Experiments on strongly correlated materials under extreme conditions

The aim of this work has been to investigate the electronic phase transitions and the electronic dynamic behavior of 'strongly correlated materials'. In particular I will show data of three different classes of materials: iron-based superconductors, topological insulators and spin-crossover (SCO) materials, under conditions that can be considered extreme. Experiments have been performed under a variable DC magnetic field from 0 to 5 T, the temperature range was from 4.2 to 300 K and pressure applied was ranging from 0 to 3 GPa. Experiments were carried out the in the majority of the cases by measuring the DC and the AC magnetic susceptibility, while the last case regarded the infrared spectroscopy. The results of the multi-harmonic magnetic susceptibility experiments on iron based superconductors show: a) in the ‘1111’ NdFeAsO1-0.14F0.14, iron pnictide, the coexistence of a superconducting (Tc = 47 K) and a magnetic phase (Tm = 90 K); b) the superconducting phase in the ‘1111’ iron pnictide exhibits good bulk 3D pinning characteristics with a crossover to a 2D behavior for a Hdc field of 1 T. In fact these systems have a layered crystal structure with alternate superconducting iron-pnictogen active layers and rare earth-oxide spacer layers; c) a superconducting phase (Tc = 10 K) coexisting with a magnetic (Tm = 125 K) phase has been detected also in the ‘11’ FeSe1-xTex iron chalcogenide; d) the superconducting phase in the ‘11’ iron-chalcogenides also exhibits good bulk 3D pinning characteristics at low magnetic fields; e) the comparison between the ‘1111’ NdFeAsO1-0.14F0.14 iron pnictide superconductor and the ‘11’ iron chalcogenide superconductors properties, where only iron-chalcogen active planes are present, points out that the ‘1111’ iron superconductor has a stronger pinning force despite its higher thermal activation, related to the role of the (NdO) spacer layer. Concerning topological insulators two samples Bi2Te3 and Bi0.6Sb1.4Te3 have been characterized with both DC and AC magnetic susceptibility at low temperature (> 4 K) and at high magnetic field (< 5 T), to eventually probe their ideal surface metallic states vs. the bulk response. DC magnetic measurements showed a bulk diamagnetic behavior in the Bi2Te3. Moreover, the AC susceptibility response of Bi2Te3 at 170 and 970 Hz has been measured with a small excitation field. Data do not apparently show the expected decrease of the bulk diamagnetic signal (real part) due to a slight imbalance of the spin-oriented surface currents resulting in a paramagnetic contribution. The phenomenon is probably due to the impossibility to generate the extremely high frequency of the excitation field necessary to probe only the surface states. However, susceptibility data show unusually high values of the leakage currents (imaginary part) and, increasing the frequency a significant increase of the amplitude. Results may be compatible with the occurrence of a contribution from surface spin-oriented states unbalanced by the exciting magnetic field. The Bi0.6Sb1.4Te3 shows a paramagnetic behavior vs. temperature under low magnetic fields and a unusual paramagnetic/diamagnetic transition at 5 K for DC fields Hdc > 1 T. Furthermore during this work I contributed to the construction, assembly and commissioning of a new apparatus named Press-Mag-O that will allow investigation of materials under extreme conditions of low temperature (LT), high pressure (HP) and high magnetic field (HMF). This device is equipped by a highly sensitive SQUID magnetometer capable to probe the dynamic magnetic properties (AC susceptibility in the range 1 Hz - 5 kHz) concurrently with IR spectroscopy vs. pressure (< 20 GPa), DC magnetic field (up to 8 T) and temperature (4.2 - 300 K). In this respect, regarding the characterization of SCO materials, in the framework of a cooperation INFN/MEC, the High-Spin (HS) to Low-Spin (LS) transition induced by pressure has been monitored at room temperature in a iron coordination polymer. The sample has been characterized by IR spectroscopy and the occurrence of a transition between the two spin states has been clearly observed. The thesis is structured as follows: after a short introduction on the strongly correlated materials studied, in chapter 1 the AC and DC magnetic susceptibility technique and the instruments used are described. Moreover the glass pinning mechanism in type II superconductors and the critical state models used in the data analysis are discussed here. Also a brief introduction on the ‘SINBAD’ (Synchrotron Infrared Beamline At DANE) beamline is included in this section. Finally, the Cu-Be DAC cell to perform high-pressure experiments is presented. In chapters 2 and 3, are presented the magnetic characterization of the ‘1111’ NdFeAsO1-0.14F0.14 iron pnictide and the ‘11’ FeSe0.88, FeSe0.5Te0.5, FeSe0.25Te0.75, FeTe0.8S0.2 iron chalcogenide superconductors. A comparison of the properties using data of multi-harmonic magnetic susceptibility experiments under high magnetic fields (< 5 T) at low temperature (> 4.2 K) is presented and discussed. In chapter 4 is presented the magnetic behavior of the topological insulators Bi2Te3 and Bi0.6Sb1.4Te3 with AC and DC magnetic susceptibility at low temperature (> 4 K) and at high magnetic field (< 5 T). In chapter 5 are summarized the characteristics and the performances of the Press-Mag-O apparatus under commissioning at the Frascati National Laboratories (LNF) of the National Institute of Nuclear Physics (INFN). Moreover, to illustrate the potential of this instrument, IR spectroscopy data of the SCO HS to LS transition in the [Fe(Butrz)3](BF4)2·2H2O polymer under high pressure (< 3 GPa) are showed.