises the charge carriers from one energy level to the next; none of it goes to random motion, because there are no other degrees of freedom. Thus, one expects any lasing from the dots to occur with high efficiency and at lower threshold current than in either quantum wells or quantum wires.
Evidence Accumulates for Unusual Behavior in Underdoped High-Tc Superconductors
Authors: Levi Barbara Goss
Published: January 2008

As you cool a conventional superconductor, it acts as a metal until it reaches the critical temperature Tc, below which it rapidly goes superconducting. But some of the high-temperature, copper oxide materials—in particular, the underdoped ones, whose densities of charge carriers are below that which gives the highest Tc—seem to be preparing for the transition to a superconducting state well before they reach Tc. Experiment after experiment on underdoped materials at temperatures above Tc has revealed some kind of crossover or change in behavior, which some feel is related to the eventual onset of superconductivity at lower temperatures. When the temperature does reach Tc, the material goes superconducting, but some of its properties exhibit relatively little additional discontinuity in behavior.
Dispersion of magnetic plasmon polaritons in perforated trilayer metamaterials
Authors: Li Tao , Wang Shu-Ming , Liu Hui , Li Jia-Qi , Wang Fu-Ming , Zhu Shi-Ning , Zhang Xiang
Published: January 2008

Multiple magnetic plasmon polariton (MPP) modes were recently explored in a well-known system—metal∕insulator∕metal layered structure perforated with periodic holes array [Appl. Phys. Lett. 90, 251112 (2007)]. Now, we consequently study the dispersions of the MPP modes in similar systems with rectangular hole arrays by analyzing the detailed optical transmittances at oblique incidences. Significantly, our results provide a definite polarization-dependent dispersion property of MPP modes: strong dispersive MPP(±1,±1) modes with the degeneration broken up and a remained degenerate MPP(0,±1) mode for s-polarization and almost flat dispersions of all MPP modes for p-polarization. Such a phenomenon is explained by the different coupling intensities among the artificial “magnetic atoms.” This finding helps us to make a deeper understanding on the artificial magnetic excitations in this trilayer metamaterial.
Magnetic properties and microstructure of directly quenched Nd9.5Fe75.5−xMxB15 (M=Mo, Nb, Ta, Ti, V, and Zr; x=0–4) bulk magnets
Authors: Chang H. W. , Shih M. F. , Chang C. W. , Hsieh C. C. , Fang Y. K. , Chang W. C. , Sun A. C.
Published: January 2008

Magnetic properties and microstructure of directly quenched Nd9.5Fe75.5−xMxB15 (M=Mo, Nb, Ta, Ti, V, and Zr; x=0–4) bulk magnets in rod form with a diameter of 0.7mm have been studied. The substitution of the selected refractory elements for Fe in the Nd9.5Fe75.5MxB15 alloy can suppress the formation of the soft phases, including Nd2Fe23B3 and one unknown phase, leading to the presence of a large amount of the Nd2Fe14B phase. Besides, the grain size at the core side of the rods is refined from a micrometer scale for a ternary magnet to 50–120nm for the magnets with refractory element substitution. As a result, the magnetic properties are improved remarkably from Br=5.0kG, Hci=1.0kOe, and (BH)max=1.7MGOe for a ternary magnet to Br=5.7–6.5kG, Hci=2.0–14.6kOe, and (BH)max=3.1–8.7MGOe for refractory element substituted magnets. The optimal magnetic properties of Br=6.5kG, Hci=10.3kOe, and (BH)max=8.7MGOe can be achieved in the Nd9.5Fe72.5Ti3B15 magnet, while the highest coercivity of Hci=14.6kOe, with Br=5.9kG and (BH)max=7.4MGOe, is obtained in the Nd9.5Fe71.5Nb4B15 magnet.
A comparative study of the ΔH(M,ΔM) method reliability for square and triangular lattices
Authors: Liu Yang , Dahmen Karin A. , Berger A.
Published: January 2008

The ΔH(M,ΔM) method and its ability to determine the intrinsic switching field distributions of perpendicular recording media are numerically studied. Strong evidence is presented that this method, which is based on the mean-field approximation, has a well-defined reliability range, corroborating earlier results from micromagnetic and hysteron simulations. Specifically, it is shown that this well-behaved failure appears to be universal and independent from the actual lattice structure.
Thermal activation of bulk and interfacial order in exchange biased systems
Authors: Vallejo-Fernandez G. , Fernandez-Outon L. E. , O’Grady K.
Published: January 2008

In this work, we report on thermal activation measurements of both the interfacial spins and the bulk of the antiferromagnet (AF) in exchange bias trilayers consisting of two ferromagnetic (F) layers of different thicknesses separated by an AF layer. Systems with two different AF thicknesses have been measured. Thermal activation of the interfacial spins was achieved by heating in a negative field with only the thicker F layer in negative saturation, while thermal activation of the bulk was achieved by heating the AF with both F layers reversed. By following a detailed measurement procedure where all measurements are made at a temperature at which the AF is free of thermal activation, the hysteresis loop of the thicker layer could be shifted along the field axis, while the loop corresponding to the thinner F layer did not move. From the thermal activation measurements, it is clear that the order at the interface accounts for approximately ∼50% of the exchange bias, while the remaining 50% is due to the order in the bulk. For both samples, the median blocking temperature was found to be greater for the interfacial spins by ∼25K. The energy barriers to reversal for both bulk and interfacial spins have been calculated from the magnetic data and we find that the distributions are of similar form but with that for the interfacial spins occurring at higher values.
Direct measurements of the entropy change and its history dependence in Ni–Mn–Ga alloys
Authors: Sasso Carlo P. , Kuepferling Michaela , Giudici Laura , Basso Vittorio , Pasquale Massimo
Published: January 2008

The role of hysteresis on materials with giant magnetocaloric effect (GMCE) was pointed out by comparing isothermal entropy change measured on a Ni55Mn20Ga25 single crystal presenting a first order magnetostructural phase transition. The entropy change is determined by a laboratory Peltier calorimeter with magnetic field of up to 2T. The measurements performed have allowed to compare the entropy change in partial or complete phase transitions induced by temperature or magnetic field, respectively. The sequence in the temperature and field change affects dramatically the observed MCE demonstrating that a control on both quantities must be accurately performed in order to extract the whole available entropy change. This kind of behavior can be explained if hysteresis in the first order phase transition is taken into account and a correct field-temperature sequence is proposed in order to maximize the MCE.
Switching and thermal stability properties of bilayer thin films: Single versus multigrain cases
Authors: García-Sánchez F. , Chubykalo-Fesenko O. , Mryasov O. , Asselin P. , Chantrell R. W.
Published: January 2008

We investigate magnetization reversal and thermal stability (energy barriers) properties of composite media, for a broad range of saturation magnetization values, comparing the case of individual grain and multigrain systems in order to assess the effect of intergranular interactions. For the hard phase we incorporate partially ordered FePt model with realistic representation of exchange interactions as well as a “future recording media” with high anisotropy and low magnetization. The results are presented as a function of the interfacial exchange parameter. We demonstrate that the energy barriers of an individual grain and multigrain thin film show different behavior due to the domain wall formation at the remanence for thin films with low magnetization soft layer. The individual grain presents the most favorable figure of merit for large soft layer magnetization. In contrast, for the multigrain thin film, the best performance corresponds to the case of both low soft and hard layer magnetizations.
Laser-induced magnetization dynamics in Co∕IrMn exchange coupled bilayers
Authors: Dalla Longa F. , Kohlhepp J. T. , de Jonge W. J. M. , Koopmans B.
Published: January 2008

The response of a ferromagnet/antiferromagnet exchange coupled bilayer to femtosecond laser heating is studied by means of pump-probe time-resolved magneto-optical Kerr effect (TR-MOKE) in the polar geometry on a Co (10nm)∕IrMn (0–15nm wedge) sample. When an external field is applied in the film plane, perpendicular to the exchange bias direction, a damped precessional motion of the ferromagnetic spins can be triggered by laser excitation. We observe that the exchange bias field HEB,dyn, extracted from the TR data, systematically differs from the value HEB,st, obtained by static MOKE loop measurements, for thin (<7nm) IrMn.
Oxidation kinetics of Nd–Fe–B permanent magnets with modified microstructure
Authors: Liu W. Q. , Yue M. , Liu C. Y. , Zhang D. T. , Zhang J. X.
Published: January 2008

Oxidation kinetics of Nd–Fe–B permanent magnets prepared by both spark plasma sintering (SPS) and conventional sintering techniques has been studied at elevated temperature in dry air environment. Compared with the conventional sintered magnet, the SPS magnet possesses much lower oxidation rate, which leads to less reduction of magnetic properties of the SPS magnet than that of the conventional sintered magnet. Microstructure observation shows that the SPS magnet possesses unique microstructure due to the special sintering mechanism of SPS technique. In detail, the grain size of the Nd2Fe14B main phase is fine and uniform, only a few Nd-rich phases form along the grain boundaries of Nd2Fe14B phase, while most of them agglomerate into triple junctions as small particles. Such microstructure modification restrains effectively the intergranular corrosion along Nd-rich phase in the SPS magnet. As a result, the SPS magnet possesses excellent corrosion resistance at elevated temperature.
Wax-bonded NdFeB micromagnets for microelectromechanical systems applications
Authors: Wang Naigang , Bowers Benjamin J. , Arnold David P.
Published: January 2008

A microfabrication technique is reported for the fabrication of wax-bonded micromagnets in the 100μm–1mm range. This dimensional size is not typically achievable by thin-film deposition techniques or by conventional macroscale magnet processing, and is of interest for magnetic microelectromechanical systems applications. The magnet fabrication approach incorporates a wax powder binder to “lock” the magnetic microparticles together to improve the coercivity. In practice, a NbFeB powder and wax powder are packed into pre-etched trenches in a silicon wafer and subsequently heated, allowing the wax to melt and bond the magnetic powder. Using this method, 500×500×320μm3 magnets have been demonstrated with a coercivity of 737kA∕m and a maximum energy product of 16.6kJ∕m3.
Magnetic property measurements on single wall carbon nanotube polyimide composites
Authors: Sun Keun J. , Wincheski Russell A. , Park Cheol
Published: January 2008

Magnetization measurements as a function of temperature and magnetic field were performed on polyimide nanocomposite samples containing various weight percentages of single wall carbon nanotubes. It was found that the magnetization of the composite, normalized to the mass of nanotube material in the sample, decreased with increasing weight percentage of nanotubes. It is possible that the interfacial coupling between the carbon nanotube (CNT) fillers and the polyimide matrix promotes the diamagnetic response from CNTs and reduces the total magnetization of the composite. The coercivity of the samples, believed to originate from the residual magnetic catalyst particles, was enhanced and had stronger temperature dependence as a result of the composite synthesis. These changes in magnetic properties can form the basis of a new approach to investigate the interfacial properties in the CNT nanocomposites through magnetic property measurements.
Lifetime and resistivity modifications induced by helium implantation in silicon: Experimental analysis with an ac profiling technique
Authors: Daliento S. , Mele L. , Spirito P. , Gialanella L. , Limata B. N.
Published: January 2008

The effect of He ion bombardment on silicon based devices can be seen observing a change of their electric properties, that are relevant for their application, as well as through the modification of some local properties, in particular, resistivity and recombination lifetime, that in turn reflect the microscopic modification of their structure. The knowledge of the relation between these two aspects of the modifications induced by ion implantation would represent a powerful tool to design new devices and processes. This paper presents the results of an experimental analysis, where the modifications of the electrical properties of power devices, induced by helium implantation, were correlated to both the local recombination lifetime and the resistivity depth profile measured using a differential ac technique. Various measurements were performed for a wide range of temperatures to obtain information on the energy levels of the recombination centers which are responsible for the observed variations of the local properties of the device. The investigation was performed varying both the beam energy (from 3.5to5.8MeV) and dose (1×108–5×1011ions∕cm2), providing a complete picture of the implantation effects. Finally, an application to the switching behavior of a fast recovery power diode is presented.
Tuning in to Noise
Authors: Bulsara Adi R. , Gammaitoni Luca
Published: January 2008

Two sweeping generalizations can be made about most natural systems: They are intrinsically nonlinear and they operate in noisy environments. Examples abound, ranging from weather systems to oscillating chemical reactions to the movements of an eel. The most complex example is arguably the human central nervous system, flooded as it is with the “noise” of modern life.
Reflections on the Fate of Spacetime
Authors: Witten Edward
Published: January 2008

Our basic ideas about physics went through several upheavals early this century. Quantum mechanics taught us that the classical notions of the position and velocity of a particle were only approximations of the truth. With general relativity, spacetime became a dynamical variable, curving in response to mass and energy. Contemporary developments in theoretical physics suggest that another revolution may be in progress, through which a new source of “fuzziness” may enter physics, and spacetime itself may be reinterpreted as an approximate, derived concept. (See figure 1.) In this article I survey some of these developments.
Solar Neutrino Experiments: The Next Generation
Authors: Bahcall John N. , Calaprice Frank , McDonald Arthur B. , Totsuka Yoji
Published: January 2008

In the next few years, three massive new solar neutrino detectors will generate large amounts of precise data that should have a major impact on our understanding of how the Sun shines and how neutrinos behave. They are Super Kamiokande, in the mountains west of Tokyo; the Sudbury Neutrino Observatory (SNO) in a northern Ontario mine and Borexino, in the Apennines east of Rome. Each of these detectors was conceived and is being built by a sizable international collaboration. Each is housed in an underground laboratory shielded from cosmic ray products other than neutrinos and very energetic muons by a mile or so of earth. Super Kamiokande, the most massive of the three, is a 50-kiloton water-Cerenkov detector. (See figure 1 and the cover of this issue.) In all of these new detectors, sophisticated electronics will record and analyze the individual neutrino collision events. Each detector will register more neutrino interactions in two months than all of the previous solar neutrino experiments have detected in a quarter of a century.
Making a Difference: Ethnic Diversity in Physics
Authors: Stith James H.
Published: January 2008

For years now, it seems, the physics community has talked about the need to increase the number of African Americans, Hispanics and Native Americans within its ranks, and both money and effort have been spent to achieve that goal. As measured by standardized test scores, there is evidence that we have had some success in improving the overall quality of science education for minorities. The number of minority students who enter college intending to major in science has also increased. However, there has been no significant corresponding rise in the number of physics BS or PhD degrees earned by minorities.
Quantum Point Contacts
Authors: van Houten Henk , Beenakker Carlo
Published: January 2008

Punctuated equilibrium, the notion that evolution in nature is stepwise rather than continuous, sometimes applies to evolution in science as well. The seed of a scientific breakthrough may slumber for a decade or even longer without generating much interest. The seed may be a theoretical concept without clear predictions to test experimentally, or an intriguing but confusing experiment without a lucid interpretation. When the seed finally germinates, an entire field of science can reach maturity in a few years.
Quantitative BEC Results Reported at DAMOP Meeting
Authors: Collins Graham P.
Published: January 2008

Now that the rantare of simply creating a Bose–Einstein condensate in a dilute atomic gas has died down (see the “Reference Frame” column by Daniel Kleppner on page 11), the leading research groups are proceeding in earnest to map out the properties of their condensates. Four experimental groups reported their latest results in invited talks at a session on Bose–Einstein condensation (BEC) at the meeting of the American Physical Society's Division of Atomic and Molecular Physics (DAMOP) held in Ann Arbor in May.
Two Little Knowns: A Color Exhibition and a Science Museum
Authors: Hurvich Leo M.
Published: January 2008

In a search some three years ago for an old catalog of optical instruments, I went to the science and technology division of the New York Public Library and requested a certain volume indexed under what seemed an appropriate category. What the libra 17 messenger delivered to me turned out to be a diverse collection of pamphlets on optics and laboratory instruments. The catalog I had hoped to locate was not in the collection, and the items were of no interest to me—with one exception: a seemingly misfiled eight-page pamphlet that came as a stunning surprise. Its cover page announced a “Printer's Proof of General Outline Plan for Exhibition on Color.” The exhibition was scheduled to begin on 1 January 1931 at the “Museums [sic] of the Peaceful Arts (A Museum of Science and Industry)” at 220 East 42nd Street in New York City. The existence of such a museum was as surprising to me as the mounting of the exhibition.
Extrasolar Planets
Authors: Boss Alan P.
Published: January 2008

T he extraordinary discovery of a number of planetary-mass bodies orbiting nearby stars similar to the Sun has completely transformed the field of extrasolar planet detection. This sudden transformation has been brought about by a handful of dedicated observers, working quietly with modest-sized telescopes, often for decades at a time. They include Michel Mayor and Didier Queloz at the Geneva Observatory, Geoffrey Marcy and R. Paul Butler at San Francisco State University and George Gatewood at the University of Pittsburgh. Many of the new objects have already been independently confirmed. The history of failure to confirm earlier claims for extrasolar planets mercifully has been forgotten at long last, as astronomers and planetary scientists rush to find even more planets.
Optical Frequency Measurement is Getting a Lot More Precise
Authors: Schwarzschild Bertram
Published: January 2008

At the Max Planck Institute for Quantum Optics in the Munich suburb of Garching, Theodor Hansch and colleagues have measured the ultraviolet transition frequency between the 1S and 2S states of atomic hydrogen to be 2.466 061 413 187 34(84)×1015 Hz. With an uncertainty of only 3 parts in 1013, this result exceeds the accuracy of the best previous measurement of the 1S-2S transition by two orders of magnitude. It is, in fact, the most accurate measurement to date of any frequency in the visible or ultraviolet. It's so accurate that simply repeating the measurement a year from now would provide a better and more direct verification (or falsification) of the constancy of the fine-structure constant over cosmological time than any astrophysical data we have.
Hysteresis Steps Demonstrate Quantum Tunneling of Molecular Spins
Authors: Schwarzschild Bertram
Published: January 2008

Hysteresis is Hysteresis is a familiar macroscopic phenomenon in the behavior of magnetic materials: When a sample is subjected to a varying external magnetic field, its magnetization curve does not, in general, retrace its steps as the external field in a fixed direction is swept from a large negative value to a large positive value and then back again. But until the summer of 1995, no one had ever seen quantum steps in a hysteresis loop.
Experiment Signals a New Phase of Quantum Dot Measurements
Authors: Levi Barbara Goss
Published: January 2008

The study of electron transport through minuscule conducting regions known as quantum dots has revealed a variety of fascinating phenomena, in which one can see the effects of individual electrons. The plots of current versus voltage, for example, are no longer the straight lines so characteristic of macroscopic wires, but rather discrete staircases, with each successive plateau representing the presence of one additional electron on the dot.
Porous Silicon: From Luminescence to LEDs
Authors: Collins Reuben T. , Fauchet Philippe M. , Tischler Michael A.
Published: January 2008

Silicon is at the heart of the microelectronics revolution. Its dominance over other semiconductors is intimately tied to its superior materials and processing properties and to the tremendous base of technology that has developed around it. Another semiconductor is not likely to displace silicon as the material of choice in electronic applications. Silicon, however, is an extremely inefficient light emitter, and for this reason has not enjoyed the same level of dominance in optical applications.
Atomic Parity Experiment Has Its Moment
Authors: Levi Barbara Goss
Published: January 2008

For over eight years, researchers at JILA and the University of Colorado, Boulder, have worked patiently and painstakingly to improve the precision with which they can detect a rare, parity-violating atomic transition. The group, led by Carl Wieman, has finally been rewarded with a measurement of the transition amplitude that is precise to 0.35%, a factor of seven better than their 1988 measurement of the same transition. The dominant uncertainty in the value of this term is now the uncertainty in the atomic theory, which reached 1.2% in the early 1990s. The combined uncertainties bring the data into the realm where they place useful constraints on the Standard Model of electro weak interactions. As icing on the cake, the Boulder experiment also gives the first good evidence for a nuclear feature predicted years ago: the nuclear anapole moment.
Can Helium Mixing Explain the Solar Neutrino Shortages?
Authors: Schwarzschild Bertram
Published: January 2008

Has the time come to abandon hope of an astrophysical—as distinguished from particle-theoretic—solution of the solar neutrino puzzle? Two recent papers espouse different positions on this important issue. In the 18 November Physical Review Letters Wick Haxton (University of Washington) and Andrew Curnrning (a visiting undergraduate from Cambridge University), argued that there might still be an astrophysical explanation for the observed shortfall of B8 decay neutrinos from the Sun and for the even greater shortfall of solar neutrinos from electron capture by Be7: namely the slow mixing of He3 down into the solar core. “This is a very speculative departure from the standard model of how the Sun works,” Haxton told us. “But we're all willing to carefully examine some rather exotic particlephysics solutions. So it may be equally important to explore unexpected solarphysics solutions.”
High-Gain Avalanche Photodiode
Authors: Levi Barbara Goss
Published: January 2008

As optical communications systems get faster and faster, approaching 10 Gbit/second, near-infrared receivers are struggling to keep pace with them. What's needed are avalanche photodetectors with enough gain to amplify the much shorter pulses that come in at these higher bit rates. Researchers from the University of California, Santa Barbara, and the Royal Institute of Technology in Kista, Sweden, hope that their new avalanche photodiode (APD) will fill the bill: Its gain-bandwidth product is over 300 GHz. That's nearly double the best performance demonstrated up to now by an APD (the old record was held by a superlattice design introduced in the late 1980s) and almost quadruple that of commercial receivers.
Hazards of Managing and Disposing of Nuclear Waste
Authors: Kastenberg William E. , Gratton Luca J.
Published: January 2008

Although it is arguably for the benefit of society, the translation of basic nuclear processes into technological achievements produces nuclear wastes that can become environmental hazards if they are not properly cared for. Light-element fusion, heavy-element fission and radioactive decay have provided us with nuclear weapons, nuclear power plants and nuclear medicine. But they all produce radioactive materials that are unusable, no longer needed, or unwanted. Ultimately, these materials require long-term isolation from the biosphere.
Special Issue: Radioactive Waste
Authors: Ahearne John F.
Published: January 2008

What can be done with radioactive waste? This extremely difficult question has not really been answered anywhere in the world; yet it must be answered. Like death and taxes, radioactive waste is with us—it cannot be wished away. This special, five-article issue of PHYSICS TODAY reviews the technical problems and risks, outlines the reasons why the basic question posed above has not been answered and suggests some possible new approaches that could enable us to make progress.
High-Redshift Absorption Lines Show Convincingly that Gamma-Ray Bursters Are Very Far Away
Authors: Schwarzschild Bertram
Published: January 2008

Last month in these pages we wrote, “Gamma-ray astronomers expect that [the 28 February gamma-ray burst] is but the first of many GRBs that will soon be revealing much more of themselves…” (page 17). “Soon” turned out to be an understatement. On 8 May, just as we were going to press, the new Italian-Dutch BeppoSAX satellite recorded another GRB, one that will surely make its way into the textbooks.
Electromagnetically Induced Transparency
Authors: Harris Stephen E.
Published: January 2008

Electromagnetically induced transparency is a technique for eliminating the effect of a medium on a propagating beam of electromagnetic radiation. EIT may also be used, but under more limited conditions, to eliminate optical self-focusing and defocusing and to improve the transmission of laser beams through inhomogeneous refracting gases and metal vapors, as figure 1 illustrates. The technique may be used to create large populations of coherently driven uniformly phased atoms, thereby making possible new types of optoelectronic devices.
Temperature Scales Below 1 Kelvin
Authors: Soulen Robert J. , Fogle William E.
Published: January 2008

The quest to measure temperature accurately began in ancient Greece with the invention of the thermoscope—an open, oil-inglass device that was the forebear of the familiar, sealed, liquid-in-glass thermometer. Since that time, many new types of thermometer have been invented to serve contemporary science and commerce. The two extremes of temperature will, however, always remain out of reach of our devices. The highest temperature attained by a physical phenomenon, that of the universe at its birth, is certainly beyond our measurement capabilities, although not our ability to estimate—on the order of 1023 kelvin. The third law of thermodynamics forbids experiments from ever reaching the lower limit, absolute zero, although they may approach it arbitrarily closely.
Nonclassical Excitation in Spectroscopy with Squeezed Light
Authors: Ficek Zbigniew , Drummond Peter D.
Published: January 2008

The amplitude and phase of all light fields are subject to fluctuations. Much of this irregularity arises from random and uncontrolled changes that occur in any light source and lead to random changes in the light wave's frequency and amplitude. Most of these factors can in principle be eliminated by a careful design of the light source. Even if all of these defects in the source are removed, however, light fields are still subject to fluctuations arising from the laws of quantum mechanics. Even in a highly stabilized laser, the resulting coherent electromagnetic field has an uncertainty in the phase and magnitude of its amplitude. All conventional sources of light fields have at least this noise level, which, in a fully quantum description, arises from vacuum fluctuations. At one time it was believed that this vacuum or coherent-state noise could not be eliminated.
When the Electron Falls Apart
Authors: Anderson Philip W.
Published: January 2008

It is ironic that, in the year when we celebrate the centenary of the discovery of the electron, the most exciting developments in the theory of electrons in solids have to do with the “fractionalization” of the electron—the discovery of particles that behave as though the electron had broken apart into three or five or more pieces each containing one-third or one-fifth of its charge, or into separate particles, one containing its charge and one its spin. (See figure 1.) No longer is the quantum theory of solids confined to the boring old electron; we now have a remarkable variety of fractional parts of electrons: composite fermions, composite bosons, spinons and holons, in addition to the heavy electrons, quasiparticles and small polarons of older stages of condensed matter theory.
Special Issue: The Ubiquitous Electron
Authors: Lubkin Gloria B.
Published: January 2008

As early as about 1830 electrical discharges in gases were intriguing a number of experimental physicists in Europe. In 1881, at the Cavendish Laboratory at the University of Cambridge, J. J. Thomson began experimenting with gaseous discharges, and continued to do so for the next 50 years.
A Hint of T Violation in a High-Tc Superconductor
Authors: Levi Barbara Goss
Published: January 2008

Unexpected behavior in a tunneling experiment on one of the high-temperature superconductors has led the experimenters to conclude that they are seeing broken time-reversal symmetry. The evidence, if confirmed, would indicate a violation of time reversal only at the surface, but it nevertheless has created a lot of interest, especially among theorists who predicted some type of symmetry breaking in unconventional superconductors even in the bulk.
Stanford Wants to Build a TeV Linear Collider with Japan
Authors: Schwarzschild Bertram
Published: January 2008

Four years ago, accelerator physicists at the Stanford Linear Accelerator Center (SLAC) began construction of the Next Linear Collider Test Accelerator (NLCTA), a 42-meter-long experimental prototype segment of what they call the “Next Linear Collider.” The NLC they hope to build early in the next century would be a face-to-face pair of 10-km linacs firing electrons and positrons at each other with collision energies up to a TeV (1012 electron volts).
The Cosmic Rosetta Stone
Authors: Bennett Charles L. , Turner Michael S. , White Martin
Published: January 2008

Today the universe is characterized by a richness of complexity. Structure exists on scales from stars to superclusters of galaxies and beyond. Ordinary “baryonic” matter, in the form of protons, nuclei and their accompanying electrons, is found in stars, diffuse hot gas, cold gas and other forms; the admixture varies greatly with environment.
Marietta Blau: Between Nazis and Nuclei
Authors: Galison Peter L.
Published: January 2008

At first sight, nothing could be simpler than nuclear emulsions, those thin strips of film designed to trap the tracks of passing charged entities—nuclei, protons, electrons and the other objects that inaugurated the field of particle physics. But the method's seeming simplicity hides a complex history. Scientifically, emulsions posed myriad problems and required years of effort by a dedicated corps of emulsion physicists and chemists, who had to learn how to make the film sensitive to minimally ionizing particles, and how to store, process, dry and ultimately analyze the ramified skein of tracks. Developed in the 1930s by Marietta Blau, an Austrian physicist who fled her homeland following the Anschluss in March 1938, the nuclear emulsion method was taken over by Cecil Powell, who transformed it during the 1940s into a cottage industry, with female “scanners” and an international team of physicists and chemists. From Powell's laboratory in Bristol, England, the method migrated to the burgeoning, industrial-scale accelerator centers at Berkeley and Brookhaven, until even there, emulsions were displaced by the mammoth bubble chambers of the 1950s and 1960s.
Quantum Teleportation Channels Opened in Rome and Innsbruck
Authors: Collins Graham P.
Published: January 2008

If you've heard the reports that teleportation has been achieved, and you're anxious about the implications for investments in the transportation sector, you can relax and instead look to physics futures for the payoff. Quantum teleportation as it currently exists involves the delicate dismantlement of an individual photon's quantum state and its reconstruction about a meter away. Although that may sound less exciting than the transport of starship captains from orbit to planet surface it should lead to new tests of the non-existence of what Albert Einstein, Boris Podolsky and Nathan Rosen called local “elements of reality.”
SOHO Observations Implicate ‘Magnetic Carpet' as Source of Coronal Heating in Quiet Sun
Authors: Day Charles
Published: January 2008

If you image the Sun in x rays or the extreme ultraviolet, you'll see its corona, a halo of million-degree gas whose spectacular structures, known as active regions, are associated with photospheric sunspots. Between the active regions, and covering most of the solar surface when sunspots are absent, is the so-called quiet corona, which glows less brightly and varies more modestly. An accurate explanation of exactly how the corona is heated has long eluded researchers, and, until now, they have found it relatively easier to make progress in the case of the active regions than the quiet Sun.
The Editors Look Back
Authors: Feder Toni
Published: January 2008

P HYSICS TODAY, the flagship magazine of the American Institute of Physics (AIP), turns 50 this month. Over the decades, as the magazine's staff has grown in size from three to seventeen, its coverage of history, international and political news has also grown, and the staff still finds itself stretching to cover both the shifting and expanding field of physics and the physics community. And, as recent chats with the six past and present editors reveal, many of the rewards and challenges of working on PHYSICS TODAY have remained steady through the years.
Energy Budget of Deep-Focus Earthquakes Suggests They May Be Slip-Sliding Away
Authors: Ladbury Ray
Published: January 2008

To understand why the Bolivian earthquake of 9 June 1994 has so rapidly become one of the most thoroughly studied seismic events in history, one must know a little about both the event and the significance of when and where it occurred. (See PHYSICS TODAY, October 1994, page 17.) To begin with, the Bolivian quake occurred in the mantle, nearly 640 km beneath the Amazon basin east of the Andes. Such a large “deep-focus” earthquake—and this one, with magnitude 8.3, was the largest such event ever recorded—can make the planet ring like a bell, exciting overtones of Earth's normal modes of oscillation that are sensitive to inhomogeneities in the core and mantle. Add in the fact that the Bolivian quake occurred after the deployment of several global and local networks of high-quality digital seismological stations, and it becomes clear that the quake offered researchers an unprecedented opportunity to illuminate the three-dimensional structure of Earth's mantle and core. (See the box at right.)
Shadows and Mirrors: Reconstructing Quantum States of Atom Motion
Authors: Leibfried Dietrich , Pfau Tilman , Monroe Christopher
Published: January 2008

Imagine that a pair of coins are tossed in a black box. The box reports only one of the following three results at random: (1) the outcome of the first coin (heads or tails), (2) the outcome of the second coin (heads or tails), or (3) whether the outcomes of the two coins matched or were different. Our task is to construct a joint probability distribution of the four possible outcomes of the coins (HH, TT, HT, TH) based on many observations of the black box outputs. Now suppose that after many trials, the black box reports that each coin comes up heads two-thirds of the time when measured individually, yet the coins never match when they are compared. (Clearly the results of the coin tosses have been correlated—perhaps a joker in the black box flips the coins and then changes the outcomes appropriately). We seek a distribution that both reflects this correlation and obeys the marginal distributions of each coin as two-thirds heads, one-third tails (see the three tables on page 23). The only way to satisfy both requirements is to force the joint probability P(TT) of getting two tails to be negative! Mathematically, this is because P(HH)+P(TT) is observed to be zero, yet we expect P(HH)to be greater than P(TT), because the individual coins are weighted toward heads.
Materials of a Different Stripe
Authors: Levi Barbara Goss
Published: January 2008

The stripe phases in the copper oxide superconductors are rather fleeting structures. But stripes are solid features of related materials—nickel and manganese oxides—whose insulating phases feature a static ordering of charges. These other oxides are cousins of the layered lanthanum copper oxides, La2CuO4, all having the basic perovskite structure. They are generated essentially by replacing the copper atoms with other transition metals. Despite the similar structures, the behaviors of these materials are very different.
New Measurements Constrain Models of Mantle Upwelling Along a Midocean Ridge
Authors: Levi Barbara Goss
Published: January 2008

It has long been known that as tectonic plates pull apart along the mid-ocean ridge system, the mantle wells up to fill the gap. Partial melting of the mantle produces magma that percolates upward to the surface and solidifies into new crust within a few kilometers of the ridge. But the details of this process have been a mystery: Over what volume is the mantle melted to form this magma? What is the concentration of the melt? How connected are the pockets of molten material? To answer such questions, an international collaboration designed the Mantle Electromagnetic and Tomography (MELT) experiment, placing seismometers, electrometers and magnetometers on the ocean floor to span the midocean ridge known as the East Pacific Rise (see figure below). After several years of gathering and analyzing data, members of the collaboration presented their results at the Boston meeting of the American Geophysical Union at the end of May.