Condensed Matter Physics

A 300 GHz frequency synthesizer incorporating a triple-push VCO with Colpitts-based active varactor (CAV) and a divider with three-phase injection is introduced. The CAV provides frequency tunability, enhances harmonic power, and buffers/injects the VCO fundamental signal from/to the divider. The locking range of the divider is vastly improved due to the fact that the three-phase injection introduces larger allowable phase change and injection power into the divider loop. Implemented in 90 nm SiGe BiCMOS, the synthesizer achieves a phase-noise of -77.8 dBc/Hz (-82.5 dBc/Hz) at 100 kHz (1 MHz) offset with a crystal reference, and an overall locking range of 280.32-303.36 GHz (7.9%).

We measure the coherent nonlinear response of excitons in a single-layer of molybdenum disulphide embedded in hexagonal boron nitride, forming a h-BN/MoS 2 /h-BN heterostructure. Using four-wave mixing microscopy and imaging, we correlate the exciton homogeneous and inhomogeneous broadenings. We find that the exciton dynamics is governed by microscopic disorder on top of the ideal crystal properties. Analyzing the exciton ultra-fast density dynamics using amplitude and phase of the response, we investigate the relaxation pathways of the resonantly driven exciton population. The surface protection via encapsulation provides stable monolayer samples with low disorder, avoiding surface contaminations and the resulting exciton broadening and modifications of the dynamics. We identify areas localized to a few microns where the optical response is totally dominated by homogeneous broadening. Across the sample of tens of micrometers, weak inhomogeneous broadening and strain effects are observed, attributed to the remaining interaction with the h-BN and imperfections in the encapsulation process.

Scientific curiosity to uncover original optical properties and functionalities of atomically thin semiconductors, stemming from unusual Coulomb interactions in the two-dimensional geometry and multi-valley band structure, drives the research on monolayers of transition metal dichalcogenides (TMDs). While recent works ascertained the exotic energetic schemes of exciton complexes in TMDs, we here employ four-wave mixing microscopy to indicate that their subpicosecond dynamics is determined by the surrounding disorder. Focusing on a monolayer WS2, we observe that exciton coherence is lost primarily due to interaction with phonons and relaxation processes towards optically dark excitonic states. Notably, when temperature is low and disorder weak excitons large coherence volume results in huge oscillator strength, allowing to reach the regime of radiatively limited dephasing and we observe long valley coherence. We thus elucidate the crucial role of exciton environment in the TMDs on its dynamics and show that revealed mechanisms are ubiquitous within that family.

P a ol a 2 0 0 9. Th e r ol e of p-d o pi n g in t h e g ai n dy n a mi c s of InAs/G aAs q u a n t u m d o t s a t low t e m p e r a t u r e . Ap plie d P hy sic s Le t t e r s 9 4 (4) , 0 4 1 1 1 0 . 1 0 . 1 0 6 3/ 1. 3 0 7 5 8 5 5 file P u blis h e r s p a g e : h t t p:// dx. d oi.o r g/ 1 0. 1 0 6 3/ 1. 3 0 7 5 8 5 5 Pl e a s e n o t e: C h a n g e s m a d e a s a r e s ul t of p u blis hi n g p r o c e s s e s s u c h a s c o py-e di ti n g, fo r m a t ti n g a n d p a g e n u m b e r s m a y n o t b e r efl e c t e d in t hi s v e r sio n. Fo r t h e d efi nitiv e v e r sio n of t hi s p u blic a tio n, pl e a s e r ef e r t o t h e p u blis h e d s o u r c e . You a r e a d vis e d t o c o n s ul t t h e p u blis h e r's v e r sio n if yo u wis h t o ci t e t hi s p a p er. This ve r sio n is b ei n g m a d e a v ail a bl e in a c c o r d a n c e wi t h p u blis h e r p olici e s. S e e h t t p://o r c a . cf. a c. u k/ p olici e s. h t ml fo r u s a g e p olici e s. Co py ri g h t a n d m o r al ri g h t s fo r p u blic a tio n s m a d e a v ail a bl e in ORCA a r e r e t ai n e d by t h e c o py ri g h t h ol d e r s .

The dephasing time of the lowest bright exciton in CdSe=ZnS wurtzite quantum dots is measured from 5 to 170 K and compared with density dynamics within the exciton fine structure using a sensitive threebeam four-wave-mixing technique unaffected by spectral diffusion. Pure dephasing via acoustic phonons dominates the initial dynamics, followed by an exponential zero-phonon line dephasing of 109 ps at 5 K, much faster than the $10 ns exciton radiative lifetime. The zero-phonon line dephasing is explained by phonon-assisted spin flip from the lowest bright state to dark-exciton states. This is confirmed by the temperature dependence of the exciton lifetime and by direct measurements of the bright-dark-exciton relaxation. Our results give an unambiguous evidence of the physical origin of the exciton dephasing in these nanocrystals.

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Trion photoluminescence spectra were measured in undoped high-quality GaAs quantum wells. The lineshape is asymmetric with a temperature-dependent tail towards lower photon energies. We have solved numerically the trion Schrodinger equation in the quantum well and found good agreement for both the trion binding energy and the luminescence lineshape at different temperatures. To analyse the relative weight of trion and exciton emission, an equilibrium mass-action law for the respective densities is studied. Using calculated radiative lifetimes, the emission decay after pulsed excitation is found to deviate strongly from a single-exponential shape in case the trions are initially saturated.

We measured the gain dynamics of the ground-state transition at 20 K in an undoped and identically fabricated p-doped InAs/GaAs quantum-dot amplifier. The dynamics in the doped device is dominated by a very short (∼0.1 ps) and a very long (∼300 ps) time constant. These were attributed to hole and electron dynamics, respectively, and quantitatively described by a microstate model. By comparing the dynamics for the same modal gain in the two devices, the gain recovery was initially faster in the p-doped sample, attributed to ultrafast hole-hole scattering, but slower at later times due to the lack of an electron reservoir.

Magnetically frustrated spin systems compose a significant proportion of topological quantum spin liquid candidates. Evidence for spin liquids in these materials comes largely from the detection of fractionalised spin-1/2 quasiparticles, known as spinons. However, the one-dimensional Heisenberg chain, which is topologically trivial, also hosts spinons. Thus, observing spinons does not necessarily signify long-range entanglement. Here, we show that spinons arising from one-dimensional physics leave a clear fingerprint in magnetic Raman scattering. We achieve this by calculating the magnetic Raman intensity of coupled Heisenberg chains. Our findings are in excellent agreement with the magnetic Raman scattering measurements on the anisotropic triangular antiferromagnet Ca3ReO5Cl2.

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Precursors can serve as a bound on the speed of information with dispersive medium. We propose a method to identify the speed of optical wave fronts using polarization-based interference in a solid-state device, which can bound the accuracy of the speed of wave fronts to less than 10 -4 with conventional experimental conditions. Our proposal may have important implications for optical communications and fast information processing.

The $$s,p{-}d$$ s , p - d exchange coupling between the spins of band carriers and of transition metal (TM) dopants ranging from Ti to Cu in ZnO is studied within the density functional theory. The $$+U$$ + U corrections are included to reproduce the experimental ZnO band gap and the dopant levels. The p–d coupling reveals unexpectedly complex features. In particular, (i) the p–d coupling constants $$N_0\beta$$ N 0 β vary about 10 times when going from V to Ni, (ii) not only the value but also the sign of $$N_0\beta$$ N 0 β depends on the charge state of the dopant, (iii) the p–d coupling with the heavy holes and the light holes is not the same; in the case of Fe, Co and Ni, $$N_0\beta$$ N 0 β s for the two subbands can differ twice, and for Cu the opposite sign of the coupling is found for light and heavy holes. The main features of the p–d coupling are determined by the p–d hybridization between the d(TM) and p(O) orbitals. In contrast, the s–d coupling constant $$N_0\alpha$$ N 0 ...

Previous luminescence and absorption experiments in Co-doped ZnO revealed two ionization and one intrashell transition of d(Co 2+ ) electrons. Those optical properties are analyzed within the generalized gradient approximation to the density functional theory. The two ionization channels involve electron excitations from the two Co 2+ gap states, the t 2↑ triplet and the e 2↓ doublet, to the conduction band. The third possible ionization channel, in which an electron is excited from the valence band to the Co 2+ level, requires energy in excess of 4 eV, and cannot lead to absorption below the ZnO band gap, contrary to earlier suggestions. We also consider two recombination channels, the direct recombination and a two-step process, in which a photoelectron is captured by Co 3+ and then recombines via the internal transition. Finally, the observed increase the band gap with the Co concentration is well reproduced by theory. The accurate description of ZnO:Co is achieved after including +U corrections to the relevant orbitals of Zn, O, and Co. The +U (Co) value was calculated by the linear response approach, and independently was obtained by fitting the calculated transition energies to the optical data. The respective values, 3.4 and 3.0 eV, agree well. Ionization of Co induces large energy shifts of the gap levels, driven by the varying Coulomb coupling between the d(Co) electrons, and by large lattice relaxations around Co ions. In turn, over ∼ 1 eV changes of Co 2+ levels induced by the internal transition are mainly caused by the occupation-dependent U (Co) corrections.

Electronic, magnetic, and structural properties of several II A -V compounds are analyzed using ab initio calculations. We consider four crystal structures: zinc-blende, NiAs, rocksalt, and Zn 3 P 2 . Our results indicate that II A -V nitrides in the rocksalt phase are stable ferromagnetic half-metals characterized by the total spin polarization of free holes in the valence band. The previously considered zinc-blende phase of II-V crystals is ferromagnetic as well, but is less stable by about 0.3 eV/atom. The stability of the Zn 3 P 2 phase is close to that of the rocksalt, but the spin polarization vanishes in this case. Analysis of the electronic structure shows that ferromagnetism of II A -V compounds originates in the spin polarization of the p shell of anions, as described by Hund's rule, which persists in solids after formation of bonds. Furthermore, the results for isolated atoms indicate why the spin polarization is the most stable in II A -V nitrides, which are discussed in detail.

We report studies on the dc magnetization of YBCO thin ®lms in simultanously applied dc and ac ®elds. The eect of the ac ®elds is to decrease the irreversible magnetization drastically leading to complete collapse of the hysteresis loops for relatively small ac ®elds (25 Oe). The magnitude of the decrease depends on the component of the ac ®eld parallel to the c-axis. The decrease is non-linear with ac amplitude and is explained in the framework of the critical state response of ultrathin ®lms in perpendicular geometry. The ac ®elds increase the relaxation rapidly at short times while the long time response appears unaected.

We have investigated the hysteresis in the AC susceptibility of granular YBCO superconductors when the DC field is cycled. A crossover point is observed where the susceptibility X' in decreasing fields becomes more diamagnetic than for increasing fields, and this is investigated as a function of temperature, AC field amplitude and frequency. The behavior is consistent with the variation of the AC penetration depth with these variables and with a reduced flux density of the intergrain regions on field reversal. The zero field cooled behavior of the second harmonic X2 as well as the behavior of X' in field cooled conditions, are also reported. The DC field dependence of X' is analyzed using the low amplitude, linear response region approximations.

The radiation embrittlement of the welds of old VVER 440 reactor pressure vessels promoted research on technical ways to ensure their safe operation during design service life. One of those is the recovery annealing of the reactor pressure vessel. Today, very few data are available concerning re-irradiated materials. These data are essential in order to understand the mechanism of re-embrittlement and to be able to predict the evolution of the microstructure and the mechanical properties of the steel during inservice re-irradiation. For that purpose a research programme was launched to carry out atom probe studies of the microstructure of weld metals in irradiated, irradiated-annealed and re-irradiated conditions. This work demonstrates that neutron irradiation promotes the formation of ultra-fine copper-enriched clusters and solute segregation to crystal defects in these Cr-Mo-V steels. The study of the annealed materials allows us to explain how the mechanical properties of the irradiated materials can be recovered. Also, study of the re-irradiated material points out the non-detection of copper-enriched clusters and the phosphorus behaviour during re-irradiation.

A series of Cu-C o ferrite based bulk particles (Cu (1-x) Co(x) Fe2O4) in which x is varied from 0.0 to 0.6 were prepared by Solid State reaction route. The influence of the Co content on the structural properties of cubic Nickel ferrites (CuF e2O4) was investigated using Raman spectroscopy. Slight reduction in the lattice parameter of Co doped CuF e2O4 has been observed as compared to the parent CuF e2O4. From Raman scattering spectra, a shoulder like feature has been observed in both parent and doped com pounds which reveals that octahedral site is occupied by Co, Cu and Fe ions and tetrahedral site is occupied by only Fe ion.

Structural, electrical and magnetic properties of nickel manganite ceramics obtained by sintering fine powders prepared by a complex polymerization method are given in this work. The phase composition of the synthesized material was examined by x-ray powder diffraction (XRPD). Field-emission scanning electron microscopy (FE-SEM) was used to analyze the obtained powder morphology. Scanning electron microscopy (SEM) was used to analyze the microstructure of sintered ceramics. The activation energy of conduction E a and the coefficient of temperature sensitivity B 25/100 were calculated from direct current (DC) resistivity measurements. The magnetization dependence of temperature M(T) and alternating current (AC) susceptibility data obtained from SQUID measurements clearly demonstrate that quadruple magnetic phase transitions can be readily detected at T

The asymmetric magnetization reversal in exchange biased Fe/MnF 2 involves coherent (Stoner-Wohlfarth) magnetization rotation into an intermediate, stable state perpendicular to the applied field. We provide here experimentally tested analytical conditions for the unambiguous observation of both longitudinal and transverse magnetization components using the magneto-optical Kerr effect. This provides a fast and powerful probe of coherent magnetization reversal as well as its chirality. Surprisingly, the sign and asymmetry of the transverse magnetization component of Fe/MnF 2 change with the angle between cooling and measurement fields.

We have discovered a positive unidirectional exchange anisotropy in antiferromagnetic (FeF 2 ) and ferromagnetic (Fe) bilayers cooled through the antiferromagnetic critical temperature T N in large magnetic fields. For low positive cooling fields, the ferromagnet's magnetization (M-H) loop center shifts to negative fields, as is normally observed in other systems. In contrast, large cooling fields can cause the shift to be positive. This can be explained if the FeF 2 surface spins couple to the external magnetic cooling field above T N and the FeF 2 -Fe interaction is antiferromagnetic. [S0031-9007 (96)

The exchange bias shift of the hysteresis loop, H E , in antiferromagnetic/ferromagnetic layer systems can be easily controlled ͑within certain limits͒ by cooling in zero field from different magnetization states above the antiferromagnetic Ne ´el temperature, T N . This indicates that for moderate cooling fields, H E is determined by the magnetization state of the ferromagnet at T N , and not by the strength of the cooling field.

We present an investigation of the effect of ferromagnetic layer thickness on the exchange bias and coercivity enhancement in antiferromagnet/ferromagnet bilayers. At low temperatures both the exchange bias and coercivity closely follow an inverse thickness relationship, contrary to several recent theoretical predictions. Furthermore, the temperature dependence of the coercivity as a function of the ferromagnet thickness provides clear evidence for the existence of two distinct regimes. These regimes were probed with conventional magnetometry, anisotropic magnetoresistance, and polarized neutron reflectometry. At low thickness the coercivity exhibits a monotonic temperature dependence, whereas at higher thickness a broad maximum occurs in the vicinity of the Ne ´el temperature. These regimes are delineated by a particular ratio of the ferromagnet to antiferromagnet thickness. We propose that the ratio of the anisotropy energies in the two layers determines whether the coercivity is dominated by the ferromagnetic layer itself or the interaction of the ferromagnetic layer with the antiferromagnet.

The temperature dependence of the exchange bias (H E ) near the FeF 2 Ne ´el temperature (ϳ 78.4 K͒ was correlated with structural measurements in FeF 2 -Fe bilayers. Low-angle x-ray diffraction and atomic force microscopy show that samples with larger height fluctuations have larger lateral grain sizes. Samples with larger lateral grain sizes exhibit a surface critical exponent (␤ S ϳ0.8) while samples with smaller grains and smaller height fluctuations have a decreased ␤ S , indicating a more three-dimensional-like phase transition or an increase in the FeF 2 surface exchange interaction. ͓S0163-1829͑97͒05230-2͔

We have studied the effect of the interface structure on the exchange bias in the FeF 2 -Fe system, for FeF 2 bulk single crystals or thin films. The exchange bias depends very strongly on the crystalline orientation of the antiferromagnet for both films and crystals. However, the interface roughness seems to have a strong effect mainly on the film systems. These results indicate that the exchange bias depends strongly on the spin structure at the interface, especially on the angle between the ferromagnetic and antiferromagnetic spins. We have also found a strong dependence of the hysteresis loops shape on the cooling field direction with respect to the antiferromagnetic anisotropy axis, induced by a rotation of the ferromagnetic easy axis as the sample is cooled through T N . For the single crystal systems the results imply the existence of a perpendicular coupling between the antiferromagnetic and ferromagnetic spins at low temperatures. ͓S0163-1829͑99͒02610-7͔

The induced moment in antiferromagnetic ͑AFM͒-ferromagnetic ͑FM͒ (FeF 2 -Fe and MnF 2 -Fe) bilayers has been studied from the shift along the magnetization axis of the exchange-biased hysteresis loops. The magnetization shift depends strongly on the cooling field and microstructure of the AFM layer. The shift for small cooling fields can be opposite to the cooling field, indicating that, in some cases, the presence of the FM layer induces an antiferromagnetic coupling at the interface. Samples with negative magnetization shifts ͑antiferromagnetic coupling͒ exhibit large changes in exchange bias H E as a function of cooling field and positive exchange bias. Samples with positive magnetization shifts ͑ferromagnetic coupling͒ show almost no change in H E with cooling field and the exchange bias field remains always negative. These results confirm the theoretical assumption that an antiferromagnetic interface coupling is necessary to observe positive exchange bias.

The dependence of exchange bias on antiferromagnet thickness has been measured in FeF 2 /Fe and MnF 2 /Fe bilayers. The two fluoride systems have identical crystal structures, similar lattice constants, but anisotropy fields that differ by a factor of 20. Hence, by comparing the antiferromagnetic layer thickness dependence of the exchange bias in the two systems we are able to directly establish the effect of the antiferromagnet anisotropy. We find that the critical antiferromagnet thickness for the onset of exchange biasing is an order of magnitude smaller for the more anisotropic fluoride, confirming the often-used assumption that the anisotropy dictates the critical thickness. By measuring the temperature dependence of the exchange bias and the structural morphology of the layers we are able to prove that the effects we observe are not due to the blocking-temperature thickness dependence or the onset of discontinuity in thin antiferromagnet layers.

The effect of the antiferromagnetic spin flop on exchange bias has been investigated in antiferromagnetic (MnF 2 ) -ferromagnetic ͑Fe͒ bilayers. Cooling and measuring in fields larger than the antiferromagnetic spinflop field, H SF , causes an irreversible reduction of the magnitude of the exchange bias field, H E . This indicates that, contrary to what is normally assumed, the interface spin structure does not remain ''frozen in'' below T N if large enough fields are applied.

We have discovered a positive unidirectional exchange anisotropy in antiferromagnetic (FeF 2 ) and ferromagnetic (Fe) bilayers cooled through the antiferromagnetic critical temperature T N in large magnetic fields. For low positive cooling fields, the ferromagnet's magnetization (M-H) loop center shifts to negative fields, as is normally observed in other systems. In contrast, large cooling fields can cause the shift to be positive. This can be explained if the FeF 2 surface spins couple to the external magnetic cooling field above T N and the FeF 2 -Fe interaction is antiferromagnetic. [S0031-9007 (96)

This report examines the current status and the future directions of the field of nanomagnetism and assesses the ability of hard X-ray synchrotron facilities to provide new capabilities for making advances in this field. The report first identifies major research challenges that lie ahead in three broadly defined subfields of nanomagnetism: confined systems, clusters and complex oxides. It then examines the relevant experimental capabilities that are currently available at hard X-ray synchrotron light sources, using the Advanced Photon Source (APS) at Argonne as an example. Finally, recommendations are made for future development in X-ray facilities that will enhance the study of nanomagnetism, including new experimental directions, modifications that would enable in situ sample preparation, and measurements at high magnetic fields and/or low temperatures. In particular, in situ sample preparation is of high priority in many experiments, especially those in the area of surface magnetism.

We review the phenomenology of exchange bias and related effects, with emphasis on layered antiferromagnetic (AFM)-ferromagnetic (FM) structures. A compilation of materials exhibiting exchange bias and some of the techniques used to study them is given. Some of the applications of exchange bias are discussed. The leading theoretical models are summarized. Finally some of the factors controlling exchange bias as well as some of the unsolved issues associated with exchange bias are discussed.

We have measured the anisotropic magnetoresistance of Fe films exchange coupled to antiferromagnetic MnF2 layers. Exchange bias and coercivity obtained from magnetoresistance are in close agreement with superconducting quantum interference device magnetometry data. In addition the magnetoresistance reveals an asymmetry in the magnetization reversal process, despite the fact that the magnetization hysteresis loops show little shape asymmetry. These results correlate well with an earlier study of magnetization reversal asymmetry by polarized neutron reflectometry. The data imply that the magnetization reverses by coherent rotation on one side of the loop and by nucleation and propagation of domain walls on the other.

The exchange bias shift of the hysteresis loop, HE, in antiferromagnetic/ferromagnetic layer systems can be easily controlled (within certain limits) by cooling in zero field from different magnetization states above the antiferromagnetic Néel temperature, TN. This indicates that for moderate cooling fields, HE is determined by the magnetization state of the ferromagnet at TN, and not by the strength of the cooling field.

We have studied the exchange anisotropy of ferromagnetic Fe films grown on antiferromagnetic FeF2 single crystals. The behavior of the hysteresis loops of the Fe above and below the Néel temperature TN of FeF2 indicates a 90° rotation of the ferromagnetic easy axis due to the antiferromagnetic ordering. By examining the Fe hysteresis loops together with the FeF2 susceptibility behavior we infer that below TN the ferromagnetic and antiferromagnetic spins are coupled perpendicular to each other. This behavior can be explained by recent micromagnetic calculations on exchange bias systems, or by magnetoelastic effects.