Low Temperature Shoulder Research Articles

Thermodynamics of the spin-half square kagome lattice antiferromagnet

Over the last decade, the interest in the spin-$1/2$ Heisenberg antiferromagnet (HAF) on the square kagome (also called shuriken) lattice has been growing as a model system of quantum magnetism with a quantum paramagnetic ground state, flat-band physics near the saturation field, and quantum scars. A further motivation to study this model comes from the recent discovery of a gapless spin liquid in the square kagome magnet $\mathrm{K}{\mathrm{Cu}}_{6}\mathrm{Al}\mathrm{Bi}{\mathrm{O}}_{4}{(\mathrm{S}{\mathrm{O}}_{4})}_{5}\mathrm{Cl}$ [M. Fujihala et al., Nat. Commun. 11, 3429 (2020)]. Here, we present large-scale numerical investigations of the specific heat $C(T)$, the entropy $S(T),$ as well as the susceptibility $\ensuremath{\chi}(T)$ by means of the finite-temperature Lanczos method for system sizes of $N=18,24,30,36,42,48$, and $N=54$. We find that the specific heat exhibits a low-temperature shoulder below the major maximum which can be attributed to low-lying singlet excitations filling the singlet-triplet gap, which is significantly larger than the singlet-singlet gap. This observation is further supported by the behavior of the entropy $S(T)$, where a change in curvature is present just at about $T/J=0.2$, the same temperature where the shoulder in $C$ sets in. For the susceptibility the low-lying singlet excitations are irrelevant, and the singlet-triplet gap leads to an exponentially activated low-temperature behavior. The maximum in $\ensuremath{\chi}(T)$ is found at a pretty low temperature ${T}_{\mathrm{max}}/J=0.146$ (for $N=42$) compared to ${T}_{\mathrm{max}}/J=0.935$ for the unfrustrated square-lattice HAF signaling the crucial role of frustration also for the susceptibility. We find a striking similarity of our square kagome data with the corresponding ones for the kagome HAF down to very low $T$. The magnetization process featuring plateaus and jumps and the field dependence of the specific heat that exhibits characteristic peculiarities attributed to the existence of a flat one-magnon band are discussed as well.

Effect of Eu doping on the thermoluminescence of UV and gamma irradiated Mg2 B2 O5 nanophosphors.

This article presents the effect of europium (Eu) doping on the thermoluminescence (TL) of ultraviolet (UV-254 nm) and gamma irradiated triclinic Mg2 B2 O5 nanophosphors. The diffuse reflectance predicts slight decrease in band gap from 5.18 to 4.99 eV with increasing Eu (1%, 3%, and 5%) content in Mg2 B2 O5 . The TL glow curves of UV irradiated samples comprised of a main peak around 500 K with weak intensity peak/shoulders in low temperature region. Interestingly Eu (3%) doped Mg2 B2 O5 shows maximum TL intensity with suppression of low temperature shoulder peaks and almost linear UV dose dependent TL response. However, in the case of gamma irradiated Eu (1% and 3% doped) samples, TL glow curve comprises of a main peak around 425-445 K and closely lying peak around 500-515 K with relatively lesser intensity. In case of Eu (5%) doped samples, TL peak around 508 K starts dominating over peak around 425 K. TL of both UV and gamma irradiated samples showed the presence of various deep and shallow defect states within the bandgap of materials having different kinetic parameters, which were determined using TLanal software based on Kiti's general order equation. The present study shows that Eu doped Mg2 B2 O5 nanophosphors can be tuned for UV and gamma dosimetry.

Calorimetric observation of lysozyme degradation at elevated temperature in water and DMSO-water mixtures

The reversibility of protein denaturation is an essential factor for biotechnology. Previous differential scanning calorimetry (DSC) studies demonstrated the development of the low-temperature shoulder on the calorimetric denaturation peak of lysozyme in successive heating-cooling cycles, which implies irreversible denaturation. However, this effect was not thoroughly investigated. In the present work, we have quantitatively studied the effect of incubation at the elevated temperature on the state of lysozyme in water and mixtures of water with dimethyl sulfoxide (DMSO) using DSC. The changes to the state of the lysozyme molecule indicated by DSC thermograms were also evaluated by circular dichroism (CD), Fourier transform infrared spectroscopy (FTIR) and dynamic light scattering (DLS) techniques. It is noted that with the increase of duration or temperature of incubation, the low-temperature peak on DSC thermograms grows, while the height of the unfolding peak of native structure decreases. The increase in the height of the low-temperature peak in DSC scans correlates with the development of the sideband associated with the absorption of the carboxyl group in the infrared spectra. This result suggests that the low-temperature endothermic peak corresponds to the unfolding of the deamidated protein. At the same time, DLS measurements indicate absence of aggregation, while FTIR and CD data demonstrate that deamidated protein maintains a native-like structure. The evaluation of the DSC thermograms allowed to determine the rates and activation energy of the degradation of protein molecules at the elevated temperatures. The addition of DMSO slows down the protein degradation but has little effect on the apparent activation energy of the process.

Low-temperature immobilization of water in Antarctic Turgidosculum complicatulum and in Prasiola crispa. Part I. Turgidosculum complicatulum.

The studies of low-temperature immobilization of bound water in Antarctic lichenized fungus Turgidosculum complicatulum were performed using 1H NMR and DSC over a wide range of thallus hydration. 1H NMR free induction decays were decomposed into a solid component well described by the Gaussian function and two exponentially decaying components coming from a tightly bound water and from a loosely bound water fraction. 1H NMR spectra revealed one averaged mobile proton signal component. 1H NMR measurements recorded in time and in frequency domain suggest the non-cooperative bound water immobilization in T. complicatulum thallus. The threshold of the hydration level estimated by 1H NMR analysis at which the cooperative bound water freezing was detected was Δm/m0 ≈ 0.39, whereas for DSC analysis was equal to Δm/m0 = 0.375. Main ice melting estimated from DSC measurements for zero hydration level of the sample starts at tm = -(19.29 ± 1.19)°C. However, DSC melting peak shows a composed form being a superposition of the main narrow peak (presumably melting of mycobiont areas) and a broad low-temperature shoulder (presumably melting of isolated photobiont cells). DSC traces recorded after two-hour incubation of T. complicatulum thallus at -20 °C suggest much lower threshold level of hydration at which the ice formation occurs (Δm/m0 = 0.0842). Presumably it is a result of diffusion induced migration of separated water molecules to ice microcrystallites already present in thallus, but still beyond the calorimeter resolution.

Magnetism of theN=42kagome lattice antiferromagnet

For the paradigmatic frustrated spin-half Heisenberg antiferromagnet on the kagome lattice we performed large-scale numerical investigation of thermodynamic functions by means of the finite-temperature Lanczos method for system sizes of up to N=42. We present the dependence of magnetization as well as specific heat on temperature and external field and show in particular that a finite-size scaling of specific heat supports the appearance of a low-temperature shoulder below the major maximum. This seems to be the result of a counterintuitive motion of the density of singlet states towards higher energies. Other interesting features that we discuss are the asymmetric melting of the 1/3 magnetization plateau as well the field dependence of the specific heat that exhibits characteristic features caused by the existence of a flat one-magnon band. By comparison with the unfrustrated square-lattice antiferromagnet the tremendous role of frustration in a wide temperature range is illustrated.

Comment on “Origin of low-temperature shoulder internal friction peak of Snoek–Köster peak in a medium carbon high alloyed steel” by Lu et al. [Solid State Communications 195 (2014) 31

We want to discuss the interpretation of low-temperature shoulder internal friction peak of Snoek-Koster peak (LTS-SK). Lu et al. (2015) [1] attributed it to the interaction between the carbon atoms and twin boundaries in martensite. Nevertheless, the decrease of the amplitude of LTS-SK peak due to carbon segregation is correlated with the interstitial carbon content in solid solution in martensite (Hoyos et al., 2015 [2]). Therefore, this peak can also be attributed to the presence of an internal friction athermal background, which is proportional to the concentration of interstitial carbon in solid solution (Tkalcec et al., 2015 [2,3]). In addition, they used an alloyed steel, in which epsilon carbide precipitated above of the LTS-SK peak temperature. As this behavior cannot be generalized for carbon and high alloyed steels, the carbide precipitates could made an additional contribution to the internal friction. (C) 2015 Elsevier Ltd. All rights reserved.

Origin of low-temperature shoulder internal friction peak of Snoek-Köster peak in a medium carbon high alloyed steel

A distinct internal friction peak located at the low-temperature shoulder of Snoek-Köster peak (LTS-SK) was found in Fe–0.39C–9.8Ni–1.56Si–2.0Mn steel and its evolution with respect to various aging treatments was investigated. The LTS-SK internal friction peak was found to occur when aged below 373K. TEM observation confirmed that the ε-carbide precipitated beyond 373K, providing an evidence that the LTS-SK peak cannot be caused by ε-carbide precipitation. The corresponding evolution on the S-K peak and thermoelectric power (TEP) illustrated that the carbon content in the solid solution decreases due to carbon atoms segregation on the surrounding dislocations during low-temperature aging. The origin of the LTS-SK peak is likely attributed to the interaction between the carbon atoms and twin boundaries in martensite.

Internal friction of hydrated soda-lime-silicate glasses

The internal friction of hydrated soda-lime-silica glasses with total water content (C(W)) up to 1.9 wt. % was studied by dynamic mechanical analysis (DMA) using temperature-frequency sweeps from 723 K to 273 K and from 1 s(-1) to 50 s(-1). Total water content and concentrations of H2O molecules (C(H2O)) and OH groups (C(OH)) in the DMA specimens were determined by infrared spectroscopy. For low water contents (C(W) ≈ C(OH) < 0.25 wt. %) two discrete internal friction peaks below the glass transition (α relaxation) were assigned to the low-temperature motion of alkali ions (γ relaxation) and cooperative movements of dissimilar mobile species under participation of OH at higher temperature (β(OH) relaxation). For large water contents (C(W) > 1 wt. %), where significant amounts of molecular water are evident (C(H2O) > 0.15 wt. %), however, internal friction spectra change unexpectedly: the β(OH) peak heights saturate and a low temperature shoulder appears on the β-relaxation peak. This emerging relaxation mode (β(H2O) relaxation) was assigned to the motions of H2O molecules. β(H2O) relaxation was found to be faster than β(OH) but slower than γ relaxation. Activation energy of the different relaxation modes increased in the order γ < β(H2O) < β(OH) < α.

Investigation of the origin of glassiness in La0.5Sr0.5CoO3

A comprehensive magnetic study has been carried out on the two sets of La0.5Sr0.5CoO3 samples with a view to understand the origin of low temperature glassiness in the ferromagnetic state. The samples prepared by the conventional solid-state synthesis method show a low temperature shoulder in both dc magnetization as well as in the ac susceptibility measurements, which exhibit characteristics of glassiness such as the frequency dependence and memory effect. These observations suggest the existence of a distinct low temperature cluster-glass like phase within dominant ferromagnetic phase. But, once the same sample is properly homogenized by repeated grinding and annealing process, the low temperature glassy phase disappears, and it shows a pure ferromagnetic behavior. Our comparative study clearly reveals that the reentrant spin-glass like nature is not intrinsic to La0.5Sr0.5CoO3 system, in fact this is an outcome of the compositional inhomogeneity.

Local spin dynamics in doped cobalt(II)-radical magnetic chains studied by 1H NMR

We investigated the spin dynamics of Zn(II)-doped “single-chain magnet” (SCM) Co(hfac) 2NITPhOMe (in short CoPhOMe) by means of 1H NMR measurements. CoPhOMe was the first magnetic chain that showed a slowly relaxing magnetization (commonly called spins’ or magnetization “slowing down” mechanism), which follows the predictions of the Glauber model: the electronic spin freezing sets in before any transition to long-range magnetic order. The slowing down of the magnetization is also evidenced in the Zn-doped samples, which present chains that reach an average length of some tenth of spins for the highest doping, together with growing finite-size effects as the doping concentration is increased. The nuclear spin–lattice relaxation rate T 1 - 1 ( T ) at different applied magnetic fields, displayed a high-temperature peak and a low-temperature shoulder that become lower and shifts toward higher temperatures with increasing the field. This behaviour is qualitatively in agreement with the recent AC susceptibility data that revealed the Glauber dynamics and finite-size effects, like the collective reversal of the spins of each chain segment.

High Sensitivity Differential Scanning Calorimetry Study of DNA-Cationic Liposome Complexes

To investigate plasmid DNA interactions with liposomes prepared from dimyristoylglyceroethylphosphocholine (EDMPC) and DOPE using high sensitivity differential scanning calorimetry (HSDSC). Large unilamellar liposomes of EDMPC with DOPE (mol ratio 0-50%) were prepared. Plasmid DNA was added to give a final DNA/lipid (-/+) charge ratio of 0.5. Samples were placed into an HSDSC and cooled to 3 degrees C, held isothermally for 30 min and then the temperature was ramped to 120 degrees C at a rate of 1 degree C/min. On heating EDMPC liposomes, the main phase transition occurred at 21.2 degrees C, with a low temperature shoulder on the endothermic peak. At low DOPE concentrations the main phase transition temperatures and enthalpies of transition were lower than for pure EDMPC, with a peak corresponding to a pure EDMPC phase occurring at DOPE concentrations of 12-17 mol%. At 50 mol%, no main transition endotherm was observed. DNA solution produced two endothermic peaks with numerous 'satellite' peaks indicating thermal denaturation. DNA binding to EDMPC changed the shape of the thermogram, indicating alteration in lipid packing within the bilayer. DNA induced demixing in the bilayers of DOPE-containing liposomes. HSDSC provided information for characterizing liposome formulations and DNA interactions with such vesicles.

Crystallization of poly(vinylidene fluoride) during ultra-fast cooling

Melt-crystallization of polyvinylidene fluoride (PVDF) was investigated in non-isothermal mode at ultra-high cooling rates ranging between 30–3000 K/s as well as at constant temperatures after quenching at 6000 K/s. An increase of the cooling rate above 150 K/s leads to the formation of β phase manifested by a low temperature shoulder of crystallization exotherm in addition to the α modification. At the cooling rates above 2000 K/s there is only low temperature exothermic peak that is attributed to the crystallization of pure β modification. Isothermal crystallization was possible to realize at 110 °C as the lowest, resulting in α form. Much higher crystallization rate in submicrogram samples, as compared to standard DSC experiments, is also reported.

Specific Heat and Magnetic Relaxation Analysis of ${\rm MgB}_{2}$ Bulk Samples With and Without Additives

The effects of SiC and Carbon doping on the superconducting properties of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> polycrystalline samples have been analysed by means of specific heat and magnetic relaxation measurements. It is known that the addition of nanometric powders of SiC and C leads to the enhancement of B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">irr</sub> and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> . However, the underlying physical mechanism is not completely understood. Magnetic relaxation measurements did not show detectable effects of both the additions on the pinning properties of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> . It follows that doping acts mainly introducing disorder into the superconductor and thus raising B <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c2</sub> . In the case of MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1.9</sub> C <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.1</sub> , specific heat measurements show that the C substitution on the B sites modifies the low temperature shoulder related to the second gap. This effect is not visible in the sample doped with SiC. From the distribution of T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> determined from the deconvolution of the calorimetric data, we argue that SiC leads to an inhomogeneous distribution of C.

Crystallization effects on autoclave foaming of polycarbonate using supercritical carbon dioxide

AbstractIn this study, the conditions leading to the formation of cells and to the onset of crystallization of polycarbonate were examined with the use of supercritical carbon dioxide for the production of foams from preforms. Small plaques cut from extruded sheets were treated with supercritical carbon dioxide in an autoclave at temperatures varying from 60 to 200°C and from 4.5 to 30 MPa pressure. Visual observations and stereoscan electron microscopy examination revealed that penetration of supercritical carbon dioxide takes place via the advancing layer mechanism and that, for the particular grade of polycarbonate used in this study, the nucleation of the cells can take place at temperatures as low as 60°C. It requires, however, long treatment times and high pressures, and the growth of foam cells is severely restricted. Nucleation and growth of cells occurred much more readily at somewhat higher temperatures. With treatments at around 80°C, the onset of crystallization started to impose considerable hindrance to the formation of uniform and evenly distributed cells. This becomes increasingly evident at higher temperatures, between 100 and 180°C, owing to the formation of large spherulitic crystalline domains. A very effective nucleation and growth mechanism for the formation of cells was obtained, on the other hand, with treatments at 200°C due to the absence of crystallization phenomena. The degree of crystallinity increased with increasing treatment temperature, and the resulting morphology gave rise to two broad melting peaks. These are displaced to higher temperatures and become closer, merging into one peak with a low‐temperature shoulder. These events were monitored by thermal analysis and wide‐angle X‐ray diffraction examinations. © 2007 Wiley Periodicals, Inc. Adv Polym Techn 25:225–235, 2006; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20075

Global Kinetics of Wood Char Devolatilization and Combustion

Thermogravimetric curves in air of wood chars obtained from different species and conventional or fast pyrolysis show a low-temperature shoulder (devolatilization) followed by a high-temperature peak (combustion). A n-order global reaction provides a very poor description of the differential curves and, in agreement with previous literature, requires relatively low activation energies (114.5 and 167 kJ/mol for conventional and fast pyrolysis chars, respectively). The combination with an additional first-order reaction for the devolatilization stage produces accurate predictions of both integral and differential curves. It also gives rise to much higher activation energies of the combustion reaction (183 or 229 kJ/mol, depending on the severity of the pyrolysis conditions). The activation energy of the devolatilization reaction also increases with the severity of the pyrolysis conditions (from 114.5 to 218.5 kJ/mol), whereas the differences in the char reactivity deriving from the wood species can be taken into account by preexponential factors and order of the combustion reaction. The use of two first-order reactions for the devolatilization stage does not improve significantly the accuracy of the predictions.

The oxidation mechanism of Pd(1 0 0)

The oxidation of Pd(1 0 0) was characterized using temperature programmed desorption (TPD), low-energy electron diffraction (LEED), and in situ variable-temperature scanning tunneling microscopy (STM). The results indicate that Pd(1 0 0) oxidation proceeds through four stages involving up to five surface phases. In the first stage, oxygen chemisorbs atop the Pd surface resulting in p(2×2) and c(2×2) overlayers that desorb at 800 and 700 K, respectively. As the overlayers saturated, island and peninsula growth was observed in STM movies. The islands are one Pd atom high and so the growth is attributed to Pd atoms ejected onto the terraces which then either nucleated islands or attached to preexisting steps. During this second stage, no change in the surface periodicity was observed. After growth stops, the surface reconstructs in the third stage. Above 475 K, STM images revealed the formation of a ( 5 × 5 ) R27° reconstruction. The images of the ( 5 × 5 ) R27° structure are twofold symmetric, consistent with the formation an epitaxial PdO(0 0 1)-like layer. In addition, a (5×5) reconstruction was observed with LEED prior to the ( 5 × 5 ) R27° reconstruction. As the oxygen coverage entered the regime where these reconstructions were observed, a new desorption peak appeared at 650 K. This temperature is higher than that expected for PdO dissociation but lower than that observed for chemisorption suggesting that the Pd–O bond strength in the reconstructed surfaces is intermediate between the bulk oxide and chemisorbed oxygen. In the fourth stage, the LEED patterns began to fade, the STM images showed three-dimensional clusters, and a low-temperature shoulder consistent with PdO decomposition developed in TPD traces. Thus the surface roughens as the bulk oxide forms. The mechanism outlined above is similar to that recently observed for Pd(1 1 1) oxidation.

Rhodium-related deep levels in n-type MOCVD GaAs

Preliminary results on the study of deep levels associated with 4d-transition metal, rhodium, in crystalline GaAs grown by metal-organic chemical vapour deposition (MOCVD) technique are reported for the first time. Deep level transient spectroscopy on n-type GaAs doped in situ with Rh during MOCVD growth reveals a broad majority carrier emission peak. The peak corresponds to a band of deep levels extending over the energy range 0.57–0.65 eV below the conduction band edge with lower-energy states having lower electron capture cross-sections. The deep levels show a pronounced dependence of electron emission rate on the junction electric field. Minority carrier (hole) emission spectra at zero bias show a pronounced Rh-related deep-level peak with a low-temperature shoulder. The dominant level in the lower half-gap is found to have a position E v+0.44 eV, with a field-dependent emission rate signature.

Dynamic mechanical signatures of a polyester‐urethane and plastic‐bonded explosives based on this polymer

AbstractThe complex shear moduli of the segmented polyurethane Estane 5703p, Livermore explosive (LX)‐14, and plastic bonded explosive (PBX)‐9501, which use this polymer as a binder, have been investigated. Segmented polyurethanes, such as Estane 5703, contain microphase‐separated hard segments in a rubbery matrix of soft segments. LX‐14 is composed of 95.5% 1,3,5,7‐tetranitroazacyclooctane (HMX) explosive with 4.5% Estane 5703 binder. PBX‐9501 is composed of 94.9% HMX, 2.5% Estane 5703p binder, 2.5% nitroplasticizer (NP), and about 0.1% antioxidant Irganox 1010. In the temperature range from −150 to 120°C, two relaxations were observed as peaks in the loss modulus and tangent delta in Estane 5703p and LX‐14. A third relaxation was found in PBX‐9501. The low temperature relaxation associated with vitrification of the poly(ester urethane) soft segment occurred in the shear loss modulus (G″) at −29 and −26°C in Estane and LX‐14, respectively, at 1 Hz. In PBX‐9501 the Estane soft segment glass transition peak, Tg(SS), in the loss modulus occurred at −40 ± 3°C at 1 Hz. The reduction in soft segment glass transition in PBX‐9501 is clear evidence of plasticization of the soft segment by NP. The apparent activation energy of the maximum in the loss modulus for LX‐14 and PBX‐9501 over the frequency range from 0.1 to 10 Hz was 230 kJ/mole (55 kcal/mole). The hard segment glass transition, Tg(HS), was observed as a peak in the loss modulus at about 70°C. In LX‐14 the transition was observed at lower temperatures (56–58°C at 1 Hz) depending on thermal history. There was a low temperature shoulder on the Tg(HS) of Estane 5703 associated with soft segment crystallinity. Modulated differential scanning calorimetry (MDSC) was used to verify the Tg(HS) in Estane and 50/50 mixtures of Estane with NP. In PBX‐9501 the hard segment glass transition occurred between 65 and 72°C. The presence of NP in PBX‐9501 gave rise to a new transition, Teu(NP), between 8 and 15°C. This peak is believed to be associated with the eutectic melting of the plasticizer. Returns of fielded PBX‐9501 that were 6 and 11 years old were also measured. Small variations in Tg(SS) and the rubber plateau modulus were observed in these aged samples, consistent with migration of plasticizer and/or very low levels of chain scission. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 1009–1024, 2002

CO/Rh(111): Vibrational frequency shifts and lateral interactions in adsorbate layers

High resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and thermal desorption spectroscopy (TDS) were used to study lateral interactions in the adsorbate layer of the CO/Rh(111) system. The vibrational spectra show that CO adsorbs exclusively on top at low coverage. At about half a monolayer a second adsorption site, the threefold hollow site, becomes occupied as well. A steady shift to higher frequencies of the internal C–O vibrations is observed over the whole coverage range. The frequency of the metal CO (M–CO) vibration in the on-top mode hardly shifts at low coverage. However, upon the emergence of the second adsorption site the M–CO vibrations experience a shift to lower frequencies. The population of the second site is also accompanied by the development of a low temperature shoulder in the TD spectra, indicating an increasingly repulsive interaction in the adsorbed CO layer. Vibrational spectra of isotopic mixtures of CO12 and CO13 were used to assess the origin of the observed frequency shifts. They confirm that frequency shifts of the C–O stretching vibration at total CO coverage of 0.33 ML in the (√3×√3)R30° structure arise purely from dipole–dipole coupling. Dilution of an isotopic species effectively suppresses frequency shifts arising from dipole–dipole coupling. Therefore, experiments with a small amount of CO13 as a tracer to monitor the frequency shifts in the CO12 adlayer were carried out over the entire coverage range of CO12. The results demonstrate that dipole–dipole coupling causes the frequency shifts at low coverage (&amp;lt;0.5 ML), whereas chemical effects set in at higher coverage (0.5–0.75 ML), connected with the population of the threefold sites. The results illustrate that HREELS in combination with isotopic dilution is a powerful tool in the assessment of lateral interactions between adsorbed molecules.

Collapse transition of isolated Lennard-Jones chain molecules: Exact results for short chains

In this work we study flexible Lennard-Jones (LJ) interaction-site chain molecules with fixed bond length L. We obtain “exact” numerical results for the equilibrium configurational and energetic properties of LJ n-mer chains with n=3, 4, and 5 for a range of L (0.8⩽L/σ⩽21/6). These results include intramolecular site–site distribution functions and fluctuations in site–site distances, radius of gyration, and average potential energy. We also compute the single-chain specific heat, chemical potential, and theta temperature. With decreasing temperature the chain molecules undergo a collapse transition from an extended disordered state to a compact highly ordered state, exhibiting an associated peak in the specific heat. For n=3 and 4 the collapse occurs in a single stage whereas for the n=5 chains a multistage collapse is possible. In particular, the specific heat peaks associated with the collapse of 5-mer chains with bond lengths in the range 0.85⩽L/σ⩽1.0 display either a low temperature shoulder or secondary maximum. We show that this complex specific heat behavior arises when the initial collapsed state consists of a set of distinct and nondegenerate structural isomers. The secondary peak or shoulder is a result of the “freezing out” of all but the ground state isomer. These results are discussed in the context of the similar anomalous specific heat behavior, and low temperature structural transitions observed in recent simulation studies of square-well and LJ chains and atomic clusters.