München, Techn. University, Diss., 2001.

Stuttgart, Univ., Diss., 2006.

Zugl.: Konstanz, Univ., Diss., 2008

Stuttgart, Univ., Diss., 2004.

Techn. Universiẗat, Diss., 2007--Darmstadt.

Halbleiter-Quantenpunkte sind Objekte in der Größenordnung von Nanometern, in denen wenige Ladungsträger in alle drei Raumrichtungen durch eine Potentialbarriere eingesperrt sind. Dies führt zu einer reduzierten Wechselwirkung mit dem Halbleiter-Gitter und zu einer diskreten Zustandsdichte. Die große Polarität der Bindung dazu, dass viele Wechselwirkungen direkt durch Spektroskopie der Photolumineszenz zu beobachten sind, was sie für die Grundlagenforschung attraktiv macht. Die ungleiche Anzahl von Elektronen und Löchern erlaubt die Untersuchung einzelner, ungepaarter Ladungsträger. Mit Hilfe der polarisationsaufgelösten Mikro-PL Spektroskopie werden Spinzustände einzelner QP reproduzierbar untersucht. Im Mittelpunkt stehen dabei Wechselwirkungen der Teilchen untereinander. Über die Anregungsspektroskopie werden höherangeregte Zustände identifiziert und charakterisiert. Die hier auftretenden Austauschwechselwirkungen führen zur Mischung zu Feinstrukturen im Spektrum. Kopplungen im angeregten Lochzustand zeigen die Möglichkeit zur optischen Orientierung des residenten Elektrons auf. Die Spinkonfiguration der Elektronen im Triplettzustand erlaubt es, die Elektron-Loch Austauschwechselwirkungen des Trions zu untersuchen. Der zweite Teil dieser Arbeit befasst sich mit semimagnetischen QP. Hier sorgt die Wechselwirkung mit einer paramagnetischen Umgebung von Manganspins für neue magneto-optische Eigenschaften. Diese zeigen sich auf der Ebene einzelner QP in Form von Linienverbreiterungen durch Spinfluktuationen als auch durch den Riesen-Zeeman-Effekt am QP-Ensemble. Besonderes Augenmerk liegt hier auf dem Einfluss der reduzierten Dimensionalität und der größeren Oberflächen der QP auf die Austauschmechanismen. Die starke Temperaturabhängigkeit der Spinumgebung wird ausgenutzt, um das Spinaufheizen als auch die Spin-Gitter-Relaxationsystematisch zu studieren. Dabei wird die PL der QP als Monitor benutzt. / Semiconductor quantum dots are objects on the nanometer scale, where charge carriers are confined in all three dimensions. This leads to a reduced interaction with the semiconductor lattice and to a discrete density of states. In the examined QD in II-VI seminconductor systems the large polar character of the bindings enables to observe particle interactions by spectroscopy of the photo-luminescence, making QD attractive for basic research. An odd number of carriers allows to study the latter in an unpaired state. By using polarization-resolved micro-PL spectroscopy, the spin-states of single, isolated QD can be studied reproducibly. Of special interest are exchange interactions in this few-particle system named trion. By excitation spectroscopy energetically higher states can be identified and characterized. The exchange interactions appearing here lead to state mixing and fine structure patterns in the spectra. Couplings in excited hole states show the way to the optical orientation of the resident electron spin. The spin configuration of the trion triplet state can be used to optically control the resident electron spin. Semimagnetic QD are focused in the second part of this work. The interaction with a paramagnetic environment of manganese spins leads to new magneto-optical properties of the QD. They reveal on a single dot level by line broadening due to spin fluctuations and by the giant Zeeman effect of the dot ensemble. Of special interest in this context is the influence of the reduced system dimension and the relatively larger surface of the system on the exchange mechanisms. The strong temperature dependence of the spin environment is used to systematically study the spin-lattice relaxation. Here, the PL of the QD ensemble monitors the spin temperature. The time constants in the mu range define the range for the incoherent switching of the Mn magnetization.

Since the last few decades frustrated spin systems have attracted much interest. These studies are motivated by the rich variety of their unusual magnetic properties and potential applications. In this thesis, excitation spectra of the weakly coupled dimer system Ba3Cr2O8, the spin-1/2 chain material with distorted diamond structure Cu3(CO3)2(OH)2 (natural mineral azurite), and the quasi-twodimensional antiferromagnet with triangle spin structure Cs2CuBr4 have been studied by means of high-field electron spin resonance. Two pairs of gapped modes corresponding to transitions from a spin-singlet ground state to the first excited triplet state with zero-field energy gaps, of 19.1 and 27 K were observed in Ba3Cr2O8. The observation of ground-state excitations clearly indicates the presence of a non-secular term allowing these transitions. Our findings are of crucial importance for the interpretation of the field-induced transitions in this material (with critical fields Hc1 = 12.5 T and Hc2 = 23.6 T) in terms of the magnon Bose-Einstein condensation. The natural mineral azurite, Cu3(CO3)2(OH)2, has been studied in magnetic fields up to 50 T, revealing several modes not observed previously. Based on the obtained data, all three critical fields were identified. A substantial zero-field energy gap, Δ = 9.6 K, has been observed in Cs2CuBr4 above the ordering temperature. It is argued that contrary to the case for the isostructural Cs2CuCl4, the size of the gap can not be explained solely by the uniform Dzyaloshinskii-Moriya interaction, but it is rather the result of the geometrical frustration stabilizing the spin-disordered state in Cs2CuBr4 in the close vicinity of the quantum phase transition between a spiral magnetically ordered state and a 2D quantum spin liquid.

Since the last few decades frustrated spin systems have attracted much interest. These studies are motivated by the rich variety of their unusual magnetic properties and potential applications. In this thesis, excitation spectra of the weakly coupled dimer system Ba3Cr2O8, the spin-1/2 chain material with distorted diamond structure Cu3(CO3)2(OH)2 (natural mineral azurite), and the quasi-twodimensional antiferromagnet with triangle spin structure Cs2CuBr4 have been studied by means of high-field electron spin resonance. Two pairs of gapped modes corresponding to transitions from a spin-singlet ground state to the first excited triplet state with zero-field energy gaps, of 19.1 and 27 K were observed in Ba3Cr2O8. The observation of ground-state excitations clearly indicates the presence of a non-secular term allowing these transitions. Our findings are of crucial importance for the interpretation of the field-induced transitions in this material (with critical fields Hc1 = 12.5 T and Hc2 = 23.6 T) in terms of the magnon Bose-Einstein condensation. The natural mineral azurite, Cu3(CO3)2(OH)2, has been studied in magnetic fields up to 50 T, revealing several modes not observed previously. Based on the obtained data, all three critical fields were identified. A substantial zero-field energy gap, Δ = 9.6 K, has been observed in Cs2CuBr4 above the ordering temperature. It is argued that contrary to the case for the isostructural Cs2CuCl4, the size of the gap can not be explained solely by the uniform Dzyaloshinskii-Moriya interaction, but it is rather the result of the geometrical frustration stabilizing the spin-disordered state in Cs2CuBr4 in the close vicinity of the quantum phase transition between a spiral magnetically ordered state and a 2D quantum spin liquid.

Within the last decades low-dimensional frustrated quantum spin systems have attracted great interest in the field of modern research. In these systems a competition of various magnetic interactions takes place, leading to an energetically degenerated magnetic ground state, and thus to the occurrence of exotic, unconventional physical properties at low temperatures. This thesis focuses on the quasi one-dimensional frustrated spin chain system linarite, PbCuSO4(OH)2. In this compound the basic building blocks are CuO4 plaquettes which are connected to each other along one crystallographic direction, analogue to a chain. The frustration in linarite is established due to the competition between the magnetic interactions. The nearest-neighbor magnetic spins are coupled ferromagnetically along the chain via a coupling constant J1, while the next-nearest neighbors are coupled antiferromagnetically via a coupling constant J2. For this configuration it is not possible to satisfy all magnetic couplings simultaneously, hence the system is magnetically frustrated. In this work, comprehensive thermodynamic and nuclear magnetic resonance (NMR) studies demonstrate that linarite is one of the richest and most fascinating compounds in the class of low-dimensional frustrated magnets. By means of susceptibility, magnetization, specific heat, magnetocaloric effect, magnetostriction, and thermal-expansion measurements a rich magnetic phase diagram could be mapped out below a temperature of 2.8 K. The phase diagram contains five different magnetic regions/phases for an external magnetic field pointing along the chain direction. Based on the thermodynamic studies it was possible to calculate the exchange integrals within the frustrated J1-J2 model and extensions of it by using various theoretical approaches. The magnetic microscopic nature of the different long-range magnetic phases present in linarite were investigated by NMR measurements and by collaborative neutron scattering experiments. The ground state (phase I) is identified as an incommensurate elliptical helical structure. Via a theoretical modelling the 1H-NMR spectrum of the ground state could be explained, revealing a rearrangement of the zero-field structure in an external magnetic field of 2.0 T used for the NMR studies. By further increasing the external field the system undergoes a complex spin flop transition in two steps (phase I - phase III - phase IV). In phase III a phase separation takes place where one part of the spins form a circular spiral structure while the remaining fraction form a simple antiferromagnetic structure. In phase IV the remaining circular spiral structure vanishes, so that all spins collectively form the antiferromagnetic collinear phase. The most peculiar physical properties studied in this thesis take place in region V at high fields, showing only tiny features in the thermodynamic properties. The magnetic spins in region V form a sine-wave modulated spin-density structure as identified via NMR and neutron investigations. It is discussed whether region V is related to a multipolar phase or if the spin-density wave structure could possibly coexist with such a phase.

Within the last decades low-dimensional frustrated quantum spin systems have attracted great interest in the field of modern research. In these systems a competition of various magnetic interactions takes place, leading to an energetically degenerated magnetic ground state, and thus to the occurrence of exotic, unconventional physical properties at low temperatures. This thesis focuses on the quasi one-dimensional frustrated spin chain system linarite, PbCuSO4(OH)2. In this compound the basic building blocks are CuO4 plaquettes which are connected to each other along one crystallographic direction, analogue to a chain. The frustration in linarite is established due to the competition between the magnetic interactions. The nearest-neighbor magnetic spins are coupled ferromagnetically along the chain via a coupling constant J1, while the next-nearest neighbors are coupled antiferromagnetically via a coupling constant J2. For this configuration it is not possible to satisfy all magnetic couplings simultaneously, hence the system is magnetically frustrated. In this work, comprehensive thermodynamic and nuclear magnetic resonance (NMR) studies demonstrate that linarite is one of the richest and most fascinating compounds in the class of low-dimensional frustrated magnets. By means of susceptibility, magnetization, specific heat, magnetocaloric effect, magnetostriction, and thermal-expansion measurements a rich magnetic phase diagram could be mapped out below a temperature of 2.8 K. The phase diagram contains five different magnetic regions/phases for an external magnetic field pointing along the chain direction. Based on the thermodynamic studies it was possible to calculate the exchange integrals within the frustrated J1-J2 model and extensions of it by using various theoretical approaches. The magnetic microscopic nature of the different long-range magnetic phases present in linarite were investigated by NMR measurements and by collaborative neutron scattering experiments. The ground state (phase I) is identified as an incommensurate elliptical helical structure. Via a theoretical modelling the 1H-NMR spectrum of the ground state could be explained, revealing a rearrangement of the zero-field structure in an external magnetic field of 2.0 T used for the NMR studies. By further increasing the external field the system undergoes a complex spin flop transition in two steps (phase I - phase III - phase IV). In phase III a phase separation takes place where one part of the spins form a circular spiral structure while the remaining fraction form a simple antiferromagnetic structure. In phase IV the remaining circular spiral structure vanishes, so that all spins collectively form the antiferromagnetic collinear phase. The most peculiar physical properties studied in this thesis take place in region V at high fields, showing only tiny features in the thermodynamic properties. The magnetic spins in region V form a sine-wave modulated spin-density structure as identified via NMR and neutron investigations. It is discussed whether region V is related to a multipolar phase or if the spin-density wave structure could possibly coexist with such a phase.