Measurements of photoinduced light absorption in Ca₃Mn₂Ge₃O₁₂ revealed some unusual features: a saturation with the pumping intensity and a broad straggling of relaxation times with a predominance of very long times. These experimental facts cannot be understood in terms of photoinduced absorption centers accociated with impurities or lattice defects, but are naturally explained within the notion of random electric fields of active charges. Active charges are produced by light pumping via the dissociation of coupled pairs of charges (consisting of a Mn-hole coupled with a compensating negative impurity or a negatively charged vacancy) which exist in the groun stare. Such active charges create electric fields in a larger volume than coupled pairs, thus enhancing the probability for forbodden optical transitions. On the other hand, the random fields of active charges promote hopping of holes and hence the relaxation of photoinduced effects. A broad distribution of the magnitudes of random fields gives rise to a very broad range of the hole hopping rates. There is also a much faster annihilation process immediately conditioned by light pumping. The simultaneous action of these relaxation channels, depending on the number of active charges, pumping intensity and temperature, explains the entire experimental picture qualitatively and in part qualitatively. Photoinduced dichroism as well as birefringence, observed under polarized pumping, are caused by an anisotropic distribution of photoproduced holes over polarozation directions.

Optical reflectivity studies of the ferromagnetic metal (FMM) to antiferromagnetic insulator (AFI) phase transition are performed on Nd0,5Sr0,5MnO3 manganite in a wide temperature and magnetic field range. The formation of a domain structure in the AFI state during the FMM-AFI phase transition is observed. It is shown that the two types of domains observed are energetically equivalent states. On the basis of the experimental results and symmetry analysis we conclude that these domains are crystal twins. The twin domain structure of the AFI state in the Nd0,5Sr0,5MnO3 is visible in reflected unpolarized light due to a different tilting of the surface in the domains. The two-phase domain structure FMM + AFI formed in the vicinity of the phase transition is also studied. It is found that a thermodynamically equilibrium two-phase stripe domain structure does not develop. The absence of the magnetic intermediate state is due to the large energy of the interphase wall, which results in the stripe structure period being much larger than the size of the sample.

The effect of linearly polarized light illumination on the metamagnetic phase transition in the antiferromagnetic garnet Ca3Mn2Ge3O12 is studied. The crystal is exposed to light propagating both along the tetragonal axis [001] and along the [100] direction. In both cases, a change of the field <$E H sub t > of the metamagnetic phase transition is observed under illumination, and this change depends on the orientation of plane of polarization of the light with respect to the crystal axes. In the first case, <$E bold roman {k ~||~ H ~||}> [001], the value of <$E H sub t > decreases on exposure to light with the polarization <$E bold roman E ||> [110] and increases on exposure to light with the polarization <$E bold roman E ~||~ [1 1 Bar 0]>. In the second case, <$E bold roman k ~||~ bold roman H ~||> [100], the value of <$E H sub t > decreases irrespective of the orientation of the plane of polarization of the light with respect to the crystal axes. However, the magnitudes of the change of <$E H sub t > are different for light with the polarization <$E bold roman E ||> [011] and with the polarization <$E bold roman E ~||~ [01 Bar 1]>. The change of the field of the metamagnetic phase transition in the second case was much larger than in the first case. A phenomenological theory of the photomagnetic effects observed in the antiferromagnetic garnet Ca3Mn2Ge3O12 is developed. It is shown that the effect of light illumination on the metamagnetic phase transition is related to the photoinduced magnetic moment in this antiferromagnet.

The magnetic moment induced by linearly polarized light in the garnet Ca3Mn2Ge3O12 is detected experimentally by means of a SQUID magnetometer.

Досліджено кінетику релаксації персистентного додаткового оптичного поглинання, спричиненого опроміненням гранатів <$E roman {NaCa sub 2 Mn sub 2 V sub 3 O sub 12}>, <$E roman {Ca sub 3 Mn sub 2 Ge sub 3 O sub 12}> і <$E roman {Ca sub 3 Gd} sub 2-x {roman Mn} sub x roman {Ge sub 3 O sub 12}> видимим світлом. Показано, що кінетика релаксації пов'язана зі структурою гранатів. Нетривіальна особливість кінетики, спільна для всіх досліджуваних гранатів, зумовлена спільним елементом структури - аніонною групою <$E roman O sub 12>, що утворює підгратку кисневих іонів <$E roman O sup 2->. Релаксаційна кінетика відбиває зародження та рух діркових поляронів у кисневій підгратці, а від катіонного складу залежить стрибковий бар'єр поляронів, що змінюється в не дуже широких межах. Роль кисневої підгратки переконливо підтверджено енергетичними міркуваннями.

The crystal structure of the manganite Nd0,5Sr0,5MnO3 is studied at temperatures T = 300 and 77,3 K by means of an x-ray diffractometer. It is shown that the transition from the ferromagnetic metallic state to the antiferromagnetic insulating charge-ordered state is accompanied by a lowering of the symmetry of the structure from orthorhombic to monoclinic. The space-group symmetry of the orthorhombic and monoclinic phases is identified as Imma and <$E P2 sub 1 "/" m>, respectively. Twinning of the crystal and the formation of a twin domain structure with coherent boundaries in the (00l) crystallographic planes are found.

It is shown that long-lived photoinduced dichroism in garnets is caused by photoproduced charges with anisotropic structure, keeping long memory of the pumping light polarization, while photoinduced absorption is due to all photoproduced charges irrespective of their intrinsic structure. The charges with anisotropic structure are identified as two-center oxygen holes. The formation of an oxygen hole is preceded by the excitation of a charge-transfer state with electron partially transferred to a cation C (<$E roman V sup 5+> for NaCa2Mn2V3O12 garnet) from an adjacent oxygen anion. To turn this excited state into a free hole state requires some time <$E tau sub roman hole> during which the hole axis can be reoriented resulting in a diminution of dichroism. The time <$E tau sub roman hole> shortens with increasing ionization potential of the cation С (very high for <$E roman V sup 5+>). Such a mechanism explains qualitatively a set of unusual experimental facts, in particular, a very strong dichroism observed just in the NaCa2Mn2V3O12 garnet, where photoinduced changes of all optical properties disappear after switching off the irradiation significantly faster than those in other garnets examined.

The magnetic and magnetoresonance properties of a new single-crystal compound Ni5(TeO3)4Cl2 are studied. The measurements of the magnetization and magnetic susceptibility of the crystal in a wide temperature range (5 - 300 K) made it possible to conclude that Ni5(TeO3)4Cl2 is a quasi-two-dimensional antiferromagnet with the easy magnetization axis <$E bold roman a sup *> directed perpendicular crystallographic plane bc and a magnetic ordering temperature <$E T sub N~symbol Ы~21> K. The resonance measurements at 4,2 K in wide range of frequencies (25 - 105 GHz) and magnetic field (up to 200 kOe) permitted us to obtain the frequency-field dependence of AFMR spectrum for a field applied along the easy magnetization axis <$E bold roman a sup *>. It is shown that the magnetic field directed along the antiferromagnetism axis (<$E bold roman H || bold roman a sup *>) induces the magnetic phase transition of a spin-flop type which is found to be <$E H sub sf~symbol Ы~120> kOe. The magnetic resonance experimental data are described qualitatively in model of the biaxial antiferromagnet.

Using Raman scattering and optical birefringence we have investigated a low-temperature phase transition in single crystal of the two-dimensional Na5RbCu4(AsO4)4Cl2. Phonon anomalies point to a first order nature of the transition. The observed transition is most probably related to a order-disorder transition of the Rb ion positions along the z axis within the ionic framework of mixed alkali metal chloride lattices.

Magnetization studies for <$E {roman FeSe} sub 1-x {roman Te} sub x> (<$E x~symbol @~0,~0,5>, and 1,0) compounds were carried out in magnetic fields up to 50 kOe and in the temperature range 2 - 300 K. The superconducting transition was observed at <$E T sub c~symbol @~8> K and 13,6 - 14,2 K in FeSe0,963 and FeSe0,5Te0,5, respectively. For the most samples, a nonlinear behavior of the magnetization curves in the normal state gives evidence of a commonly observed substantial presence of ferromagnetic impurities in the compounds under study. By taking these impurity effects into account, the intrinsic magnetic susceptibility <$E chi> of FeSe0,963, FeSe0,5Te0,5, and FeTe was estimated to increase gradually with Te content. For FeTe a drastic drop in <$E chi (T)> with decreasing temperature was found at <$E T sub N~symbol @~70> K, which is presumably related to antiferromagnetic ordering. To shed light on the observed magnetic properties, ab initio calculations of the exchange enhanced magnetic susceptibility are performed for FeSe and FeTe within the local spin density approximation.

Індекс рубрикатора НБУВ: В368.313

Рубрики:

Магнітні властивості надпровідників

Шифр НБУВ: Ж14063 Пошук видання у каталогах НБУВ 
Повний текст  Наукова періодика України 

We report Raman scattering studies and magneto/structural symmetry analysis of the many sublattices ferrimagnet Cu2OSeO3 with a cubic symmetry and a linear magnetoelectric effect. There is no spectroscopic evidence for structural lattice distortions below <$E T sub C~=~60> K, which are expected due to magnetoelectric coupling. Using symmetry arguments we explain this observation by considering a special type of ferrimagnetic ground state which does not generate a spontaneous electric polarization. Interestingly, Raman scattering shows a strong increase of electric polarization of media through a dynamic magnetoelectric effect as a remarkable enhancement of the scattering intensity below <$E T sub C>. New lines of purely magnetic origin have been detected in the magnetically ordered state. A part of them are attributed as scattering on exchange magnons. Using this observation and further symmetry considerations we argue for strong Dzyaloshinskii - Moriya interaction existing in the Cu2OSeO3.

Для новітніх надпровідників системи <$E {roman FeSe} sub 1-x {roman Te} sub x> виконано розрахунки електронних структур з перших принципів з метою пояснення експериментально знайдених аномальних магнітних властивостей у нормальному стані. Розрахунки показали, що сполуки <$E {roman FeSe} sub 1-x {roman Te} sub x> є близькими до магнітної нестабільності з домінуючим посиленим спіновим парамагнетизмом. Встановлено, що магнітна сприйнятливість монотонно зростає зі збільшенням концентрації Te. Температурні залежності магнітної сприйнятливості <$E chi> і її анізотропію <$E DELTA chi> досліджено для FeSe, і встановлено зростання сприйнятливості з температурою в діапазоні 4,2 - 300 K. Для FeTe виявлено істотне збільшення <$E chi> під тиском. Розрахунки парамагнітної сприйнятливості виявили її сильну залежність від об'єму елементарної комірки V і, особливо, від відстані між шарами заліза та халькогену. Проведені розрахунки пояснюють експериментальні залежності <$E chi (T)> і <$E chi (P)> для FeSe і FeTe відповідно.

Індекс рубрикатора НБУВ: В368.2

Рубрики:

Високотемпературна надпровідність. Високотемпературні надпровідники

Шифр НБУВ: Ж26988 Пошук видання у каталогах НБУВ 
Повний текст  Наукова періодика України 

Магнітну сприйнятливість <$E chi> сполук FeSe і FeTe, які утворюють найпростішу систему FeSe(Te) нових надпровідників на основі заліза, вивчено в нормальному стані під дією гідростатичного тиску. Значний позитивний ефект тиску на <$E chi> визначено за низьких температур для обох сполук. За кімнатної температури цей ефект виявляється також сильним, але негативним - для FeSe і позитивним - для FeTe. Розрахунки з перших принципів залежності електронної структури й магнітної сприйнятливості від тиску показали, що FeSe і FeTe близькі до магнітної нестійкості з переважанням обмінно-посиленого спінового парамагнетизму. Обчислена парамагнітна сприйнятливість проявляє сильну залежність від об'єму елементарної комірки й висоти Z шару халькогену над площиною заліза. Великі позитивні ефекти тиску на <$E chi> в FeTe і FeSe, які спостерігалися за низьких температур, пов'язані зі значною чутливістю парамагнетизму до внутрішнього параметра Z. Показано, що наявні експериментальні дані про сильну й немонотонну залежність температурного надпровідного переходу від тиску в FeSe якісно корелюють із розрахованою поведінкою густини електронних станів на рівні Фермі.

We present a comparative study of the low-dimensional compounds alpha- and beta-TeVO4. Our data clearly show that the change in the local coordination geometry of V<^>4+ ions comparing alpha- and beta-TeVO4 leads to a drastic differences in the magnetic properties. Despite sharing the same crystal structure, the two compounds realize different magnetic exchange topologies. Both compounds exhibit a transition from ferro- to antiferromagnetic correlations with decreasing temperature. This effect however is driven by different mechanisms for both compounds. Additionally, a dimensional crossover is found in beta-TeVO4.

We report Raman light scattering in the phase separated superconducting single crystal Rb0,77Fe1,61Se2 with <$E T sub c~=~32> K over a wide temperature region 3 - 500 K. The observed phonon lines from the majority vacancy ordered Rb2Fe4Se5 (245) antiferromagnetic phase with <$E T sub N~=~525> K demonstrate modest anomalies in the frequency, intensity and halfwidth at the superconductive phase transition. We identify phonon lines from the minority compressed <$E roman {Rb sub delta Fe sub 2 Se sub 2}> (122) conductive phase. The superconducting gap with <$E d sub {x sup 2 - y sup 2}> symmetry has been detected in our spectra. In the range 0 - 600 cm<^>-1 we observe a weak but highly polarized <$E B sub roman 1g>-type backgroundwhich becomes well-structured upon cooling. A possible magnetic or multiorbital origin of this background is discussed. We argue that the phase separation in M0,8+xFe1,6+ySe2 is of pure magnetic origin. It occurs below the Neel temperature when the magnetic moment of iron reaches a critical value. We state that there is a spacer between the majority 245 and minority 122 phases. Using ab initio spin-polarized band structure calculations we demonstrate that the compressed vacancy ordered Rb2Fe4Se5 phase can be conductive and therefore may serve as a protective interface spacer between the purely metallic <$E roman {Rb sub delta Fe sub 2 Se sub 2}> phase and the insulating Rb2Fe4Se5 phase providing percolative Josephson-junction like superconductivity all throughout of Rb0,8+xFe1,6+ySe2. Our lattice dynamics calculations show significant differences in the phonon spectra of the conductive and insulating Rb2Fe4Se5 phases.

Polarized absorption spectra of HoFe3(BO3)4 single crystal in the range of 8500 - 24500 cm<^>-1 were studied asa function of temperature beginning from 2 K. The ground and excited electron states of Ho<^>3+ were identified.The abrupt changes of the spectra at the reorientation phase transition at 4,7 K were observed. The exchange splitting of some excited states were revealed and measured. They changed at the reorientation phase transition. Several vibronic transitions were observed. The splitting of absorption lines corresponding to the C2 local symmetry of the Ho ion was not observed. Moreover, spectra of some absorption bands correspond to splitting in the cubic crystal field. There are some absorption lines, whose polarization can not be explained both in D3 and C2 local symmetries. Some lines appear or disappear as a result of the transition from the easy axis to the easy plane state of the crystal. All these observations testify to the substantial changes of the local magnetic and structural properties in the excited states of the Ho<^>3+ ion and to the strong influence of the magnetic moments orientationon the polarization of the electron transitions.