Search

Time-resolved photoluminescence has been studied in molecular beam epitaxy grown Cd 1− x Mn x Te/Cd 1− y Mg y Te and CdTe/Cd 1− x Mn x Te single and multiple quantum wells. The excitonic properties were investigated at several temperatures and magnetic fields. The dynamics of the exciton magnetic polaron are greatly influenced by the quantum confinement of the excitons in the single quantum wells. The formation of a spin superlattice structure was realized in Cd 0.95 Mn 0.05 Te/Cd 0.95 Mg 0.05 Te multi-quantum wells under an external magnetic field.

Using the pulsed excitation with a femtosecond laser and streak-camera detection we have studied the processes of spin relaxation in the system of localized excitons in low dimensional superlattices formed by the insertion of CdSe submonolayers into the ZnSe matrices. Polarization of exciton luminescence in the magnetic field arises due to the thermal redistribution of population in the system of splitted exciton spin sublevels. The distribution of polarization across the PL band countour is governed by the complicated interplay of energy and spin relaxation processes. The study of PL decay kinetics in polarized light allows us to determine with the help of a simple two-level model the times of exciton spin relaxation. Spin relaxation times strongly decrease with the increase of magnetic field indicating the phonon assisted mechanism of spin relaxation. The detailed analysis of the dependence of spin relaxation times on the exciton localization energy is not possible with the model used due to the complicated character of exciton population kinetics for deeply localized states. Deeply localized states are to a great extent populated through the tunnelling relaxation of excitons with higher energies. The transfer of polarization in the processes of exciton energy relaxation has been clearly observed in the study of spectrally resolved decay kinetics in polarized light.

We report an experimental evidence of the existence of free excitonic magnetic polaron (FEMP) states from the study of the dynamics of free and bound excitons in Cd 1−x Mn x Te (x=0.03) by time-resolved photoluminescence measurements. We found, for the first time, that FEMP are more stable than the polarons formed from bound excitons. The polaron binding energy for the free excitons is evaluated to be 4 meV , which is considerably larger than that of 1 meV found for the magnetic polarons of the bound excitons; these values were determined from the suppression of the excitonic energy relaxation in high magnetic fields. As a consequence, the peak energies of the optical emissions of free excitons and donor-bound excitons cross in the time-resolved photoluminescence spectra.

Optical pump-probe experiments were carried out in Cd1−xMnxTe/ZnTe multiple quantum wells to study the dynamics of the excitons and the exciton magnetic polarons in these materials. In the pump-probe spectra, three signals were detected. The main signal is negative and monitors the energy relaxation of the excitons created by the pump pulse. The two other pump-probe signals are positive and their intensity grows during the time when the excitons created by the pump pulse recombine. The low-energy positive pump-probe signal is found to track the average Mn spin polarization in the sample. Experiments performed with circular polarization show that long-range order is induced by a sufficiently intense pump beam. But in our samples, the spins of the excitons are found to be oriented mainly antiparallel to the long-range magnetization generated after the pump excitation. Furthermore, we found that, at high excitation densities, the excitons couple antiferromagnetically to the long-range ‘background’ magnetization of the Mn spins.

The formation of a spin superlattice structure was realized in Cd0.95Mn0.05Te/Cd0.95Mg0.05Te under a magnetic field. The experimental evidence of the spin superlattice structure was found from time-resolved polarized magneto-photoluminescence measurements, in the Zeeman splittings and the exciton magnetic polaron formation. The Zeeman splitting directly indicates the spatial separation of spin-up and spin-down electrons and heavy holes, which are localized in the Cd0.95Mg0.05Te and Cd0.95Mn0.05Te layers, respectively. The formation of exciton magnetic polarons under a magnetic field can be attributed to the spin-dependent confinement where a large polaron binding energy of 3.5 meV has been observed in 2.5 T to be enhanced by the field-induced localization of the excitons.

We carried out time-resolved magneto-photoluminescence experiments in diluted magnetic semiconducting Cd1−xMnxTe MBE grown epilayers to study the excitonic localization mechanisms. We found that, for samples with a high non-radiative recombination rate, the luminescence intensity and the decay time decrease significantly under a magnetic field. This shows that the magnetic localization dominates over the non-magnetic localization when magnetic polarons are formed.

We present optical pump–probe experiments carried out in Cd1—xMnxTe/ZnTe multiple quantum wells to study the dynamics of excitons, exchange-coupled to Mn spins in the form of magnetic polarons. From the analysis of the transient absorption spectra, we found that the magnetic polaron formation a) suppresses the formation of biexcitons (this simplifies the lineshape analysis of the pump–probe spectra) and b) retards the energy relaxation of the excitons by 14 ps in these semimagnetic structures.

Optical pump–probe experiments were carried out in Cd 1− x Mn x Te/ZnTe multiple-quantum wells to study the carrier dynamics and the exciton formation in these materials. Starting from 15 ps after the pump excitation, the bleaching of the absorption of the heavy-hole excitons in the wells yields a strong negative pump–probe signal. But before this occurs, the pump–probe signal has a completely different lineshape resembling a lying `S'. The time evolution of the pump–probe signal is understood to have arisen from a coexistence of carriers and excitons. We treated the excitons in the wells as Mahan excitons and obtained a good fit to the experimental data. The cooling of the carriers after the pump excitation can be monitored as an increasing Fermi energy of the `cold’ carriers at the band edge. The Fermi sea is then depleted starting from 15 ps after the pump excitation due to the formation of excitons.

Time-resolved photoluminescence has been studied on molecular beam epitaxy grown Cd 1− x Mn x Te epilayers and Cd 1− x Mn x Te/Cd 1− y Mg y Te single- and multiple-quantum wells. The formation of exciton magnetic polarons is detected in the single-quantum wells of various well widths. The transient photoluminescence of magnetic polarons is significantly affected by the external field. The photoluminescence intensity of the exciton is found to show two-exponential decay characteristics. The magnetic field dependence of the two decay times is attributed to the conversion of the dark excitons to the bright excitons by a spin-flip process. The temperature dependence of exciton decay times is interpreted as the thermal activation process in the multiple-quantum well.

We report on a field-dependent photoluminescence (PL) emission rate for the transitions between band states in modulation-doped CdTe/Cd1−xMgxTe single quantum wells in the integer quantum Hall region. The recombination time observed for the magneto-PL spectra varies in concomitance with the integer quantum Hall plateaus. Furthermore, different PL decay times were observed for the two circular polarizations, i.e. for the transitions between the Zeeman split subbands of the Landau levels. We analyzed the data in comparison with the experimentally determined spin polarization of the conduction electrons and the Zeeman splitting of the valence band. Furthermore, we discuss the relevance of the spin polarization of the conduction electrons, the electron–hole exchange interaction and the spin-flip processes of the hole states for the PL decay time.

We present a study of the exciton dynamics in modulation doped ${\mathrm{G}\mathrm{a}\mathrm{A}\mathrm{s}/\mathrm{G}\mathrm{a}}_{1\ensuremath{-}x}{\mathrm{Al}}_{x}\mathrm{As}$ heterojunctions and GaAs epilayers. The comparison permits to identify the features characteristic for the heterojunctions. In particular, we analyze the rise time of the transient photoluminescence (PL) intensity. In general, we find a longer PL rise time for the lower-energy excitons indicating that the time required for the energy relaxation process increases with increasing binding energy of the excitons. Moreover, the rise time of the free excitons turns out to be conspicuously longer in heterojunctions than in epilayers although the time integrated PL spectra of the two systems are similar. From our analysis we conclude that the long rise time observed in the heterojunctions is closely connected with the vertical drift of the photoexcited carriers driven by the interface potential. In fact, we find that the carriers (i.e., holes in n-doped samples) photoexcited in the heterojunctions drift vertically from the heterointerface to the flat band region, where they finally recombine after forming excitons (bimolecular formation) with oppositely charged and locally excited carriers (i.e., electrons in n-doped samples).

Exciton dynamics has been studied in quantum wells, quantum dots and quantum wires of diluted magnetic semiconductors by time-resolved photoluminescence spectroscopy. The transient exciton photoluminescence with resonant excitation in Cd 1− x Mn x Te/ZnTe ( x =0.1) multiple quantum wells shows the formation of magnetic polarons in the quantum well excitons. Free and localized states of the excitonic magnetic polarons are distinguished from the difference of the lifetimes and the binding energies. The magnetic polaron formation accompanied with emission or absorption of LO phonons was also observed. Quantum dots of Cd 1− x Mn x Se grown by the self-organization mode showed the luminescence of confined excitons in the dots. The Mn ions in the quantum dots induce large Zeeman shifts and the excitonic magnetic polaron formation in the dots. Quantum wires of Cd 1− x Mn x Se showed polarized exciton emissions parallel to the wires. The polarization was decreased by the external magnetic field perpendicular to the wires. The excitonic magnetic polaron states in the quantum wires are also discussed.

We present the magneto-optical polar Kerr spectra of cleaved single crystals of the cerium monopnictides taken at 1.5 K and in strong magnetic fields in the photon energy range from 0.23 to 5.3 eV. These spectra are dominated by two features of unprecedented magnitude, i.e. a peak in the IR around 0.5 eV and a second feature in the UV around 5.3 eV. The magneto-optical signals of cerium compounds have an outstanding figure of merit, several orders of magnitude larger than all other classes of magnetic materials. This is discussed with a phenomenological model, which yields vanishingly small magneto-optical signals for both, well localized and itinerant electronic states, and only states on the borderline between localized and itinerant have a strong magneto-optical activity. This is optimally realized for the 4f 1 state in cerium compounds

To investigate about the effective mass of the carriers at the Fermi level, we carried out cyclotron resonance experiments in single crystals of the intermediate valent systems Yb 4 (As 0.6 P 0.4 ) 3 and Yb 4 (As 0.71 Sb 0.29 ) 3 at several frequencies above 95 GHz. In the former material, a strong double cyclotron resonance absorption indicates cyclotron masses of 0.60 and 0.87 m e , while in the latter material no cyclotron absorption was observed up to 18 T. This excludes the possibility of Yb 4 (As 0.6 P 0.4 ) 3 to be a heavy fermion and indicates a lower limit of 5.3 m e for the cyclotron mass in Yb 4 (As 0.71 Sb 0.29 ) 3 .

The recently discovered novel giant polar magneto-optical Kerr rotation of 90° in CeSb is compared to previous record Kerr signals of the order of 10°, and to another newly discovered giant Kerr signal of 22° in CeS. An analysis of the temperature and field dependence of the off-diagonal conductivity spectra of CeSb is presented which allows an empirical assignment of several structures to electronic transitions. In particular, a folding of bands due to magnetic superstructures in a ferrimagnetic phase is shown to account for three additional peaks.

Recent measurements of the magneto-optical Kerr effect of the cerium monochalcogenides and monopnictides have brought a deeper insight into the character of the 4f 1 state. In these materials, the 4f → 5d transitions give rise to very sharp and larger Kerr rotation peaks. Among these, one finds the three largest ever measured Kerr rotation angles of 90°, 22.1° and 9.2° for cleaved single crystals of CeSb, CeS and CeBi, respectively. Furthermore, for the 4f 1 level, the effect of pf mixing has been found to be significant.

We report the first observation of the magneto-optical signal of an empty spin-polarized 4f state in a paramagnetic metal. At 1.5K and in a magnetic field of 10T the Kerr rotation spectrum of LaSe shows a remarkably sharp {bold {ital positive}} peak of 2.1{degree} at 2.77eV and a broader and smaller structure at 3.01eV. A comparison with CeSe, which displays a similar sharp peak with opposite sign but no second peak, gives evidence that the LaSe signal is due to 5d{r_arrow}4f (J=5/2) and 5d{r_arrow}4f (J=7/2) transitions, while the magneto-optical signal in CeSe corresponds to a 4f(J=5/2){r_arrow}5d transition. {copyright} {ital 1997} {ital The American Physical Society}

We report on experimental evidence of different electronic phases in the superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$: simple metallic CuO chains and highly correlated ${\mathrm{CuO}}_{2}$ planes. ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ is a genuine untwinned compound; hence, we were able to determine the anisotropy of the resistivity along the a and b directions. Along the b direction (chain and plane conduction channels), a normal metallic temperature behavior of the chain dominates. For the a direction (only plane conduction channel), the resistivity reveals an unconventional behavior with a kink at 160 K, becoming linear at higher temperatures. Further, we present results of the dynamical (optical) conductivity of the ${\mathrm{CuO}}_{2}$ plane as a function of frequency and temperature. The frequency dependence of the optical conductivity is consistent with a model of ferromagnetic polarons in an antiferromagnetic matrix. This is further confirmed by a depolarization experiment which is sensitive to crystal regions with different spin polarizations. The temperature behavior of the thermal occupation of the ground state is in agreement with a real-space condensation.

Strong flux creep effects are observed in CeCu2Si2 at low magnetic inductions and temperatures in the range 6 mK

We present a study of the dynamics of the excitonic magnetic polarons in Cd1−xMnxTe epilayers. The transient photoluminescence measured by a streak camera under resonant excitation is analyzed with a special attention devoted to line shape of the emission signal. A nonradiative spectral region is observed in zero field and is strongly suppressed in a field of 6 T. This observation is understood to be directly related to the alignment of the Mn2+ spins and is therefore closely related to the formation of magnetic polarons. A few Mn spins are found to be aligned already within 10 ps, but the total Mn spin polarization in the sample is found to increase exponentially with a rise time of 37 ps in Cd0.73Mn0.27Te epilayers. Furthermore, the values of 19.2 and 12.4 meV were determined for the energies of the excitonic magnetic polarons in Cd0.73Mn0.27Te and Cd0.79Mn0.21Te epilayers, respectively.

Time-resolved photoluminescence (PL) and optical pump–probe experiments were carried out in Cd1−xMnxTe/ZnTe multiple quantum wells to study the dynamics of the exciton magnetic polaron in this material. In the pump–probe data, three signals were detected. The main signal is negative and monitors the energy relaxation of the excitons created by the pump pulse. The other two pump–probe signals are positive and their intensity grows while the excitons created by the pump pulse recombine. The low-energy positive pump–probe signal is found to track the average Mn spin polarization in the sample. On the contrary, the high-energy positive peak indicates the enhancement of the density of states due to correlation effects.

Exciton localization is studied with time-resolved photoluminescence (PL) in Cd 1− x Mg x Te epilayers. The energy relaxation rate of excitons is found to be energy dependent and reduces drastically below an effective excitonic mobility edge. This particular property results in the saturation of the PL intensity rise time below the mobility edge and a two-exponential decay for the PL intensity and the energy relaxation of the excitons.

The lifetime of excitons in Cd1−xMnxTe/Cd1−yMgyTe single quantum wells was investigated by time-resolved photoluminescence spectroscopy. Spin-flips between dark and bright excitons remarkably change the excitonic lifetime yielding a two-exponential decay in the photoluminescence intensity. The temperature dependence of the two decay times shows that the spin-flip rate depends strongly on the formation of excitonic magnetic polarons. By forming polarons, the spin-flip rate from dark excitons to bright excitons is reduced effectively by the spin- barriers of the polarized Mn ions, which extends significantly the luminescence decay time.

The decay properties of excitons in Cd 1− x Mn x Te/Cd 1− y Mg y Te single and multiple quantum wells are investigated by the time-resolved photoluminescence measurements. We observed the conversion of the dark excitons to the bright excitons by a spin-flip process. This yields a two-exponential decay of the photoluminescence intensity. The magnetic field dependence of the two-exponential decay is discussed in the frame of the spin-flip scattering and the giant Zeeman splitting in single quantum wells. The temperature dependence of exciton decays is interpreted by the increase of exciton scattering rate due to thermal activation in multiple quantum well.

The asymmetric line shape of the transient photoluminescence (PL) spectra of high-quality Zn1−x−yMgxCdySe epilayers is deconvoluted to a narrow and a broad Gaussian peak at each time delay. The fitted energy difference between the two peaks corresponds to one longitudinal optical (LO) phonon energy. We assign the narrow peak to the recombination of free excitons (FXs) and the broad peak to the recombination of excitons localized in the broad band tail characteristic for this quaternary compound. The broad PL peak is found to arise from the energy relaxation of the FX at the exciton mobility edge to the localized states by LO phonon emission. Therefore, the population peak of the localized excitons is formed at one LO phonon energy below the FX for all times after the laser excitation.

Optical pump-probe experiments were performed in Cd0.9Mn0.1Te/ZnTe multiple quantum wells to study the dynamics of the exciton magnetic polaron in this material. Two transient absorptions of opposite sign were observed. The first signal is negative and indicates the decrease of optical absorption, as the excitons are approaching saturation. The second signal is positive and is interpreted as the absorption of “new excitons” after the initial excitons created by the pump pulse have decayed leaving behind a magnetic polarized Mn environment. The spin relaxation time of the Mn ions has been determined to be 601 ps. Finally, we discuss the effect of the polaron–polaron interaction, relevant in our experiment because we saturate the excitons in the quantum wells.

The 4f correlations in Yb4(As0.88Sb0.12)3 and Yb4(As0.71Sb0.29)3 give rise to an exceptional field-dependence of the optical reflectivity, measured for photon energies between 3 meV and 0.8 eV at 15 K under magnetic fields up to 6 T. From the optical spectra, the energy difference between the occupied and empty 4f14 state in these materials is estimated to be 0.42 eV. Furthermore, we have detected a strongly field dependent Drude-like absorption in the far infrared and an unusual field enhancement of the phonon peaks.

The IR magneto-optical Kerr effect of CeSb is rich in anomalies. In particular, the Kerr rotation reaches the new record value of 90° at 0.46 eV at a temperature of 1.5 K with an external field of 5 T. At this maximum, the Kerr rotation jumps with no intermediate values from +90 ° to −90° . We present measurements of the field dependencies of the Kerr rotation taken at 1.5 K for fields up to 10 T on both sides of the Kerr rotation jump, at 0.25 and 0.57 eV. Both field dependencies show three magnetic phase transitions. But the general shape of the field dependence of the Kerr rotation θ K is quite different from the magnetization curve M , breaking the usually accepted proportionality between θ K and M . The origins of this anomaly are discussed.

The Hall resistivity ϱH in single crystals of CeS, CeSe and CeTe has been measured between 1.8 and 300 K. In all the compounds ϱH, being always negative, is proportional to the magnetic field up to 70 kOe in the paramagnetic region. The temperature dependence of the Hall coefficient RH reflects that of the magnetic susceptibility above ∼ 150 K, below which it exhibits a more complicated behavior. The ordinary Hall coefficient derived from a plot of RH(T − θP) against T − θP above 150 K suggests one conduction electron per Ce ion.

We report on measurements of the room temperature reflectivity between 0.03 and 12.4 eV and of the magneto-optical polar Kerr rotation in the energy range between 0.6 and 5.3 eV at 1.35 K in a magnetic field of 10 T on single crystals of CeSe. Although the magnetic moment amounts only to 0.376μB/Ce, a remarkable Kerr rotation τK of 5.5° has been measured at the photon energy of 2.6 eV. We assign this peak in θK to a 4f1 → 5d (EF) transition, enhanced by the low values of the optical constants in the plasma minimum of CeSe. A comparison with the magneto-optical spectrum of CeTe and with the optical spectra of CeS and CeTe is made.

We investigated the dynamics of free and bound excitonic magnetic polarons in ${\mathrm{Cd}}_{1\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Te}$ $(x=0.03)$ single crystals by time-resolved photoluminescence experiments. We found that the magnetic polarons formed from free excitons are more stable than those formed from bound excitons. As a consequence, the peak energies of the optical emissions of free excitons and donor-bound excitons cross in the time-resolved photoluminescence spectra. Furthermore, we observed that nonmagnetic localization is not required to form free magnetic polarons.

We report the first discovery of 90\ifmmode^\circ\else\textdegree\fi{} magneto-optical Kerr rotation in CeSb at 1.5 K in a field of 5 T. This is 6.5 times more than the previous record rotation of 14\ifmmode^\circ\else\textdegree\fi{} and it is the absolute maximum observable rotation in a single reflection. Using newly derived equations for large rotations and ellipticities, the off-diagonal conductivity has been computed from the magneto-optical spectra and the optical spectra determined for the same external parameters. The spectra are discussed in terms of the field and temperature dependent ground state of CeSb.

After the observation of heavy-fermion characteristics in the charge-ordered phase of Yb 4 As 3 , it is surprising that Yb 4 (As 0.71 Sb 0.29 ) 3 shows heavy-fermion behavior, though the charge ordering is suppressed. To shed more light onto this enigma, we measured the optical reflectivity of Yb 4 (As 0.71 Sb 0.29 ) 3 at several temperatures down to 15 K in the photon energy range from 1 meV to 5.5 eV. The comparison of the optical functions of Yb 4 As 3 and Yb 4 (As 0.71 Sb 0.29 ) 3 revealed decisive differences in the far-IR region. For the latter material, we found an hybridization gap of 3.3 meV, absent in Yb 4 As 3 . The effective mass of the conduction carriers takes the temperature-independent value of 2 m e in Yb 4 As 3 , while m * shows a logarithmic temperature dependency and reaches 80 m e at 15 K in Yb 4 (As 0.71 Sb 0.29 ) 3 . This proves that two different mechanisms lead to a large γ value in Yb 4 As 3 and Yb 4 (As 0.71 Sb 0.29 ) 3 .

We use magneto-optical techniques to determine empty f levels by making an optical transition in a spin-polarized sample by a strong external magnetic field. In a La compound, e.g. LaSe in a field of 10 T at 1.5 K, we observe the optical transition from the occupied and spin polarized 5d states into the empty and spin—orbit split 4f 1 state. In the magneto-optical spectrum of LaSe we find two sharp lines at 2.77 and 3.01 eV, the stronger one having +2.1° Kerr rotation. The sign of the Kerr rotation is inverted between CeSe (occupied 4f 1 ) and LaSe (empty 4f 1 ) permitting a distinction between transitions into empty or from occupied f states.

Very fast magnetic relaxation processes down to 7 mK have been observed in UPt3 single crystals with H⊥c-axis. The decay curves cannot be described with a logarithmic law but are well fitted by a stretched exponential time dependence. At low magnetic fields, two different vortex dynamics regimes are observed: an almost temperature independent flux motion regime for T 400 mK. These novel relaxation effects point to particular features of the vortices or of the flux line lattice in the two low-field superconducting phases of UPt3.

The temperature and field dependence of the complex polar Kerr effect and the optical reflectivity of CeSb single crystals are presented. The evolution of the magneto-optical effect to reach the new record value of 90 degree single Kerr rotation at 1.5 K and 5 T in the near infrared spectral range is discussed and qualitative arguments to account for this giant effect are given.