Žekš, Boštjan (1940–)

Žekš Boštjan, fizik, r. 26. jun. 1940 v Ljubljani zdravniku Zoltanu in Mariji r. Torkar, živi na Vrhniki. V Lj. je obiskoval 1951–9 klas. gimn. in 1959–63 štud. tehn. fiziko, 1966 dipl., 1970 magistriral ter bil 1972 prom. z dis. Dinamične lastnosti feroelektrikov tipa red–nered. Na odseku za teorijsko fiziko Inst. J. Stefan v Lj. je bil 1966–73 asistent in 1975–7 znanstv. sodelavec, na inšt. za fiziko univ. v Recifeju (Brazilija) 1973–5 red. prof., na Med. fak. v Lj. 1977–82 docent, od 1982 izr. in od 1987 red. prof. za biofiziko. – Prejel je nagrado Sklada B. Kidriča (1980) in Kidričevo nagrado (1988). 1987 je bil izvoljen za dop. člana SAZU.

Izpopolnjeval se je 1972–3 (Humboldtov štipendist) na inšt. za teorijsko fiziko univ. v Frankfurtu, avg.–dec. 1979 na univ. v Recifeju, apr.–maj 1983 v Saarbrücknu, jun.-sept. 1983 v Colchestru (Vel. Britanija). Udeležil se je štev. kongresov (ref. obj. v kongr. zbornikih), mdr.: Pariz 1971, 1976, Rennes 1972, Bangalore (Indija) 1973, 1978, Philadelphia 1975, Sarajevo 1976, Haludovo 1977, Bordeaux, Kruševac, Lj. 1978, Portorož, Skopje 1979, Cannes, Ohrid 1980, Malaga 1983 (Phase diagram of ferroelectric liquid crystals in an external magnetic field), Bovec 1984 (Landau free energy expansion for chiral ferroelectric smectic liquid crystals), Novi Sad 1984, Poljska 1984 (Red cell membrane properties and the bilayer couple hypothesis of red cell shape transformations) idr.

Ž-ev znanstv. opus je obširen, po tematiki raznolik, v večini raziskav pa je prisotna Landauova teorija faznih prehodov s posebnim poudarkom na dinamiki. Dela lahko razvrstimo v več obsežnih skupin:

a) Teorija faznih prehodov v feroelektrikih. Tu je obravnaval termodinamijske lastnosti in dinamiko feroelektrikov. Veliko je prispeval k razvoju teorije feroelektrikov z vodikovimi vezmi in feroelektrikov perovskitnega tipa. Posebej je treba omeniti raziskave dinamike kristalov tipa KH₂PO₄; sodeloval je pri razvoju teorije dinamike mehkih nihanj in teorije centralnega vrha v teh kristalih. Študij feroelektrikov z vodikovimi vezmi ga je vodil do raziskav dinamike Isingovega modela v transverzalnem polju. Z R. Blincem je pokazal, da je psevdospinski formalizem primeren za opis večine dinamičnih lastnosti pri sistemih s strukturnimi faznimi prehodi tipa red–nered. Na področju feromagnetizma in antiferomagnetizma je prispeval k teoriji razredčenih antiferomagnetikov.

b) Teorija tekočih kristalov. Proučeval je mehka nihanja, dinamiko vijačnih smektičnih feroelektričnih kristalov, dielektrično disperzijo in vpliv magnetnega polja na fazne diagrame tekočih kristalov. S sodelavci je razvil generalizirano Landauovo teorijo tekočih kristalov.

c) Površinska nihanja v feroelektrikih. Sodeloval je pri analizi možnosti obstoja površinskih fluktuacij s pomočjo treh različnih teoretskih pristopov: z mikroskopsko psevdospinsko teorijo za Isingov model v transverzalnem polju, z makroskopsko Landauovo teorijo z dodanimi členi, ki popisujejo vpliv površine feroelektrika, ter končno s polaritonskim modelom.

č) Uporaba teorije katastrof pri analizi faznih prehodov plin–tekočina–trdno telo (z M. Ribaričem).

d) Teorijske raziskave v biofiziki s poudarkom na študiju elektrostatičnih lastnosti membran. Tu proučuje predvsem fazne prehode v biol. membranah in transport skozi membrane. Posebno pozornost je posvetil fenomenološkemu modelu faznega prehoda v dvojni lipidni plasti, ki je osnovan na modificirani teoriji Landaua. Pri tem upošteva tudi elektrostatični prispevek, ki izvira iz površinskih nabojev. Pomembne so raziskave o obliki eritrocitov ter o vplivu vodikovih vezi na površinsko napetost vode in na spontano polarizacijo na meji tekočina–trdno telo.

Napisal je knjigo Lattice dynamics of ferroelectrics and antiferroelectrics (Amsterdam 1974, soavtor R. Blinc; ruski prevod Moskva 1975, kitajski Peking 1982). Za Biomathematics and cell kinetics (Amsterdam 1978) je s S. Svetino prispeval Transition probability models of the cell cycle exhibiting the age distribution for cells in the indeterministic state of the cell cycle.

Razprave, nastale večinoma z domačimi ali tujimi soavtorji, so izšle mdr. v: Journal of physic and chemistry of solids, London (1966: Kinetics of the alignment of O₂ centers in stressed alkali halide crystals; 1978: Theory of ferromagnetic polariton linewidth), Physical review, New York (1971: Dynamics of the ferroelectric Rochelle Salt; 1974: Softmode dynamics in nematic liquid crystals; 1975: Gravitational radiation from supernova explosions; 1976: Thermodynamics of an eight-site order-disorder model for ferroelectrics, Soft-mode dynamics of partially deuterated KH₂PO₄ – type crystals; 1978: On the dynamics of helicoidal ferroelectric smectic C liquid crystals, Model of the structural phase transitions in (CH₃NH₃)₂CdC1₄–type compounds; 1979: NQR study of the ferroelectric phase transition in KH₂AsO₄ under high hydrostatic pressure, Ising model in a transverse tunneling field and proton-lattice interaction in H-bonded ferroelectrics, Dynamics of structural phase transitions in (CH₃NH₃)₂CdC1₄–type compounds; 1980: Microscopic theory of the ferroelastic transition in KH₃(SeO₃)₂; 1982: Dielectric constant in the incommensurate phase of Rb₂ZnCl₄, Phase transition from an intercalated to a nonintercalated structure in a lipid bilayer; 1983: Dielectric relaxation in pseudo one-dimensional ferroelectric CsD₂PO₄), Solid state communications, Oxford (1972: On the vanishing of ferroelectricity in KH₂PO₄ at high pressures), Advances in physics, London (1972: Dynamics of order-disorder-type ferroelectrics and antiferroelectrics), Physica status solidi, Berlin (1977: Field-induced phase transitions in a randomly diluted antiferromagnet), Journal of physics C: Solid state physics, London (1979: Magnetic properties of pure and diluted uniaxial antiferromagnets; 1982: Proton-lattice interactions and the soft mode in KH₂PO₄, Surface nuclear spin waves in a semi-infinite antiferromagnetic insulator; 1984: Applicability of mean field approximation to KD₂PO₄–type ferroelectrics, Theory of surface modes in ferroelectrics; 1986: Dielectric properties of the quasi-one-dimensional hydrogenbonded ferroelectric crystal PbHPO₄), Physical review letters, New York (1979: Pseudo-one-dimensional ferroelectric ordering and critical properties of CsH₂PO₄ and CsD₂PO₄, Phase transitions in a lipid bilayer embedded in a crystalline matrix; 1982: Phase diagram of a ferroelectric chiral smectic liquid crystal near the Lifshitz point, Incommensurate-reentrant high symmetry phase transition in a layer-structure perovskite), Journal of chemical physics, New York (1979: Dynamics of the n-decylammonium chains in the perovskite type layer structure compound (C₁₀H₂₁NH₃)₂CdC1₄; 1981: Electrostatic potential of bilayer lipid membranes with the structural surface charge smeared perpendicular to the membrane-solution interface. An extension of the Gouy-Chapman diffuse double layer theory; 1985: Consideration of the spontaneous polarization of water at the solid/liquid interface), Chemical physics letters, Amsterdam (1981: The undulations of hydrated phospholipid multilayers may be due to water-mediated bilayer-bilayer interactions; 1982: The free energy, enthalpy, and enthropy of hydration of phospholipid bilayer membranes and their dependence on the interfacial separation; 1983: The contribution of hydrogen bonds to the surface tension of water), Biomedica biochimica acta, Berlin (1983: Bilayer couple hypothesis of red cell shape transformations and osmotic hemolysis; 1985: Bilayer couple as a possible mechanism of biological shape formation), Bioelectrochemistry and bioenergetics, Lausanne (1984: Polarization of water at the solid/liquid interface and the distribution on ions close to the phospholipid membranes), Studia biophysica, Berlin (1985: Lipid bilayer elasticity and the bilayer couple interpretation of red cell shape transformations and lysis).

Prim.: osebni podatki (z bibl); LSAZU 1987 (1988), 94–6. – Slika: arhiv SBL. Trontelj