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We report $^{139}$La nuclear magnetic resonance measurements on a single-crystal sample of La$_{1.875}$Ba$_{0.125}$CuO$_{4}$ under uniaxial stress. The spin order is shown to be more robust than at $x = 0.115$ doping, however, for magnetic field $H$ in the $c$ direction and the stress applied along the [110] direction ($\sigma_{[110]}$) the spin order transition temperature $T_{\text{SO}}$ is rapidly suppressed. This is in stark contrast to the behavior with stress in [100] direction ($\sigma_{[100]}$), which has virtually no effect on $T_{\text{SO}}$. For $H \parallel [1\overline{1}0]$, $\sigma_{[110]}$ stress has a weakened effect, and the rate $d T_\text{SO}/d \sigma_\text{[110]}$ is drastically reduced. Thus, $H\parallel [1\overline{1}0]$ acts as a stabilizing factor for spin-stripe order. Also, the onset temperature of the low-temperature tetragonal crystal structure $T_{\text{LTT}}$ is essentially unaffected by [110] stress, while it decreases slowly under compression along [100]. We develop a Landau free energy model and interpret our findings as an interplay of symmetry-breaking terms driven by the orientation of spins. These findings put constraints on the applicability of theoretical models for the development of spin-stripe order., Comment: 25 pages, 10 figures

We report an extensive 139La and 17O NMR study of the spin dynamics under uniaxial strain with the external magnetic field aligned along [001] direction to avoid breaking the four-fold symmetry of the CuO2 planes. We find that even the moderate strain along [110] direction induces orthorhombic distortion which is detrimental to the formation of both SO and CO in the system. The 139La NMR spin-lattice relaxation rate consequently decreases. The system also exhibits unusual scaling with the applied strain. In contrast, no discernible change in either TSO or TCO is found when the strain is applied in [100] direction. We propose a free-energy model where the stripe formation is directly coupled to the orthorhombic distortion. We also applied a magnetic field in [110] direction to quench the spin degrees of freedom in CuO2 plane. The field expectedly couples to the spin system and the observed effect of the [110] orthorhombic strain is substantially diminished.

Discovery of high-temperature superconductivity in the cuprates in 1986 marked a milestone for condensed matter science, triggering extraordinary efforts to understand the mechanism of unconventional superconductivity and to improve the properties of these new materials. It turned out that cuprates display extremely complex phase diagrams with a wealth of phases that may be related to or inhibit superconductivity. Additional complexity comes from disorder that is inherent to the perovskite structure of cuprates. Many issues remained unsolved, especially the mechanism of Cooper pairing, and the nature of the nonsuperconducting state.Soon after the discovery, cuprate superconductors have been extensively studied by NMR, with many important insights, including the discovery of a pseudogap - the most intriguing state of the phase diagram. After intensive research in the first years, the pace of NMR investigations has slowed down, but the technique continues to provide valuable new information. NMR is very sensitive to the local charge distribution due to the nuclear quadrupole interaction that measures the electric field gradient (EFG) at each nucleus, and is thus a versatile bulk probe. Recent NMR work on cuprates predominantly involves either charge ordering phenomena on long or meso-scales, or intra-unit-cell charge distribution between Cu and O ions. Charge order is an intriguing phenomenon ubiquitous in cuprates, but likely unrelated to superconductivity. On the other hand, the precise determination of intra-unit-cell charge distribution might bring new insights into the superconducting mechanism. An overview will be given of recent NMR results in cuprates with an outlook towards understanding the pairing mechanism.

Rapid technological development affects every part of human action, from energy and communications to health and household. Stepwise jumps in this continous growth happen when new concepts or new states of matter are discovered. A large playground to discover novel phases of matter can be found in strongly correlated electron systems (SCES), comprising high-temperature superconductivity, quantum magnetism, heavy fermion systems, quantum Hall effect, etc. The discovery of such new phases is the forefront of modern condensed matter research. In SCES, competing interactions, collective phenomena, and quantum criticality ensure an enormous wealth of orderings and extraordinary complex phase diagrams, where the control parameter can be temperature, magnetic field, pressure or doping. Theoretical investigations predict the existence of novel even more exotic phases with yet unexplored properties. An interesting example are nematic phases predicted in frustrated spin systems or in high temperature superconductors. They are experimentally hard to detect since they break rotational, but not translational symmetry. To detect such phases one should apply a combination of several experimental methods, and some of them are rather unconventional. Here we show how a combination of nuclear magnetic relaxation, differential heat capacity and nonlinear conductivity may help to discover and characterize such nematic phases. We will also present some other experimental methods that are available in the Laboratory for solid state NMR and high frequency measurements at our Department.

NMR measurements of Y and La nucleus in YTiO3 and Y0.7La0.3TiO3.

The discovery of high-temperature superconductivity in cuprates ranks among the major scientific milestones of the past half century, yet pivotal questions regarding the complex phase diagram of these materials remain unanswered. Generally thought of as doped charge- transfer insulators, these complex oxides exhibit pseudogap, strange-metal, superconducting, and Fermi liquid behavior with increasing hole-dopant concentration. Motivated by recent experimental observations, here we introduce a phenomenological model wherein exactly one hole per planar copper- oxygen unit is delocalized with increasing doping and temperature. The model is percolative in nature, with parameters that are highly consistent with experiments. It comprehensively captures key unconventional experimental results, including the temperature and the doping dependence of the pseudogap phenomenon, the strange-metal linear temperature dependence of the planar resistivity, and the doping dependence of the superfluid density. The success and simplicity of the model greatly demystify the cuprate phase diagram and point to a local superconducting pairing mechanism.

Haldaneov jednodimenzionalni AFM lanac cjelobrojnih spinova S = 1 je tipičan niskodimenzionalni sustav: očituje se bogatstvom faznog dijagrama, egzotičnim fazama i kvantnim faznim prijelazima. Osnovno stanje ovog sustava je singlet, a pobužđenja su spinoni koji tvore tripletnu vrpcu odvojenu energetskim procijepom od osnovnog stanja. Haldane je pokazao da procijep nastaje zbog kvantnog svojstva cjelobrojnog spina i ne postoji u lancima polucjelobrojnih spinova.[1] Pri djelovanju vanjskog magnetskog polja jedna grana tripletne vrpce se zbog Zeemanovog efekta spušta do osnovnog stanja i pri kritičnom polju Bc1 (QCP)[2] zatvara procijep. Drugo kritično polje Bc2 opisuje polarizirani sustav kod kojeg je cijela tripletna vrpca popunjena. Izmežđu te dvije kvantne kritične točke dolazi do dugodosežnog koherentnog 3D urežđenja opisanog AFM-XY hamiltonijanom. Spinovi u takvom sustavu se mogu mapirati na Bose-Einsteinov kondenzat.[3] Iznad urežđenog stanja pobuženja su 1D spinoni opisani Tomonaga-Luttinger tekucinom (TLL).[4] Organski spoj mNO2PhBNO (BoNO) je dobar predstavnik Haldaneovog lanca budući da njegov spinski hamiltonijan ne sadrži nikakve anizotropne clanove, jer je cjelobrojni spin kreiran molekulskom orbitalom, za razliku od d-orbitale iona prijelaznog metala. Takožđer, sa jačinom interakcije J=10 K, relativno visokom temperaturom urežđenja Tc=2.7 K i relativno niskim kritičnim poljima Bc1=1 T, Bc2=30 T predstavlja idealni spoj za proučavanje 1D lanca cjelobrojnog spina kao i cjelokupnog faznog dijagrama kondenzata. Bit će prikazani rezultati mapiranja faznog dijagrama mjerenjem toplinskog kapaciteta u He3 VTIju (T > 0.4 K) i NMR mjerenja iznad T > 1.6 K na niskim poljima B < 12 T. Rezultati ukazuju na visoku anizotropiju i postojanje kvantne kriticne točke na kritičnom polju Bc1. Mjerenja ukazuju na privlačnu interakciju u TLL režimu. Diskutirat će se BoNO sustav kao idealni haldaneov lanac. [1] F. D. M. Haldane, Phys. Rev. Lett. 50, 1153 (1983). [2] S. Sachdev, QuantumPhase Transitions, 2nd. ed. (CambridgeUniversity Press, Cambridge, 2011). [3] V. Zapf, M. Jaime and C. D. Batista, Rev. Mod. Phys. 86, 563 (2014). [4] T.Giamarchi, QuantumPhysics inOneDimension (OxfordUniversity Press, Oxford, 2004).

We present a Cu nuclear magnetic/quadrupole resonance study of the charge stripe ordered phase of LBCO, with detection of previously unobserved ('wiped-out') signal. We show that spin-spin and spin-lattice relaxation rates are strongly enhanced in the charge ordered phase, explaining the apparent signal decrease in earlier investigations. The enhancement is caused by magnetic, rather than charge fluctuations, conclusively confirming the long-suspected assumption that spin fluctuations are responsible for the wipeout effect. Observation of the full Cu signal enables insight into the spin and charge dynamics of the stripe-ordered phase, and measurements in external magnetic fields provide information on the nature and suppression of spin fluctuations associated with charge order. We find glassy spin dynamics, in agreement with previous work, and incommensurate static charge order with charge modulation amplitude similar to other cuprate compounds, suggesting that the amplitude of charge stripes is universal in the cuprates., Comment: 7 pages, 5 figures

Charge stripe order is recognized as an important ingredient of the physics of cuprates, yet the thermodynamics of its formation and the influence of disorder are unclear. We present a study of the development of charge stripes in the cuprate La1.8xEu0.2SrxCuO4 using three complementary experimental techniques: nuclear quadrupole resonance, nonlinear conductivity and specific heat. We find an intermediate phase between(pseudogapped) metal and charge stripes, existing in a dome-shaped region of the phase diagram and appearing through a sharp phase transition. A novel technique is used for measurements of nonlinear response, which show that the new phase is consistent with a charge nematic. This is in agreement with recent predictions of charge stripe formation with quenched disorder. Our experiments thus resolve the intricate process of charge stripe formation and provide a link to other materials with electronic nematic order.

The prospect of carbon-based ferromagnetic materials is extremely interesting from fundamental point of view, and promising for various applications. Recently, bulk ferromagnetic graphite was produced by intro- ducing high concentrations of defects into pristine graphite, providing the opportunity to investigate the microscopic origin of carbon ferromagnetism. Here we present an investigation of the intrinsic magnetic field in ferromagnetic carbon, studied by nuclear magnetic resonance (NMR) of the carbon nuclei. The nuclear resonant frequencies without external applied fields are a measure of the local intrinsic fields, giving direct insight into the magnetism of carbon-based materials. We find a clear 13C NMR signal in magnetic graphite, persisting up to temperatures above 200 K and with all the characteristics of ferromagnetic NMR, showing that the material is indeed intrinsically magnetic. We compare our results to quantitative DFT calculations of the microscopic field, showing that the magnetism originates in polarized point defects in the graphite structure. Finally, we discuss unconventional domain dynamics in ferromagnetic graphite uncovered by dynamical NMR measurements.

Low dimensional quantum magnetism is of great interest in theoretical and experimental physics research, owing to strong quantum fluctuations and consequently rich phase diagrams. Coupled spin clusters, called quasi-0D magnets, are of great interest because of their isolated intracluster interactions, as well as competing intercluster interactions that give rise to a 3D ordering. SeCuO3 is monoclinic P21/n system. Magnetism in this compound is due to a single spin on Cu 3dx2-y2 orbital. Two inequivalent Cu sites, Cu1 and Cu2 are connected by oxygen superexchange to other Cu sites, forming spin cluster of four spins, i.e. tetramer (Fig.1). The Heisenberg interaction parameters- J11 for Cu1 – Cu1, and J12 for Cu1 – Cu2 - have values of approx. 200 K [1]. SeCuO3 orders antiferromagnetically at TN = 8 K, which leaves a wide temperature range (8- 200 K) at which low dimensional magnetism exists, making it an ideal compound for study. Multiple experimental techniques were previously used [2, 3] to determine its ordered phase and low-dimensional behaviour, but no clear picture was established. Tetramer interactions are not able to fully explain observables in disordered state, and spin structure in ordered phase is not completely clear. NQR technique was used as a local probe of spin structure. Cu nucleus was chosen for its direct coupling to magnetic moment. By doing rotation measurements on single crystals, we show that only Cu1 site is observable by NMR technique, implying fast fluctuations of Cu2 site magnetic moments. Zero field NMR in the ordered phase, together with neutron measurements [4], provided insight into spin structure and onset of antiferromagnetic order. Spin-lattice relaxation measurements were conducted to study spin dynamics in 3D ordered and disordered lowdimensional phase. It provided clear and direct evidence of spin ordering into gapless antiferromagnetic phase. Evidence for 1D long range correlations are present in temperature range from 8 up to 90 K. At higher temperatures activation behavior is visible, comparable to J11 coupling energy. Our work showed that to completely explain the magnetism in disordered phase of SeCuO3, a model beyond tetramer is needed, probably including J22 chain-like interaction (in accordance with our spin – lattice relaxation), and maybe existence of Dzyaloshinskii-Moriya interaction between Cu1 and Cu2 sites. Magnetic structure in ordered state was determined for Cu1 spins, studied with NMR of the corresponding nuclear spin. [1] H. Effenberger, Z. Kristallogr. 175, 61 (1986) [2] I. Živković et al., Phys. Rev. B 86, 054405 (2012) [3] M.Herak et al., Phys. Rev. B 89, 184411 (2014) [4] V. Šurija, private correspondence

We present an investigation of local magnetic fields in ferromagnetic graphite, using nuclear magnetic resonance (NMR) of carbon nuclei. Guided by ab initio calculations, we detect a 13C NMR signal in chemically modified graphite without external magnetic fields, conclusively demonstrating the intrinsic ferromagnetic character of the material. Our work is the first detection of internal fields in carbon-based ferromagnets, and the values of the fields suggest that the magnetism is induced by defects in the material structure.[1] The zero-field NMR signal survives up to temperatures above 200 K, in agreement with previous macroscopic magnetization measurements, providing perspectives for the application of this unique material at room temperature. Furthermore, we investigate the dynamics of domain walls, through measurements of nuclear relaxation times and the ferromagnetic enhancement of the 13C NMR signal. Two dynamically distinct carbon sites are found, and we discuss their origin and the consequences for the microscopic structure of the material.

La2−xSrxCuO4(LSCO) je jedan od najistraživanijih visokotemperaturnih supravodiča, ali kao ni za druge kuprate, prijelaz iz uređenog antiferomagneta u supravodič nije popuno istra- žen. Nova eksperimentalna metoda omogućila nam je NMR mjerenja na kraćim vremenskim skalamaodstandardnih, čimejedijelomizbjegnutwipeoutefektkojisejavljaprimjerenimana niskim temperaturama na jezgrama bakra. Frekventni spektri poddopiranog kristala LSCO- 0.06susnažnopotisnutiiprošireniispod50K, štojeznatnoiznadtemperaturesupravodljivog prijelaza. Mjerenja relaksacijskog vremena T 2kroz liniju upućuju na moguću faznu separaciju, pri čemu se javljaju “brza” komponenta u središtu linije i “spora” komponenta koja se proteže u širem frekventnom području. Relaksacijsko vrijeme T2 mjereno u središtu linije raste s padom temperature, što znači da uzrok nestanka signala nije ubrzana relaksacija cijelog sustava, nego postupno razdvajanje faza u realnom prostoru, pri čemu s hlađenjem raste udio “brze” faze.

Niskodimenzionalni kvantni magnetizam je važna grana istraživanja u fizici čvrstog stanja. Materijali niskodimenzionalne magnetske strukture često imaju bogate fazne dijagrame kao posljedicu kvantnih fluktuacija te nude izvrsnu platformu za testiranje teorijskih predviđanja. Spinski klasteri, poznati i kao kvazi-0D magneti, posjeduju jednostavne unutarklasterske interakcije koje se mogu dobro teoretski opisati i eksperimentalno potvrditi. Posebno je važna međuigra unutar i vanklasterskih interakcija koje vode do 3D uređenja koje je nezaobilazno u eksperimentu. SeCuO3[1] predstavlja spinski klaster u kojem magnetizam proizlazi zbog jednog elektrona u posljednjoj dx2−y2 orbitali. Dva neekvivalentna položaja bakra, Cu1 i Cu2 interagiraju preko superizmjene i tvore spinski klaster (tetramer) od četiri linearno poredana spina: Cu2-Cu1-Cu1- Cu2. Parametri interakcije za Cu1-Cu1 vezu i za Cu1-Cu2 vezu imaju vrijednosti J11 = 225 K i J12 = 160 K.[2] Ispod TN = 8 K SeCuO3 se uređuje u 3D antiferomagnet. Širok temperaturni raspon 8−200 K niskodimenzionalne fizike je idealan za proučavanje kvantnih interakcija. Nekoliko eksperimentalnih tehnika su korištene da bi se razjasnio magnetizam u ovom spoju [2, 3, 4] budući da samo tetramerske interakcije nisu dovoljne za opis sustava iznad temperature uređenog stanja, već je očito da važnu ulogu imaju vantetramerske interakcije. Metoda nuklearne magnetske rezonancije na jezgri bakra pruža direktan uvid u statičko i dinamičko ponašanje spina stoga je to nezaobilazna metoda u razjašnjavanju magnetskih interakcija i dinamike. Napravljen je rotacijski spektar te je uspoređen sa simulacijama kako bi se odredili nepoznati NMR parametri. Knightov pomak i spin- rešetka relaksacija su potom mjereni što nam daje direktan uvid u spinsku statiku i dinamiku. Rezultati pokazuju nastanak spinskog singleta centralnih spinova u tetrameru (Cu1-Cu1) na temperaturama ispod J11 interakcije, te upućuju na potrebu razmatranja interklasterskih interakcija koji su odgovorni za ponašanje sustava na nižim temperaturama. [1] H. Efenberger, Z. Kristallogr. 175, 61 (1986) [2] I. Živković et al., Phys. Rev. B 86, 054405 (2012) [3] M. Herak et al., Phys. Rev. B 89, 184411 (2014) [4] S. Lee et al., Phys. Rev. B 95, 054405 (2017)

We measure the NQR signal on the Zn site in nearly optimally doped YBa2Cu3O6:92, when Cu is substituted by 3% isotopically pure 67Zn. We observe that Zn creates large insulating islands, confirming two earlier conjectures, that doping provokes an orbital transition in the CuO2 plane, which is locally reversed by Zn substitution, and that the islands are antiferromagnetic. Cluster and DFT calculations show that the D4 symmetry breaking, observed directly in the NQR signal here, is due to the same partial lifting of degeneracy of the nearest-neighbor oxygen sites as in the LTT transition in La2-xBaxCuO4, similarly well-known to strongly suppress superconductivity. These results prove that in-plane oxygen 2p5 orbital configurations are principally involved in the metallicity and SC of all high-Tc cuprates, and provide a qualitative symmetry-based constraint on the SC mechanism.

We measure the NQR signal on the Zn site in nearly optimally doped YBa2Cu3O6:92, when Cu is substituted by 3% isotopically pure 67Zn. We observe that Zn creates large insulating islands, confirming two earlier conjectures, that doping provokes an orbital transition in the CuO2 plane, which is locally reversed by Zn substitution, and that the islands are antiferromagnetic. Cluster and DFT calculations show that the D4 symmetry breaking, observed directly in the NQR signal here, is due to the same partial lifting of degeneracy of the nearest-neighbor oxygen sites as in the LTT transition in La2-xBaxCuO4, similarly well-known to strongly suppress superconductivity. These results prove that in-plane oxygen 2p5 orbital configurations are principally involved in the metallicity and SC of all high-Tc cuprates, and provide a qualitative symmetry-based constraint on the SC mechanism.

We calculate realistic Fermi surface (FS) evolution of La2−xSrxCuO4 (LSCO) with Sr doping within an extensive ab-initio framework including advanced band-unfolding techniques. We show that ordinary Kohn-Sham DFT+U can reproduce the observed metal-insulator transition and arc growth, when not restricted to the paramagnetic solution space. We elucidate both arc protection and the inadequacy of the rigid-band picture as consequences of a rapid change in orbital symmetry at the Fermi energy: the material undergoes a dimensional crossover along the Fermi surface, between the nodal (2D) and antinodal (3D) regions. In LSCO, this crossover accounts for FS arcs and the antinodal pseudogap, otherwise ubiquitous phenomena in high-Tc cuprates. The orbital transition is experimentally confirmed by replacing 4\% of planar Cu in nearly optimally doped YBa2Cu2.97Zn0.03O6.92 powder with 67Zn isotope, lowering Tc to 57 K. The NQR spectrum of 67Zn, measured for the first time, shows that each Zn dopand surrounds itself with an insulating cluster. Zn destroys the SC metal by a Coulomb ``domino effect'' which reverts the orbital transition locally and pushes a significant number of surrounding sites back towards the parent-compound configuration.

U većini materijala vrijedi Ohmov zakon - odgovor na relativno slaba pobudna polja je linearan. Nelinearni odziv upućuje na netrivijalno ponašanje sustava, poput korelacija višeg reda, i čest je u blizini kritičnih pojava. U slučaju izmjenične pobude nelinarnost odziva uzro- kuje deformaciju sinusoidnog signala, koja se može detektirati, pomoću fazno osjetljivih pojačala, kao viši harmonici. Pogotovo su zanimljivi parni harmonici, odnosno signali na parnim višekratnicima pobudne frekvencije, jer se javljaju pri lomu nekih simetrija. Zbog geometrije eksperimentalnog postava koji koristimo, pojava drugog harmonika u našem mjerenju označava narušenje simetrije inverzije vremena u materijalu. Mjerenjem na uzorcima visokotem- peraturnih supravodiča iz porodice kuprata ustanovljeno je postojanje univerzalnog signala na drugom harmoniku u okolici temperature prijelaza, što ukazuje na lom simetrije inver- zije veremena u supravodljivom stanju. Prijašnja mjerenja nelinarnog odziva oko Tc signal su uglavnom pripisivala efektima vrtloga, što smo isključili mjerenjema u vanjskom magnetskom polju. Pokazalo se da je intenzitet signala (skaliran na veličinu uzorka) ovisi samo o dopiranju te da je neovisan o polju, a ispod temperature prijelaza i o temperaturi. Zaključujemo kako drugi harmonik ne opisuje dinamiku prijelaza, već odražava svojstva samog supravodljivog stanja.

We report systematic 17O-NMR measurements on the high-Tc cuprate (Ca_xLa_{; ; ; ; ; ; 1-x}; ; ; ; ; ; ) (Ba_{; ; ; ; ; ; 1.75 - x}; ; ; ; ; ; La_{; ; ; ; ; ; 0.25 + x}; ; ; ; ; ; )Cu_3O_y, for four different families (different x). Using Knight shift data, we show that the pseudogap lines for all families are inconsistent with a quantum critical point inside the superconducting dome. In addition, at constant doping the pseudogap temperature does not vary with x, in contrast to Tc. We therefore argue that pseudogap and superconductivity are separate phenomena in these cuprates. Using Knight shift data, we show that the pseudogap opening temperature T* is much higher than Tc near optimal doping, unlike structurally similar YBCO. In addition, at constant doping the pseudogap temperature does not vary with x, in contrast to Tc. This puts constraints on the nature of the pseudogap and position of the quantum critical point inside the superconducting dome.

We present the experimental detection of a previously unseen electronic phase in the cuprate Eu-LSCO (La2-x-yEuySrxCuO4 , with y = 0.2), an intermediate between the well-known charge stripe ordered phase and the Fermi liquid. While its origin remains unknown, our experimental investigation employing several techniques reveals surprising properties of the new phase. We have used nuclear quadrupole resonance (NQR) of copper nuclei in the Cu-O planes of high quality Eu-LSCO single crystals to obtain information on the local electronic dynamics, detecting a decrease of signal intensity (partial 'wipeout' effect) in the new phase. This is indicative of strong spin fluctuations, as already observed in other charge- and spin-ordered cuprates, and the partial signal decrease hints at significant nanoscale inhomogeneity of the electron system. Insight into collective electronic behaviour is obtained from nonlinear conductivity measurements performed in a specially constructed high-sensitivity probe. Nonlinear conductivity shows a divergence at the temperature corresponding to the onset of the intermediate phase, and since all linear transport coefficients are smooth close to the onset temperature, this suggests that the nature of the order in the intermediate phase is highly unusual. While Eu-LSCO was chosen as a model system due to prominent charge stripe order and the absence of interfering structural transitions, the observation of the unconventional intermediate phase could be of much broader importance for the physics of cuprates. Although evidence mounts that 'stripe-like' electronic phases are an essential ingredient of cuprate physics, unambiguously detecting them has been notoriously difficult if long-range translational order is absent. Our combination of experimental techniques enabled us to study just such a phase in unprecedented detail, uncovering its extraordinary properties.

Najpoznatiji visokotemperaturni supravodiči, kuprati rijetkih zemalja, uz supravodljivost pokazuju veliko bogatstvo elektronski uređenih faza čija priroda često nije do kraja razjašnjena. Posebno mjesto tu zauzimaju faze s ’prugastom’ modulacijom spina i/ili naboja ; njihova dinamika i uloga ustvaranju i razaranju supravodljivog uređenja još uvijek su predmet istraživanja i rasprava. Ovdje ćemo prezentirati rezultate istraživanja prugastih i srodnih faza pomoću nuklearne kvadrupolne rezonancije (NQR) u monokristalima reprezentativnih lantanovih kuprata, La_2-x Ba_x Cu O_4 i La_2-x-y Eu_y Sr_x CuO_4. Neočekivano, rezultati se drastično razlikuju od prijašnjih mjerenja na mikronskim prahovima. Intenzitet NQR linije bakra ispod temperatura prijelaza u prugaste faze pokazuje smanjenje koje se opaža i u prahovima(tzv. wipeout), ali je u svim promatranim monokristalima pad signala s temperaturom dramatičan i znatno nagliji nego u prahu: signal pada na razinu šuma već nekoliko K ispod prijelaza. Time dokazujemo da NQR intenzitet bakra ne može biti proporcionalan parametru uređenja - suprotno tvrdnjama na osnovi mjerenja u prahovima - s obzirom da (lokalni) parametar uređenja ne može ovisiti o veličinama kristalnih zrna. Također, iznad temperatura prijelaza određenih neutronskim i rentgenskim raspršenjem u kristalima La_2-x-y Eu_y Sr_x CuO_4 opažamo dodatnu fazu koja do sada nije viđena, u kojoj dolazi do djelomičnog gubitka NQR intenziteta. Priroda te faze je nepoznata, ali pretpostavljamo da se radi o flžuktuirajućim prugama bez dugodosežnog reda (e.g. nabojni nematik) koje su nevidljive za tehnike raspršenja. NQR kao lokalna proba nam time omogućava uvid u do sada slabo istraženu dinamiku taljenja prugastih faza i međufaza u kojima su samo neke simetrije slomljene.

Mehanizam supravodljivosti u kupratima nije u potpunosti razjašnjen ni 20 godina nakon otkrića prvih supravodljivih kuprata dobivenih dopiranjem Mottovih izolatora. Teorijsko rješenje sustava je teško zbog jako koreliranih elektrona. Jedno od zanimljivijih područja je poddopirano područje (u blizini Mottovog izolatora) u kojem se javlja faza pseudoprocjepa. Radi proučavanja ponašanja pseudoprocjepa u ovisnosti o dopiranju, koristili smo modelni spoj (Ca_x La_1-x)(Ba_1.75−x La_0.25+x)Cu_3 O_y (CLBLCO), zanimljiv jer se neovisno može mijenjati interakcija J ključna za supravodljivost (x) od koncentracije vodljivih šupljina (y). Zbog toga u različitim obiteljima maksimalni Tc varira do 30%, bez većih promjena u kristalnoj strukturi i kakvoći uzorka, što potvrđuju i naša NMR i NQR mjerenja. Ponašanje koje ukazuje na otvaranje pseudoprocjepa primijećeno je u temperaturnoj ovisnosti spektara kisika te u spin-rešetka relaksacijskim mjerenjima kisika, bakra i lantana. Sustavno je bilo moguće odrediti temperaturu otvaranja pseudoprocjepa T* samo iz temperaturnog pomaka kisikove linije u NMR spektru. Potvrđeni su rezultati određivanja temperature T∗ istosmjernom magnetskom susceptibilnošću i dopunjen je fazni dijagram. Zanimljivo, iako je strukturno CLBLCO sličan YBCO sustavu, rezultati pokazuju veću sličnost sa LBCO/LSCO spojevima. Za razliku od optimalno dopiranog YBCO spoja, u CLBLCO postoji faza pseudoprocjepa čak i za optimalno dopirani uzorak.

Solid state ionic conductivity is of immense technical importance, mainly because of battery technology. Yet it is not always clear what makes a substance undergo a transition from insulator to ionic conductor, and how the mobile ions manage to travel around the lattice relatively undisturbed. Here we discuss the ionic transport in Cu2HgI4, a well known fast ion conductor that undergoes an insulator-conductor transition at 344 K. NMR measurements provide ion-selective insight into dynamics, and we show that conduction is, surprisingly, mediated by the heavy Hg2+ ions, rather than much lighter and smaller Cu+. Although disordering of the Cu sublattice occurs at the transition, copper ions seem to remain static, while mercury motion is evident from a strong narrowing of the 199Hg NMR line. However, conductivity calculated from NMR linewidths shows a systematic discrepancy from DC conductivity, so frequency-dependent nonlinear conductivity measurements were used as a probe for dynamical heterogeneity. They reveal an 'intermediate' characteristic correlation timescale tau_corr ~1 ms. The existence of dynamical correlations is confirmed by stimulated echo NMR measurements, where the fraction of ions trapped after tau_corr can be quantitatively determined. To explain the observations, we propose a simple model requiring the existence of two kinds of particles, slow and fast. A simulation based on the model shows islands of trapped fast particles forming and dissolving in time, with a characteristic timescale similar to the one observed experimentally. The observed dynamical cooperativity can be compared with recent studies of arrested and glassy materials, thus opening an unexpected connection between seemingly unrelated fields of physics.

An exotic spin nematic phase is theoretically predicted [1] to exist in 1D chains with frustrated interactions, FM between 1st and AFM between 2nd nearest neighbors when a magnetic field is applied perpendicular to the chain with magnetization close to saturation. A good candidate for the realization of such a phase is the recently discovered compound LiCuSbO4 (J1/J2 = - 2.2), which shows an absence of long range order (LRO) down to 100mK and, most importantly, an experimentally accessible saturation field of 12T [2]. We present the temperature (1.5-300K) and field (2-13T) dependence of both NMR spectra and T1 spin-lattice relaxation times of 7Li measured on a powder sample. Our results show a change in dynamical properties (T1) at low temperatures and provide new information on the possible ground state of this J1-J2 system. The temperature dependence (26mK-10K) of the muon relaxation rate together with results in ref [2] point towards the existence of short range correlations at low temperatures.

Prostiranje supravodljivih flžuktuacija u području pseudoprocjepa poddopiranih visokotemperaturnih supravodiča proučava se već dulje vrijeme raznim metodama u mnogim svjetskim laboratorijima. Primjenjujući mikrovalnu tehniku(10-20 GHz), pokazali smo da se u optimalno dopiranom i blago poddopiranom supravodiču HgBa2CuO4+δ supravodljive flžuktuacije protežu oko 10 K iznad Tc. U jako poddopiranom YBa2Cu3O6+δ doseg supravodljivih flžuktuacija je oko 20 K iznad Tc. Analogna mjerenja u području THz pokazala su u nizu poddopiranih do naddopiranih tankih filmova LaSrCuO da se supravodljive flžuktuacije protežu oko 10-15 K iznad Tc. U svrhu provjere univerzalnosti dosega supravodljivih žfluktuacija, proveli smo mjerenja na monokristalima LaSrCuO raznih dopiranja sa i bez primjene vanjskog magnetskog polja, te utvrdili da flžuktuacijsko područje prati supravodljivu kupolu kao i u drugim ranije mjerenim visokotemperaturnim supravodičima.

U niskodimenzionalnim magnetskim izolatorima predviđeno je postojanje egzotičnih magnetskih uređenja u kojima bi očekivanje vrijednosti operatora spina =0 iščezavalo za svaki spin i i smjer α(α=x, y, z). Parametar uređenja predstavljala bi neiščezavajuća srednja vrijednost nekog kompleksnijeg operatora: primjerice ≠0, takvo egzotičnomagnetsko uređenje narušavalo bi spinsku O[3] simetriju, ali bi simetrija vremenske inverzije ostala netaknuta. Po analogiji s multipolnim razvojem distribucije naboja ovakve se faze nazivaju kvadrupolnim nematicima, a eksperimentalno su teško provjerljive. Analogna egzotična stanja predviđena su u sistemima opisanim frustriranim Heisenbergovim Hamiltonijanom s FM interakcijom prvih (J1) i AFM drugih susjeda (J2), u smjeru okomitom na smjer primijenjenog magnetskog polja u području visoke magnetizacije. Elementarna pobuđenja bila bi pobuđenja vezanih magnona, što bi indirektno bilo moguće opaziti NMR tehnikom. Realizacija nematske faze u spoju LiCuVO4 još uvijek je eksperimentalno otvoreno pitanje, a nedavno otkriveni spoj LiCuSbO4, s eksperimentalno dostupnim saturacijskim poljem 12T je odličan kandidat zaprovjeru dotične faze.